Trailer power

[clrussell]

So for an upcoming trip I’m going to have to run my big fridge, and honestly it’s been in the plan it’s just getting moved up the list.

Fridge is a snow master ex85, it’ll be ran off a group 31 agm, in/ on my trailer.

Currently the trailer is stand alone because it doesn’t require much power (31 lead acid in the box running lights/ water). I just charge it on shore power when available. It’s been on multi day trips with zero issue.

The upcoming trip will be 10 days, 4 adults 2 kiddos. Hence the bigger fridge this time around.

My plan is run 8awg paired wire to the bumper, Anderson plug, then to the trailer fridge to recharge the trailer battery. This is the setup I used in my 100 with the house battery and fridge being mounted in the rig.

Does anyone see any problems with this on the 200? My 100 had both house and cranking battery agm with a simple charge controller that was tied into the crank battery and switched on with key input. Basically key off it disconnects so it won’t draw down the crank battery.

I don’t feel like the factory trailer plug will supply sufficient power to charge the 31 back up. Or will it?

Pic of setup for the thread.

 

[TeCKis300]

It'll work. But you'll get pretty minimal charging to the battery because of the longer cable runs and also the temperature compensating alternator. What the battery in the trailer will effectively see is something like 13V and charging at 5 amps. Probably will maintain but never really fully charge.

I would recommend you add a DC-DC charger on the trailer side. This should supplement your Anderson circuit nicely. It'll be useful whether you stay with your current battery or any future battery chemistry.

I would recommend the Victron linked. It'll do several things including simplifying your Anderson circuit as it has configurable engine-on monitoring to only switch on when it detects the engine running. I'll also give visibility to the trailer battery state of charge and monitoring over BT

Amazon product ASIN B0851TPKV7

 

[mobi-arc]

I'd go bigger than 8. I used 8 on my 100 and the voltage drop at distance caused the fridge to go into it's low voltage error mode prematurely. Changed it to #4 and haven't had any drama.

 

[radman]

I would recommend you add a DC-DC charger on the trailer side. This should supplement your Anderson circuit nicely. It'll be useful whether you stay with your current battery or any future battery chemistry.

I came here to recommend a dc-dc as well. I have the Victron that teck posted inside my rig charging my LiFepo4 bank, but went with the renogy in my trailer because it has a built in mppt charger.

https://a.co/d/88PwyFz

 

[clrussell]

I wondered about a charge controller. Just for the fact I could easily add a solar panel on the trailer for down time.

 

[sdnative]

I'd go bigger than 8. I used 8 on my 100 and the voltage drop at distance caused the fridge to go into it's low voltage error mode prematurely. Changed it to #4 and haven't had any drama.

The 18A Victon DCDC mentioned above would be a good fit for a group 31 LA battery. I wouldn't go any bigger than that. 6awg between the engine battery and trailer DCDC should be good, or a larger 4awg from the engine battery to the back of the truck (can supply rear cargo area loads as well) and possibly a short 6awg or 8awg run from the truck to the trailer nose box. You want minimal voltage drop (~3% or less) when the DCDC is pulling full power, else it will think the engine battery voltage is running low and switch in and out of low voltage shutoff.

This assumes you will be driving every couple of days. You can also consider a MPPT charger with solar panels on the trailer or on a kick-stand nearby.

 

[staggrlee]

Voltage drop isn't as big of a deal with a DC/DC charger, but it is more inefficient. I'd run 6AWG min, but it's easy enough to run 4AWG down the frame rail.

Victron is nice for the BT, but plan for a shunt for batt. SOC monitoring, and a standalone MPPT charger. Redarc combines DC/DC & MPPT but no BT.

The cost and complexity of discrete components outweighs the higher cost of the Redarc IMO.

The stock trailer 12V aux is a 30A feed so you can run, at best, Redarc's 12A charger which will take a long time to charge a group 31. A higher rated Victron may allow 30A fusing/smaller guage wiring.

I had a 4AWG run direct from the primary batt. via a fused, ignition-triggered relay to an Anderson plug and installed a 25A Redarc in the trailer. Could have gone 50 with the LFP battery, but worried about alternator load. I already have a 50 charging the 2nd batt.

Frame ground your negative in the rear to reduce total wire path. Nothing about this requires a common ground with the batt. ground point. Better to run a fat + and local-ground the -, than to run lighter duplex wire.

It's super important to weatherize your Anderson connection. Dirt and corrosion = resistance = melted wires and possibly fire. The SB50 rubber boots are tough i to find in my experience, but worth it.

 

[sdnative]

Voltage drop isn't as big of a deal with a DC/DC charger, but it is more inefficient. I'd run 6AWG min, but it's easy enough to run 4AWG down the frame rail.

The DC/DC will boost voltage easily enough if needed, but the issue is to make sure the voltage doesn't sag too much going from no load to full load. If the voltage gets near or below the low voltage cutoff setpoint of the charger, it will cycle in and out of charging. That is the main reason to limit voltage drop to 3% or less. That was a problem on my truck using 8awg wire with the 18A charger. I switched to larger wire and the problem went away.

Also, the Victron Orion DC/DC chargers, and possible their MPPT chargers as well, can only accept a max of 6awg wire at the terminals. So if you run a larger wire you'll need to terminate on some power posts (probably a good idea anyway even with smaller wire).

 

[Fisher23]

That fridge takes very little amps so the normal line in the trailer plug will keep the battery running the fridge charged up while traveling.

If you will be driving 3-4 hours every day then it will recharge while driving.

BUT if only driving 2 or less hours per day then you need the Anderson and a dc to dc charger. I used the 40 amp dc to dc. the 20 amp was too slow charging.
I also use a lithium battery to run the fridge. You can get the 90 amp hr batteries cheaper and they are half the size. They charge back up fast.

The solar panels and a charge controller are only good if you base camp. If you are traveling overlanding everyday do not bother.
For base camp they are excelent.

 

[daneo]

I see lots of responses, but not a lot of maths. So here I am to ruin everyone's morning! Scroll to the end for the TLDR.

I don’t feel like the factory trailer plug will supply sufficient power to charge the 31 back up. Or will it?

Let's try a few methods and see!

According to this hastily-located fuse diagram I found in a YouTube video (and YouTube never lies), the 200 has 30A available on the towing plug before it blows the fuse. Staggrlee corroborates this info. Consult your documentation, but this should be sufficient for our example. In the documentation, it shows that other trailer loads like the brakes and lights run on different wires, so I will assume that we have all 30A at our disposal for our analysis.

Fuse Box Diagram Toyota Land Cruiser 200/J200/V8 2008-2018

Fuse box diagram (location and assignment of electrical fuses) for Toyota Land Cruiser (200/J200/V8; 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018).

fuse-box.info

​

--------------------------------------------------------------------------------------------------------------------------------​

First, could you run a wire from this pin to the battery in your trailer directly? Let's try some math.
The current (I) flowing between two points can be determined by knowing the voltage difference between those two points, and the resistance separating them:

V1 = Alternator output (or start battery voltage, whichever is higher at a given point in time)
V2 = Voltage at the trailer battery
R = Resistance in all of the wires and connectors

I = V2-V1/R

Let's model the case where you're just starting up the vehicle after sitting for several days and the battery in the trailer is dead from running the fridge. That's a very feasible worst-case.

For V1, our alternator voltage, I've never measured higher than 13.8V, but we'll round up a bit for some safety factor.
V1= 14.0V

For V2, we'll look at the SnoMaster's cutoff. 10V and 10.7V sound like they're getting into deep-cycle territory, so I wouldn't run a SLA down to those voltages. We'll stick with the 11.8V setting.

V2 = 11.8V

For R, since we're modeling for a worst-case scenario, I'm going to ignore any resistance introduced by connectors. I'm also going to assume 12AWG wire throughout. I'm using 16 feet for the run of wire within the Cruiser, 5 feet for the tongue, and 5 feet inside the trailer.

Wire Resistance Calculator - Cirris Inc

www.cirris.com

R = 0.033ohms

Plug those all in:
I = 14.0-11.8/0.033
I = 66.6 Amps
But, we said that the fuse for the trailer would blow at 30 Amps. 66.6 >> 30, so the setup where a wire is run directly from the tow plug to the battery won't work And we proved it with math!

----------------------------------------------------------------------------------------------------------------------------------

Second, let's evaluate a DC-DC running from the tow plug.
30 Amps lets us choose from a few models mentioned above.

The Victron https://www.victronenergy.com/uploa...Smart-DC-DC-chargers-isolated-250-400W-EN.pdf
The Renogy 12V 20A DC to DC On-Board Battery Charger - https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/
The Redarc https://cdn.intelligencebank.com/au/share/yE9N/zJpl/oGLrq/original/BCDC1225D,+BCDC1240D+Manual+English

I'd like to note that the documentation for the Redarc is much better than the other two. Good on you, Redarc. It's also the highest current of the lot, so if it checks out, the others will work as well. For the example, I'll be using the BCDC1225D (BDCD for short).

To evaluate this setup, it's clear that the 25A that the BDCD draws is less than the 30A the fuse is rated for. Huzzah! But the concern noted by some sharp commenters above is that it's possible that the voltage seen at the BCDC will sag to the point where it shuts itself off when there's high current on the wire. Let's see.

We want to know voltage drop, so we can use the most well-known form of Ohm's Law:
V=I*R

I = 25A from the BCDC regulated draw

R = The resistance in the wire + all the connections. More resistance means more voltage sag.

For the purposes of this model, I'm going to use 0.006 ohms for the resistance based on this connector designed for 30A automotive applications. It's connector type looks similar to a tow plug. 30 Amp Unassembled Red/Black Anderson Powerpole Connectors - https://powerwerx.com/anderson-powerpole-connectors-30amp-unassembled

Note that if you do a quick google search, it's not hard to find claims that the connection of a trailer plug is anywhere from 0.3ohms to 3ohms! The very small resistances are tough to measure with handheld meters because they have an extremely small test current and the resistance is dominated by tiny, needle-like probes. Yes, even you, Fluke. When attempting to measure small resistances with a handheld meter, the measurements have to be taken with a grain of salt. To measure small resistances well, one would need to send a larger, known current through the connection and measure the voltage drop.

I'll put in 0.006 ohms for the connection at the tow plug and another 0.006 ohms for the connection at the BCDC. Note, the BCDC comes with fantastic crimp connectors, so the 0.006 ohms is being modeled high.

For these purposes, I'll assume the battery charger wouldn't live on your bumper or the trailer tongue, so We'll re-use the 0.033 ohms for the 21 feet of wire from the calculations on the previous "directly wired" setup.

R = wire resistance + plug connector resistance + BCDC crimp resistance = 0.033 + 0.1 + 0.1
So back to our voltage drop equation:
V = I*R
V = 25*(0.033 + 0.006 + 0.006)
V = 1.125 volts dropped across a good clean connection ignoring any other connections that may exist on the cruiser between the tow plug and the alternator.

Let's say our temperature-compensating alternator has dropped to 13.2 volts (lowest I've measured) and we subtract the 1.125 volts from the resistive losses. That means the BCDC would only see a voltage of 12.075.

If the "Ignition Sense" wire is left unconnected, the BDCD unit would turn itself off every 100 seconds. It will check the input voltage every 100 seconds, and since 12.075 is less than 12.7 volts, it will shut itself down. And it's not even close.

However, the BCDC (and other DC-DC chargers) have an ignition sense wire, just for this scenario! To start, let's be lazy and connect the ignition sense wire to the 12AWG wire that's carrying power to the BCDC. The threshold for the ignition sense input is 12V. Since 12.075 volts is just barely above 12, if there are higher resistances that we've not accounted for in our model, the trigger would fail. I don't like those odds.

Better would be to run the Ignition Sense wire to an ignition-switched circuit in the vehicle like here (Thanks TeCKis): Switch Panel / Switchpros Install Underhood Tap Points - https://forum.ih8mud.com/threads/switch-panel-switchpros-install-underhood-tap-points.1217437/

At which point, yes! You can charge your trailer battery from the trailer plug! The downside is you're losing 1.125V*25A = 28watts to heat up that wire and the connectors. Not a big deal, but not ideal to have ~10% losses.

------------------------------------------------------------------------------------------------------------------------------------------------------

Let's examine the thicker wire.

My plan is run 8awg paired wire to the bumper, Anderson plug, then to the trailer fridge to recharge the trailer battery. This is the setup I used in my 100 with the house battery and fridge being mounted in the rig.

We're now losing (0.006+0.006+.013)*25^2 = 15.6 watts to resistive losses in the wiring, so about twice the efficiency of using the trailer plug wiring. There are still losses in the DC-DC conversion that should be equivalent between the two methods.

--------------------------------------------------------------------------------------------------------------------------------------------------------------


So yes, you can charge your trailer battery from the tow plug. You'd need to run an Ignition Sense wire somewhere in the truck and run that alongside your tow plug on the vehicle and the corresponding wire/connector on the trailer.

And yes, you can run a thicker wire and gain some efficiency, but 15 watts isn't material in the scheme of things. Adds more cost and failure points. The wire routing on the Cruiser is already fused, and mounted in places where it won't rub, get water logged, etc. The pins at the tow plug are serviceable. For these reasons, I'd personally lean toward using the tow plug. I didn't actually expect to come to this conclusion at the start of the analysis!

You could also mount the DC-DC under the hood to save on having to run the ignition sense wire but it's not recommended. The losses will be the same as above, just downstream of the DC-DC charger. The voltage will sag at the trailer battery during high current. Once the battery starts to fill up, and current reduces, the voltage sagging will reduce. But Redarc still doesn't recommend it.

Thank you for coming to my TED talk.

 

[clrussell]

I see lots of responses, but not a lot of maths. So here I am to ruin everyone's morning! Scroll to the end for the TLDR.


Let's try a few methods and see!

According to this hastily-located fuse diagram I found in a YouTube video (and YouTube never lies), the 200 has 30A available on the towing plug before it blows the fuse. Staggrlee corroborates this info. Consult your documentation, but this should be sufficient for our example. In the documentation, it shows that other trailer loads like the brakes and lights run on different wires, so I will assume that we have all 30A at our disposal for our analysis.

Fuse Box Diagram Toyota Land Cruiser 200/J200/V8 2008-2018

Fuse box diagram (location and assignment of electrical fuses) for Toyota Land Cruiser (200/J200/V8; 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018).

fuse-box.info

View attachment 3377465

​

--------------------------------------------------------------------------------------------------------------------------------​

First, could you run a wire from this pin to the battery in your trailer directly? Let's try some math.
The current (I) flowing between two points can be determined by knowing the voltage difference between those two points, and the resistance separating them:

V1 = Alternator output (or start battery voltage, whichever is higher at a given point in time)
V2 = Voltage at the trailer battery
R = Resistance in all of the wires and connectors

I = V2-V1/R

Let's model the case where you're just starting up the vehicle after sitting for several days and the battery in the trailer is dead from running the fridge. That's a very feasible worst-case.

For V1, our alternator voltage, I've never measured higher than 13.8V, but we'll round up a bit for some safety factor.
V1= 14.0V

For V2, we'll look at the SnoMaster's cutoff. 10V and 10.7V sound like they're getting into deep-cycle territory, so I wouldn't run a SLA down to those voltages. We'll stick with the 11.8V setting.
View attachment 3377469
V2 = 11.8V

For R, since we're modeling for a worst-case scenario, I'm going to ignore any resistance introduced by connectors. I'm also going to assume 12AWG wire throughout. I'm using 16 feet for the run of wire within the Cruiser, 5 feet for the tongue, and 5 feet inside the trailer.

Wire Resistance Calculator - Cirris Inc

www.cirris.com

View attachment 3377470

R = 0.033ohms

Plug those all in:
I = 14.0-11.8/0.033
I = 66.6 Amps
But, we said that the fuse for the trailer would blow at 30 Amps. 66.6 >> 30, so the setup where a wire is run directly from the tow plug to the battery won't work And we proved it with math!

----------------------------------------------------------------------------------------------------------------------------------

Second, let's evaluate a DC-DC running from the tow plug.
30 Amps lets us choose from a few models mentioned above.

The Victron https://www.victronenergy.com/uploa...Smart-DC-DC-chargers-isolated-250-400W-EN.pdf
The Renogy 12V 20A DC to DC On-Board Battery Charger - https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/
The Redarc https://cdn.intelligencebank.com/au/share/yE9N/zJpl/oGLrq/original/BCDC1225D,+BCDC1240D+Manual+English

I'd like to note that the documentation for the Redarc is much better than the other two. Good on you, Redarc. It's also the highest current of the lot, so if it checks out, the others will work as well. For the example, I'll be using the BCDC1225D (BDCD for short).

To evaluate this setup, it's clear that the 25A that the BDCD draws is less than the 30A the fuse is rated for. Huzzah! But the concern noted by some sharp commenters above is that it's possible that the voltage seen at the BCDC will sag to the point where it shuts itself off when there's high current on the wire. Let's see.

We want to know voltage drop, so we can use the most well-known form of Ohm's Law:
V=I*R

I = 25A from the BCDC regulated draw

R = The resistance in the wire + all the connections. More resistance means more voltage sag.

For the purposes of this model, I'm going to use 0.006 ohms for the resistance based on this connector designed for 30A automotive applications. It's connector type looks similar to a tow plug. 30 Amp Unassembled Red/Black Anderson Powerpole Connectors - https://powerwerx.com/anderson-powerpole-connectors-30amp-unassembled

Note that if you do a quick google search, it's not hard to find claims that the connection of a trailer plug is anywhere from 0.3ohms to 3ohms! The very small resistances are tough to measure with handheld meters because they have an extremely small test current and the resistance is dominated by tiny, needle-like probes. Yes, even you, Fluke. When attempting to measure small resistances with a handheld meter, the measurements have to be taken with a grain of salt. To measure small resistances well, one would need to send a larger, known current through the connection and measure the voltage drop.

I'll put in 0.006 ohms for the connection at the tow plug and another 0.006 ohms for the connection at the BCDC. Note, the BCDC comes with fantastic crimp connectors, so the 0.006 ohms is being modeled high.

For these purposes, I'll assume the battery charger wouldn't live on your bumper or the trailer tongue, so We'll re-use the 0.033 ohms for the 21 feet of wire from the calculations on the previous "directly wired" setup.

R = wire resistance + plug connector resistance + BCDC crimp resistance = 0.033 + 0.1 + 0.1
So back to our voltage drop equation:
V = I*R
V = 25*(0.033 + 0.006 + 0.006)
V = 1.125 volts dropped across a good clean connection ignoring any other connections that may exist on the cruiser between the tow plug and the alternator.

Let's say our temperature-compensating alternator has dropped to 13.2 volts (lowest I've measured) and we subtract the 1.125 volts from the resistive losses. That means the BCDC would only see a voltage of 12.075.

If the "Ignition Sense" wire is left unconnected, the BDCD unit would turn itself off every 100 seconds. It will check the input voltage every 100 seconds, and since 12.075 is less than 12.7 volts, it will shut itself down. And it's not even close.
View attachment 3377503

However, the BCDC (and other DC-DC chargers) have an ignition sense wire, just for this scenario! To start, let's be lazy and connect the ignition sense wire to the 12AWG wire that's carrying power to the BCDC. The threshold for the ignition sense input is 12V. Since 12.075 volts is just barely above 12, if there are higher resistances that we've not accounted for in our model, the trigger would fail. I don't like those odds.

Better would be to run the Ignition Sense wire to an ignition-switched circuit in the vehicle like here (Thanks TeCKis): Switch Panel / Switchpros Install Underhood Tap Points - https://forum.ih8mud.com/threads/switch-panel-switchpros-install-underhood-tap-points.1217437/

At which point, yes! You can charge your trailer battery from the trailer plug! The downside is you're losing 1.125V*25A = 28watts to heat up that wire and the connectors. Not a big deal, but not ideal to have ~10% losses.

------------------------------------------------------------------------------------------------------------------------------------------------------

Let's examine the thicker wire.


View attachment 3377513

We're now losing (0.006+0.006+.013)*25^2 = 15.6 watts to resistive losses in the wiring, so about twice the efficiency of using the trailer plug.

--------------------------------------------------------------------------------------------------------------------------------------------------------------


So yes, you can charge your trailer battery from the tow plug. You'd need to run an Ignition Sense wire somewhere in the truck and run that alongside your tow plug on the vehicle and the corresponding wire/connector on the trailer.

And yes, you can run a thicker wire and gain some efficiency, but 15 watts isn't material in the scheme of things. Adds more cost and failure points. The routing on the Cruiser is already fused, and mounted in places where it won't rub, get water logged, etc. The pins at the tow plug are serviceable.

You could also mount the DC-DC under the hood to save on having to run the ignition sense wire but it's not recommended. The losses will be the same as above, just downstream of the DC-DC charger. The voltage will sag at the trailer battery, so it's charging will be about 1.125V lower that it should be. This is significant. Not recommended.

Thank you for coming to my TED talk.

My brain hurts. But thank you for the info.

I’m really leaning toward the 1225 redarc for the fact that I can put it under the hood later down the road.

The trailer battery will be mounted about 12” of wire away from the charger controller. It is a deep cycle agm. Fridge mounting about 12” of wire from battery.

The ignition sense wire could very easily be tied into the trailer park lights as I always have my lights turned on.

 

[daneo]

My brain hurts. But thank you for the info.

I’m really leaning toward the 1225 redarc for the fact that I can put it under the hood later down the road.

The trailer battery will be mounted about 12” of wire away from the charger controller. It is a deep cycle agm. Fridge mounting about 12” of wire from battery.

The ignition sense wire could very easily be tied into the trailer park lights as I always have my lights turned on.

This all sounds good to me!

Clever to use the trailer lights for the ignition switching!

I'm curious about the scenario where the ignition is in Accessory mode and the engine is off. I think parking lights would still be on in that mode, so it's possible you could be draining your start start battery at 25A if the park lights are left on.

PS: the rig looks great!

 

[TeCKis300]

Even though the vehicle trailer circuit has a 30A fuse, real capacity is well short of that. The wiring gauge, also used a resistance current limiter, but also the trailer 7-pin is not up to the task of that current. 10-15A is what it will typically carry, supporting to about something less than 20A. With weather and use, the pins degrading will create further resistance at which point these will fail in time. I had a 20A DC-DC and it didn't like it after 1/2 a year. Granted my 7-pin connector is like 15 yrs old and really needed some attention.

That fridge takes very little amps so the normal line in the trailer plug will keep the battery running the fridge charged up while traveling.

Great thought and it really brings us back to center. The OP is looking for a solid solution for supporting relatively minimal charging electronics.

Hooking up the charge wire directly from the 7-pin may be sufficient for a lead acid. For an AGM, it won't do it. With a lithium current might just flow the wrong way and will need proper isolation.

A small DC-DC directly on the 7-pin power circuit can solve all the above. The Victron 12/12-18A will do that and I think the 18A is just about max of what's reasonable.

I see lots of responses, but not a lot of maths. So here I am to ruin everyone's morning! Scroll to the end for the TLDR.


Let's try a few methods and see!

According to this hastily-located fuse diagram I found in a YouTube video (and YouTube never lies), the 200 has 30A available on the towing plug before it blows the fuse. Staggrlee corroborates this info. Consult your documentation, but this should be sufficient for our example. In the documentation, it shows that other trailer loads like the brakes and lights run on different wires, so I will assume that we have all 30A at our disposal for our analysis.

Fuse Box Diagram Toyota Land Cruiser 200/J200/V8 2008-2018

Fuse box diagram (location and assignment of electrical fuses) for Toyota Land Cruiser (200/J200/V8; 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018).

fuse-box.info

View attachment 3377465

​

--------------------------------------------------------------------------------------------------------------------------------​

First, could you run a wire from this pin to the battery in your trailer directly? Let's try some math.
The current (I) flowing between two points can be determined by knowing the voltage difference between those two points, and the resistance separating them:

V1 = Alternator output (or start battery voltage, whichever is higher at a given point in time)
V2 = Voltage at the trailer battery
R = Resistance in all of the wires and connectors

I = V2-V1/R

Let's model the case where you're just starting up the vehicle after sitting for several days and the battery in the trailer is dead from running the fridge. That's a very feasible worst-case.

For V1, our alternator voltage, I've never measured higher than 13.8V, but we'll round up a bit for some safety factor.
V1= 14.0V

For V2, we'll look at the SnoMaster's cutoff. 10V and 10.7V sound like they're getting into deep-cycle territory, so I wouldn't run a SLA down to those voltages. We'll stick with the 11.8V setting.
View attachment 3377469
V2 = 11.8V

For R, since we're modeling for a worst-case scenario, I'm going to ignore any resistance introduced by connectors. I'm also going to assume 12AWG wire throughout. I'm using 16 feet for the run of wire within the Cruiser, 5 feet for the tongue, and 5 feet inside the trailer.

Wire Resistance Calculator - Cirris Inc

www.cirris.com

View attachment 3377470

R = 0.033ohms

Plug those all in:
I = 14.0-11.8/0.033
I = 66.6 Amps
But, we said that the fuse for the trailer would blow at 30 Amps. 66.6 >> 30, so the setup where a wire is run directly from the tow plug to the battery won't work And we proved it with math!

----------------------------------------------------------------------------------------------------------------------------------

Second, let's evaluate a DC-DC running from the tow plug.
30 Amps lets us choose from a few models mentioned above.

The Victron https://www.victronenergy.com/uploa...Smart-DC-DC-chargers-isolated-250-400W-EN.pdf
The Renogy 12V 20A DC to DC On-Board Battery Charger - https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/
The Redarc https://cdn.intelligencebank.com/au/share/yE9N/zJpl/oGLrq/original/BCDC1225D,+BCDC1240D+Manual+English

I'd like to note that the documentation for the Redarc is much better than the other two. Good on you, Redarc. It's also the highest current of the lot, so if it checks out, the others will work as well. For the example, I'll be using the BCDC1225D (BDCD for short).

To evaluate this setup, it's clear that the 25A that the BDCD draws is less than the 30A the fuse is rated for. Huzzah! But the concern noted by some sharp commenters above is that it's possible that the voltage seen at the BCDC will sag to the point where it shuts itself off when there's high current on the wire. Let's see.

We want to know voltage drop, so we can use the most well-known form of Ohm's Law:
V=I*R

I = 25A from the BCDC regulated draw

R = The resistance in the wire + all the connections. More resistance means more voltage sag.

For the purposes of this model, I'm going to use 0.006 ohms for the resistance based on this connector designed for 30A automotive applications. It's connector type looks similar to a tow plug. 30 Amp Unassembled Red/Black Anderson Powerpole Connectors - https://powerwerx.com/anderson-powerpole-connectors-30amp-unassembled

Note that if you do a quick google search, it's not hard to find claims that the connection of a trailer plug is anywhere from 0.3ohms to 3ohms! The very small resistances are tough to measure with handheld meters because they have an extremely small test current and the resistance is dominated by tiny, needle-like probes. Yes, even you, Fluke. When attempting to measure small resistances with a handheld meter, the measurements have to be taken with a grain of salt. To measure small resistances well, one would need to send a larger, known current through the connection and measure the voltage drop.

I'll put in 0.006 ohms for the connection at the tow plug and another 0.006 ohms for the connection at the BCDC. Note, the BCDC comes with fantastic crimp connectors, so the 0.006 ohms is being modeled high.

For these purposes, I'll assume the battery charger wouldn't live on your bumper or the trailer tongue, so We'll re-use the 0.033 ohms for the 21 feet of wire from the calculations on the previous "directly wired" setup.

R = wire resistance + plug connector resistance + BCDC crimp resistance = 0.033 + 0.1 + 0.1
So back to our voltage drop equation:
V = I*R
V = 25*(0.033 + 0.006 + 0.006)
V = 1.125 volts dropped across a good clean connection ignoring any other connections that may exist on the cruiser between the tow plug and the alternator.

Let's say our temperature-compensating alternator has dropped to 13.2 volts (lowest I've measured) and we subtract the 1.125 volts from the resistive losses. That means the BCDC would only see a voltage of 12.075.

If the "Ignition Sense" wire is left unconnected, the BDCD unit would turn itself off every 100 seconds. It will check the input voltage every 100 seconds, and since 12.075 is less than 12.7 volts, it will shut itself down. And it's not even close.
View attachment 3377503

However, the BCDC (and other DC-DC chargers) have an ignition sense wire, just for this scenario! To start, let's be lazy and connect the ignition sense wire to the 12AWG wire that's carrying power to the BCDC. The threshold for the ignition sense input is 12V. Since 12.075 volts is just barely above 12, if there are higher resistances that we've not accounted for in our model, the trigger would fail. I don't like those odds.

Better would be to run the Ignition Sense wire to an ignition-switched circuit in the vehicle like here (Thanks TeCKis): Switch Panel / Switchpros Install Underhood Tap Points - https://forum.ih8mud.com/threads/switch-panel-switchpros-install-underhood-tap-points.1217437/

At which point, yes! You can charge your trailer battery from the trailer plug! The downside is you're losing 1.125V*25A = 28watts to heat up that wire and the connectors. Not a big deal, but not ideal to have ~10% losses.

------------------------------------------------------------------------------------------------------------------------------------------------------

Let's examine the thicker wire.


View attachment 3377513

We're now losing (0.006+0.006+.013)*25^2 = 15.6 watts to resistive losses in the wiring, so about twice the efficiency of using the trailer plug wiring. There are still losses in the DC-DC conversion that should be equivalent between the two methods.

--------------------------------------------------------------------------------------------------------------------------------------------------------------


So yes, you can charge your trailer battery from the tow plug. You'd need to run an Ignition Sense wire somewhere in the truck and run that alongside your tow plug on the vehicle and the corresponding wire/connector on the trailer.

And yes, you can run a thicker wire and gain some efficiency, but 15 watts isn't material in the scheme of things. Adds more cost and failure points. The wire routing on the Cruiser is already fused, and mounted in places where it won't rub, get water logged, etc. The pins at the tow plug are serviceable. For these reasons, I'd personally lean toward using the tow plug. I didn't actually expect to come to this conclusion at the start of the analysis!

You could also mount the DC-DC under the hood to save on having to run the ignition sense wire but it's not recommended. The losses will be the same as above, just downstream of the DC-DC charger. The voltage will sag at the trailer battery during high current. Once the battery starts to fill up, and current reduces, the voltage sagging will reduce. But Redarc still doesn't recommend it.

Thank you for coming to my TED talk.

Thanks for the maths!

 

[radman]

10-15A is what it will typically carry, supporting to about something less than 20A.

True statement. My Renogy DC-DC typically shows around 20amps using the 7-pin connector!

 

[FrazzledHunter]

Aux power to a trailer under tow is a subject that interests me. I found the below diagram somewhere else on this forum but I can't remember where. I have a 2018 and I was led to believe that 40A was available via "SUB BATT". Looking inside the fuse box cover does indeed show "SUB BATT" at 40A. I haven't tested by pulling the fuse and volt meter at the plug. Not that it's a heck of a huge difference over 30A.
So am I off base here?

EDIT: I found the thread with the picture.
7 pin trailer plug - https://forum.ih8mud.com/threads/7-pin-trailer-plug.1145054/

 

[daneo]

Even though the vehicle trailer circuit has a 30A fuse, real capacity is well short of that.

The tow plug will supply 30A (or whatever the real fused value is, 40A noted above). The plug nor the wire are current limiting devices. Losses will reduce the amount of power supplied to the load versus in the wiring and connections. Corroded connections should be addressed to insure the input voltage at the load is within spec. For the DC-DC regulator, the input voltage can only get so low before it fails to do its job of regulating the voltage up to the desired charge profile.

In the case of corroded connectors, the current is available. The problem is voltage drop before it gets to the load.

In the case of the BCDC, 9 volts are required for it to regulate its charge profile. There's quite a bit of margin from the 12.125V in the example, but yes, corroded connectors can kill that margin. A corroded Anderson plug on a 0AWG cable will suffer the same fate.

Hooking up the charge wire directly from the 7-pin may be sufficient for a lead acid.

Did the math above, and this will blow the fuse.

 

[TeCKis300]

The tow plug will supply 30A (or whatever the real fused value is, 40A noted above). The plug nor the wire are current limiting devices. Losses will reduce the amount of power supplied to the load versus in the wiring and connections. Corroded connections should be addressed to insure the input voltage at the load is within spec. For the DC-DC regulator, the input voltage can only get so low before it fails to do its job of regulating the voltage up to the desired charge profile.

In the case of corroded connectors, the current is available. The problem is voltage drop before it gets to the load.

In the case of the BCDC, 9 volts are required for it to regulate its charge profile. There's quite a bit of margin from the 12.125V in the example, but yes, corroded connectors can kill that margin. A corroded Anderson plug on a 0AWG cable will suffer the same fate.




Did the math above, and this will blow the fuse.

Hehe, I agree with you. In practice, with low gauge wire and connectors, sometimes marginal, they will become - capability limiting. Many RV full timers with higher power needs will change the 7-pin to higher quality 7-way round pins, aka semi-truck connectors, to improve reliability for higher current handling. It's another strategy I'm pondering in place of a separate anderson. I'm leaning towards the anderson as the std 7-pin is important to have compatibility with trailers.

Lead acids work with direct hook-ups and have for decades because of the batteries internal resistance.

 

[daneo]

Lead acids work with direct hook-ups and have for decades because of the batteries internal resistance.

I'm seeing that a decent-condition SLA internal resistance is ~10mOhms so the in-rush equation then reads:
I = (V2-V1)/R
I = (Alternator voltage - discharged battery voltage)/(wire resistance + battery equivalent series resistance)
I = (14.0-11.8)/(0.033 + 0.01) = 51A
Still well above the 30A (or 40A?) fuse for the trailer supply. Maybe you could cobble together enough parasitic resistances and throw a big watts/low resistance resistor on there to... oh geez. Haha. Just get the DC-DC charger.

For the starter battery, it has the benefit of having an enormous wire supplying it with current (and no fuse!), so 50-60amps is a cakewalk.

 

[sdnative]

I imagined a FLA (and I assume SLA as well) would have a higher IR than that, but I don't have anything to back that up. I think that was in the ballpark of what my LFP measured.

Don't you need round trip wire resistance? So that gives a more reasonable ~28A. A higher FLA IR will lower that even further.

 

[Julian73]

I see lots of responses, but not a lot of maths. So here I am to ruin everyone's morning! Scroll to the end for the TLDR.


Let's try a few methods and see!

According to this hastily-located fuse diagram I found in a YouTube video (and YouTube never lies), the 200 has 30A available on the towing plug before it blows the fuse. Staggrlee corroborates this info. Consult your documentation, but this should be sufficient for our example. In the documentation, it shows that other trailer loads like the brakes and lights run on different wires, so I will assume that we have all 30A at our disposal for our analysis.

Fuse Box Diagram Toyota Land Cruiser 200/J200/V8 2008-2018

Fuse box diagram (location and assignment of electrical fuses) for Toyota Land Cruiser (200/J200/V8; 2008, 2009, 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018).

fuse-box.info

View attachment 3377465

​

--------------------------------------------------------------------------------------------------------------------------------​

First, could you run a wire from this pin to the battery in your trailer directly? Let's try some math.
The current (I) flowing between two points can be determined by knowing the voltage difference between those two points, and the resistance separating them:

V1 = Alternator output (or start battery voltage, whichever is higher at a given point in time)
V2 = Voltage at the trailer battery
R = Resistance in all of the wires and connectors

I = V2-V1/R

Let's model the case where you're just starting up the vehicle after sitting for several days and the battery in the trailer is dead from running the fridge. That's a very feasible worst-case.

For V1, our alternator voltage, I've never measured higher than 13.8V, but we'll round up a bit for some safety factor.
V1= 14.0V

For V2, we'll look at the SnoMaster's cutoff. 10V and 10.7V sound like they're getting into deep-cycle territory, so I wouldn't run a SLA down to those voltages. We'll stick with the 11.8V setting.
View attachment 3377469
V2 = 11.8V

For R, since we're modeling for a worst-case scenario, I'm going to ignore any resistance introduced by connectors. I'm also going to assume 12AWG wire throughout. I'm using 16 feet for the run of wire within the Cruiser, 5 feet for the tongue, and 5 feet inside the trailer.

Wire Resistance Calculator - Cirris Inc

www.cirris.com

View attachment 3377470

R = 0.033ohms

Plug those all in:
I = 14.0-11.8/0.033
I = 66.6 Amps
But, we said that the fuse for the trailer would blow at 30 Amps. 66.6 >> 30, so the setup where a wire is run directly from the tow plug to the battery won't work And we proved it with math!

----------------------------------------------------------------------------------------------------------------------------------

Second, let's evaluate a DC-DC running from the tow plug.
30 Amps lets us choose from a few models mentioned above.

The Victron https://www.victronenergy.com/uploa...Smart-DC-DC-chargers-isolated-250-400W-EN.pdf
The Renogy 12V 20A DC to DC On-Board Battery Charger - https://www.renogy.com/12v-20a-dc-to-dc-on-board-battery-charger/
The Redarc https://cdn.intelligencebank.com/au/share/yE9N/zJpl/oGLrq/original/BCDC1225D,+BCDC1240D+Manual+English

I'd like to note that the documentation for the Redarc is much better than the other two. Good on you, Redarc. It's also the highest current of the lot, so if it checks out, the others will work as well. For the example, I'll be using the BCDC1225D (BDCD for short).

To evaluate this setup, it's clear that the 25A that the BDCD draws is less than the 30A the fuse is rated for. Huzzah! But the concern noted by some sharp commenters above is that it's possible that the voltage seen at the BCDC will sag to the point where it shuts itself off when there's high current on the wire. Let's see.

We want to know voltage drop, so we can use the most well-known form of Ohm's Law:
V=I*R

I = 25A from the BCDC regulated draw

R = The resistance in the wire + all the connections. More resistance means more voltage sag.

For the purposes of this model, I'm going to use 0.006 ohms for the resistance based on this connector designed for 30A automotive applications. It's connector type looks similar to a tow plug. 30 Amp Unassembled Red/Black Anderson Powerpole Connectors - https://powerwerx.com/anderson-powerpole-connectors-30amp-unassembled

Note that if you do a quick google search, it's not hard to find claims that the connection of a trailer plug is anywhere from 0.3ohms to 3ohms! The very small resistances are tough to measure with handheld meters because they have an extremely small test current and the resistance is dominated by tiny, needle-like probes. Yes, even you, Fluke. When attempting to measure small resistances with a handheld meter, the measurements have to be taken with a grain of salt. To measure small resistances well, one would need to send a larger, known current through the connection and measure the voltage drop.

I'll put in 0.006 ohms for the connection at the tow plug and another 0.006 ohms for the connection at the BCDC. Note, the BCDC comes with fantastic crimp connectors, so the 0.006 ohms is being modeled high.

For these purposes, I'll assume the battery charger wouldn't live on your bumper or the trailer tongue, so We'll re-use the 0.033 ohms for the 21 feet of wire from the calculations on the previous "directly wired" setup.

R = wire resistance + plug connector resistance + BCDC crimp resistance = 0.033 + 0.1 + 0.1
So back to our voltage drop equation:
V = I*R
V = 25*(0.033 + 0.006 + 0.006)
V = 1.125 volts dropped across a good clean connection ignoring any other connections that may exist on the cruiser between the tow plug and the alternator.

Let's say our temperature-compensating alternator has dropped to 13.2 volts (lowest I've measured) and we subtract the 1.125 volts from the resistive losses. That means the BCDC would only see a voltage of 12.075.

If the "Ignition Sense" wire is left unconnected, the BDCD unit would turn itself off every 100 seconds. It will check the input voltage every 100 seconds, and since 12.075 is less than 12.7 volts, it will shut itself down. And it's not even close.
View attachment 3377503

However, the BCDC (and other DC-DC chargers) have an ignition sense wire, just for this scenario! To start, let's be lazy and connect the ignition sense wire to the 12AWG wire that's carrying power to the BCDC. The threshold for the ignition sense input is 12V. Since 12.075 volts is just barely above 12, if there are higher resistances that we've not accounted for in our model, the trigger would fail. I don't like those odds.

Better would be to run the Ignition Sense wire to an ignition-switched circuit in the vehicle like here (Thanks TeCKis): Switch Panel / Switchpros Install Underhood Tap Points - https://forum.ih8mud.com/threads/switch-panel-switchpros-install-underhood-tap-points.1217437/

At which point, yes! You can charge your trailer battery from the trailer plug! The downside is you're losing 1.125V*25A = 28watts to heat up that wire and the connectors. Not a big deal, but not ideal to have ~10% losses.

------------------------------------------------------------------------------------------------------------------------------------------------------

Let's examine the thicker wire.


View attachment 3377513

We're now losing (0.006+0.006+.013)*25^2 = 15.6 watts to resistive losses in the wiring, so about twice the efficiency of using the trailer plug wiring. There are still losses in the DC-DC conversion that should be equivalent between the two methods.

--------------------------------------------------------------------------------------------------------------------------------------------------------------


So yes, you can charge your trailer battery from the tow plug. You'd need to run an Ignition Sense wire somewhere in the truck and run that alongside your tow plug on the vehicle and the corresponding wire/connector on the trailer.

And yes, you can run a thicker wire and gain some efficiency, but 15 watts isn't material in the scheme of things. Adds more cost and failure points. The wire routing on the Cruiser is already fused, and mounted in places where it won't rub, get water logged, etc. The pins at the tow plug are serviceable. For these reasons, I'd personally lean toward using the tow plug. I didn't actually expect to come to this conclusion at the start of the analysis!

You could also mount the DC-DC under the hood to save on having to run the ignition sense wire but it's not recommended. The losses will be the same as above, just downstream of the DC-DC charger. The voltage will sag at the trailer battery during high current. Once the battery starts to fill up, and current reduces, the voltage sagging will reduce. But Redarc still doesn't recommend it.

Thank you for coming to my TED talk.

Good input; however, its would be best to round voltages down for calculations be it for amperage calculation or minimum voltages after voltage drop.