Dual Battery System (1 Viewer)

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^ OK, not sure if I got that ("knows the status of charge") right, but sounds like the weak point in the approach you describe is that the charger has to know the level of charge of the battery at all times.
Short of measuring directly the voltage there, it'd have to do some historical calculations based on some presumably known initial condition. IOW, keeping track of how much energy has flown into the battery knowing how charged it was before.
The Redarc is connected directly to the load batt (and nothing else other than charge source, and possibly solar - a secondary charge source). Every 100 seconds it ceases operations, takes a measurement of the load bat, and carries on till it hits the selected charge profile limits.
It doesn't track total energy, just how far off the load batt is from the hi/lo values in the charge profile it's using. I use a monitoring device to track total energy, tho I haven't found the data to be useful other than making me feel like a wasteful human. lol
But the monitoring does contribute to my explanations here as I have about a years worth of base lining as a result, watching the Redarcs draw on its charge source and its output to the load source across all 3 charge profiles at differing charge states between the front and the rear batts.
That would be quite an iffy proposition, I think, given particular unknown battery specs and condition, inefficiencies and the like. I suppose you could charge something like that but to expect it would be charged optimally that way seems like a long shot. Then again, I'm probably missing something. Interesting topic.
For example, my AGM batts have their charge/low voltage levels on a sticker. The Redarc profile B matches those specs exactly. So if I leave the Redarc to use profile B (I can switch between profiles from the dash, and do sometimes), and if the truck runs long enough, the RedArc will not leave bulk mode until it sees the charge portion (bulk) of profile B has been reached.
Certainly can't speak for all setups, but I can say there's been no iffyness in this. When the RedArc hits a mode other than bulk, the load batt(s) have always been fully charged (or near enough ~29.6v across 2 - 12v batts rated to 14.8v/ea max). They settle down a bit moving from bulk to absorbtion, but that's SOP and part of getting the last little bit in the batt.

Capacity decay in all its manifestations can be accounted for somewhat, but it's still a net sum loss as in any other situation.
There's a reason the RedArc is ~$500 and most of it's competition is not.
C-Tek comes to mind in price parity. Not too familiar with it, tho I imagine it should work the in same way.
 
Not sure I agree with this.

The function of the DC-DC charger is two fold:
1. convert one DC voltage to another to provide proper voltage to charge the battery
2. manipulate the input voltage to control current flow into the battery being charged
Not sure we disagree with this. This is exactly what the RedArc BCDC does when the RedArc is placed in the rear of a vehicle.
1) overcomes voltage drop by stepping up a lower voltage (result of drop) to a higher, more optimum charge voltage (voltage drop compensation).
2) maintains charge current to load batt by ensuring the charge source sees a load commensurate with load charge requirements - bulk/absorb/float.
what? lol

The charger can overcome a low input voltage and convert to a higher output voltage as needed, but it expects that voltage to remain fairly constant during its full range of output. That's why the recommendation is no more than a 3% voltage drop at full capacity.
Well, given the RedArc has just 8ga wires permanently installed, and if you put it in the rear of a vehicle with an 8ga run front to back, it's already pushing close to 10% drop. Yet RedArc confirmed this is ok as it will compensate w/i 1.5v or so. I aimed for 3% with a single 2ga front to rear. But OP... it's why I recommended using both 8ga runs as a single charge source input to the RedArc. Keeps from running new cable and definitely suffices to charge a 55Ahr batt. or whatever the OP is running. Would technically be ok for up to 100ahrs imo, tho the 1240 can absolutely move some current.

Problem with too small of a wire is during low power charging everything may work fine, but when the charger is outputting high current, the voltage sensed at the input terminals may drop significantly causing the charger to shut off on low voltage shutoff threshold or engine off detection.
Agree here too.
The DC-DC charger (Redarc or any other) does not present a constant load to the alternator... The load to the alternator is only constant during the bulk phase.
The RedArc varies the load, from constant to something less depending on it's stage in the profile. Agree here too.
The charger is a very simple device. It doesn't have any purpose other than to charge the battery.
Disagree here. lol A switched relay binding to 2 batts to allow charge equalization is a simple device (i.e. start/aux batts under the hood)
A BCDC is much more than that. It maintains batt health and combats charge memory, 2 largest threats to batt longevity and runtime (imo of course).
The RedArc also incorporates other features such as an MPPT solar controller to aid in its duties when the truck is off and a Smart Alternator feature for better 'control' over what its charge source is delivering. (OPs situation specifically).
The devices connected will see the battery terminal voltage or the charger output voltage or somewhere in between (hopefully these voltage will be very close).
There are no end-use devices connected to the RedArc. Only the charge source and load source.
End use devices connected to the (RedArcs) load source (second batt in the rear) will essentially see alternator voltage (or maybe a little more) at the battery during bulk stage - like they would anyway with none of this in play and the engine running. (minus voltage drop if devices are in the rear)
The charger only measures the output voltage (which should be the same or very close to battery voltage), and manipulates the input voltage to control current flow through the charger.
In the RedArcs case, it does measure its charge source voltage to know when the truck is running or not. The RedArc does not function at batt voltage. Only alt voltage.

edit:clarity
 
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^^ Interesting point! I did not think about the possibility that the charger would shut down the current and find out the load battery voltage that way, that is very clever indeed. Had not heard of that before. So then yes, in principle the charger could adapt the output voltage to make sure the 14.4 or whatever is actually felt at the battery, not just at the charger, even at a distance. Nice! Those are smarter than usual, maybe I should look into getting one just to explore a bit, hopefully one with solar input possibilities. Can the converter accept both alternator and solar inputs at the same time, like when you'd be driving with a panel on the roof?
 
This is a good watch:

BTW, this is a good watch. 30ft runs definitely require their own calculation apart from the equipment on either end.
 
had a quick look at the REDARC line up to check the dual input units. Interesting and tempting , but I noted that -on the units I saw at least- it seems like the solar voltage input is limited to 32V, which basically means you can't use panels in series. Hope I'm wrong on that, because that is a bit limiting.
 
^^ Interesting point! I did not think about the possibility that the charger would shut down the current and find out the load battery voltage that way, that is very clever indeed. Had not heard of that before. So then yes, in principle the charger could adapt the output voltage to make sure the 14.4 or whatever is actually felt at the battery, not just at the charger, even at a distance. Nice! Those are smarter than usual, maybe I should look into getting one just to explore a bit, hopefully one with solar input possibilities. Can the converter accept both alternator and solar inputs at the same time, like when you'd be driving with a panel on the roof?
Yes, while they aren't turn-key per say, they are the key component to a premeditated power system design incorporating front to rear power distribution. Of course, their BCD30 is the mack daddy. And $$$. Couple that with their RedView(?) distro system (also $$$) and you're nearly commercial grade power.

Another make, Projecta IDC25, is a RedArc clone with RedArcs profiles and algorithms cut and pasted into it. Seems its components are also nearly identical based on the users high regard of them. Tho being clones...never know what you'll actually get if you can get your hands on one. They are (or were) about 70% the cost of RedArc.

And yes, when available they use both alternator and solar simultaneously to reach charge profile parameters.

All the RedArc units come with solar controllers now, regardless of spec. This wasn't always the case and I've found ebay will list some their units w/o the controller for very reasonable prices. Still has all the goodness we've talked about, just w/o the solar. My RedArc doesn't have the solar, but I'm using Victron for that and my LF inverter has a controller built in for use as a fail-over if need be.
 
One of the features I like about some Victrons is that the output voltages are fully adjustable at the user's discretion, to any 0.1V IIRC, not just a few fixed profiles to choose from. Is that also the case with the REDARCs?
 
had a quick look at the REDARC line up to check the dual input units. Interesting and tempting , but I noted that -on the units I saw at least- it seems like the solar voltage input is limited to 32V, which basically means you can't use panels in series. Hope I'm wrong on that, because that is a bit limiting.
Good point. Definitely a consideration.
 
One of the features I like about some Victrons is that the output voltages are fully adjustable at the user's discretion, to any 0.1V IIRC, not just a few fixed profiles to choose from. Is that also the case with the REDARCs?
My understanding is that the solar output on the BCDC models is constrained to the same built-in profiles as the alt charge source is.
The BDC30 tho I believe incorporates this feature, if not alone, with the RedView add on certainly.
In terms of total cost, the Victron would be a far more economical route to gain that feature with a BCDC otherwise in play.
 
Not sure we disagree with this. This is exactly what the RedArc BCDC does when the RedArc is placed in the rear of a vehicle.
1) overcomes voltage drop by stepping up a lower voltage (result of drop) to a higher, more optimum charge voltage (voltage drop compensation).
2) maintains charge current to load batt by ensuring the charge source sees a load commensurate with load charge requirements - bulk/absorb/float.
what? lol


Well, given the RedArc has just 8ga wires permanently installed, and if you put it in the rear of a vehicle with an 8ga run front to back, it's already pushing close to 10% drop. Yet RedArc confirmed this is ok as it will compensate w/i 1.5v or so. I aimed for 3% with a single 2ga front to rear. But OP... it's why I recommended using both 8ga runs as a single charge source input to the RedArc. Keeps from running new cable and definitely suffices to charge a 55Ahr batt. or whatever the OP is running. Would technically be ok for up to 100ahrs imo, tho the 1240 can absolutely move some current.


Agree here too.

The RedArc varies the load, from constant to something less depending on it's stage in the profile. Agree here too.

Disagree here. lol A switched relay binding to 2 batts to allow charge equalization is a simple device (i.e. start/aux batts under the hood)
A BCDC is much more than that. It maintains batt health and combats charge memory, 2 largest threats to batt longevity and runtime (imo of course).
The RedArc also incorporates other features such as an MPPT solar controller to aid in its duties when the truck is off and a Smart Alternator feature for better 'control' over what its charge source is delivering. (OPs situation specifically).

There are no end-use devices connected to the RedArc. Only the charge source and load source.
End use devices connected to the (RedArcs) load source (second batt in the rear) will essentially see alternator voltage (or maybe a little more) at the battery during bulk stage - like they would anyway with none of this in play and the engine running. (minus voltage drop if devices are in the rear)

In the RedArcs case, it does measure its charge source voltage to know when the truck is running or not. The RedArc does not function at batt voltage. Only alt voltage.

edit:clarity

I went back and re-read your original and most recent posts, and I think we are on the same page and pretty much in agreement. My discussion was only about DC-DC controller functionality of the Redarc, I was ignoring the MPPT controller functionality. Good discussion though, probably more than the PO asked for however :cheers:
 
My understanding is that the solar output on the BCDC models is constrained to the same built-in profiles as the alt charge source is.
The BDC30 tho I believe incorporates this feature, if not alone, with the RedView add on certainly.
In terms of total cost, the Victron would be a far more economical route to gain that feature with a BCDC otherwise in play.

so you are saying that there are only a few fixed charging profiles to choose from on the REDARC BCDC, not fully adjustable ones, correct?

can you elaborate a bit on how you would connect a Victron MPPT together with a REDARC BCDC dual input unit?
 
I went back and re-read your original and most recent posts, and I think we are on the same page and pretty much in agreement. My discussion was only about DC-DC controller functionality of the Redarc, I was ignoring the MPPT controller functionality. Good discussion though, probably more than the PO asked for however :cheers:
Yeah I thought we were ringing the same tree.
And who knows, maybe the PO is tracking.
He's hunting runtime from a small battery, 20ft away, over small cables, in a low re-gen (w/o solar) environment.
As it stands, somethings gotta change. Maybe we helped some. 🤔
:cheers: indeed.

OP: No. You didn't help at all. :rofl:
 
Well, one big fridge in AZ summer: 3Ah per hour average, say, needs 72Ah over 24h.
BCDC1240 gives 40A max I assume. 12 miles in city traffic, say 30 mins of alternator time. that gives you 20Ah. Say it's a 1h commute even: best case is 40Ah.
Ain't gonna work long term...
 
so you are saying that there are only a few fixed charging profiles to choose from on the REDARC BCDC, not fully adjustable ones, correct?
Correct. Profiles A, B, C. Each have slightly higher charge on/off voltage values, with A being the lowest (for vented LA i believe) and C being the highest (for Li). I use all 3 profiles at times depending on the charge state of my batt bank, but only because I have monitoring and can see when the batts are reaching their max. Then I'll switch the profile back to the one RedArc recommends for AGM batts for the final top off (B profile). Speeds things up a little when batts are at low charge state, tho requires manual monitoring and switching to keep safe. lol
can you elaborate a bit on how you would connect a Victron MPPT together with a REDARC BCDC dual input unit?
I would have their inputs switched, not connected together. The panels go to one or the other at a time. (This is how I have my (1) panel wired between the Victron and the AC inverters built-in controller. If one fails, just switch the panel over to the other controller)
A call to RedArc on that wouldn't be a bad idea. It may be switching is unnecessary.
Not my situation so haven't had to suss that out.
 
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Seems to be a lot of back and forth about the functionally of the BCDC and how to wire it here.
  • The Redarc BCDC does not link your batteries per say. It uses the alternator or solar to charge your secondary battery.
  • In addition, it has a high quality MPPT Solar Charger built in that prioritizes solar over alternator charging if available
  • All you need to do is buy a quality panel and wire the + to the solar wire on the BCDC and the - wire to any ground on the vehicle (I used the roof rack bolt)
I ran the Off Grid BCDC Setup on my 15 4Runner with a similar battery, Bluesea Panel, Dometic CFX-50 Fridge and a 120W Generic Flexible Solar Panel on my Prinsu rack.

With this setup, I ran the fridge continuously with no issue. Fridge ran 24/7 through GA temps. I am working on the same setup in my 08 GX.

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