Fusing dual battery setup? After research I have questions... (1 Viewer)

[caladin]

Truck came with a dual battery setup.
Old school, 2 trays, 2 batteries big ass connecting cable and marine switch between them.

I replaced the marine switch with a dual battery isolator/VSR with ovveride.

MaySpare 12V 140Amp Voltage... https://www.amazon.com/dp/B08BRJRY42?ref=ppx_pop_mob_ap_share

140 amps seems low now... (see below)

Connecting big fat wire wire goes from pass battery, up fender(through vsr) , along firewall, then back down fender to drivers battery... Let's call this the linkup cable, since I don't know the real name.

Is there a reason it's like this and not just across the rad support?


Have a volt meter to each battery to dash.
(primary batt is via harness, other is direct, this may be the real issue with 7nbalanced voltage.. Just loads now that I write it out. .)

Was surprised when activating the isolator to force connection that voltage doesn't balance.

To figure that out, Did research on dual battery systems, and have questions.

1. They reccomended a fuse at each end of the linkup cable in case it gets damaged and shorts.

This seems wise to me. Another thread said peak voltage draw starting could be 400amps at spike when first sending power to starter.

Does this mean I need a 500a fuse at each end, so I can start off either battery? (they are biiig fues blocks) what size should I run if not?

Fuse block


Will my isolator/vsr support starting off either battery? Or is it just to manage charging? Assuming it does not(140 amps?),

What do I need to manage charging like the vs isolator and still allow emergency staring off both batteries?

They reccomended a second charge wire directly to 2nd battery, wouldn't that be for a non switched system, that's always in parelell? Seems like it would just bypass the isolator/vsr to me.

Do I actually need it?

They suggested a DC to DC charger to 2nd battery, the isolator/vsr is handling this for me yes?(their use case was running fridge off battery in cab)

My goal is effort free 2 battery management, and just flip a switch in the cab to emergency start off both batteries.

Any other gotchas I should address?

Thanks!

 

[caladin]

I think this is what i need for charge and start.

Amazon product ASIN B0B9RT971X

 

[ZackR]

If the point of the second battery to to run a fridge I'd get a 100Ah (or bigger/smaller depending on what your loads are) lithium battery and install it inside the vehicle near the fridge. It would be charged with a DC to DC charger and or a PV panel on the roof (with its own charge controller). This would allow for a separate system that is specifically suited for "house" loads instead of trying to increase the capacity of the charging system.

The cable used by the DC to DC charger wouldn't need to be massive since it would max out at 30A (you would need to run wire size calculations based on length).

Having an additional lead acid starting battery to run a fridge really doesn't make a ton of sense since they aren't great for house loads and they weigh a ton. An additional starting battery is specifically useful for high current/short duration loads such as running a winch or starting a diesel engine in the cold.

 

[caladin]

Sorry I was unclear in their scenerion2nd battery runs accessories "in the "house".

In mine it runs lights and winch, and other external accessories and is a yellow top optima primarily for deep cycling but which can start the vehicle. Then I have a regular battery for starting "only" (anything on the main harness that I have not moved, like the headlights lights are now on the yellow top, but cabin lights are on the regular harness). Eventually I'm going to move them both to yellow tops, as they have had better luck surviving parasitic draws if the vehicle sits.

Sounds like you agree the extra dc to dc is for the "in cabin" systems like the frigde. and the extra charging wire was for perma parallel setups?

If I fuse both ends the wire that connects the two battery systems together, do I really need a 500amp fuse, or can the fuse take a spike of up to 400amp for long enough to start the vehicle? I know the initial EMF spike and motor start is big, up to 400ish amps,>

From another thread:

How many amps to start a 60?

I'm going to be installing a 500a fuse to my Warn M12k. In the case of a front end crunch that shorts the power cable to the winch I want to be protected.

forum.ih8mud.com

Looking deeper in the saem thread, 400 amps is for a 2+kw diesel starter, for a gasser it's only around 200-250amps.

 

[Zjohnsonua]

Sorry I was unclear in their scenerion2nd battery runs accessories "in the "house".

In mine it runs lights and winch, and other external accessories and is a yellow top optima primarily for deep cycling but which can start the vehicle. Then I have a regular battery for starting "only" (anything on the main harness that I have not moved, like the headlights lights are now on the yellow top, but cabin lights are on the regular harness). Eventually I'm going to move them both to yellow tops, as they have had better luck surviving parasitic draws if the vehicle sits.

Sounds like you agree the extra dc to dc is for the "in cabin" systems like the frigde. and the extra charging wire was for perma parallel setups?

If I fuse both ends the wire that connects the two battery systems together, do I really need a 500amp fuse, or can the fuse take a spike of up to 400amp for long enough to start the vehicle? I know the initial EMF spike and motor start is big, up to 400ish amps,>

From another thread:

How many amps to start a 60?

I'm going to be installing a 500a fuse to my Warn M12k. In the case of a front end crunch that shorts the power cable to the winch I want to be protected.

forum.ih8mud.com

Looking deeper in the saem thread, 400 amps is for a 2+kw diesel starter, for a gasser it's only around 200-250amps.

The 400+ amp load is called "inrush". This lasts milliseconds. Actual cranking loads are in the ~100ish range.

Winching is an entirely different can of worms and can pull north of 400 when approaching line stall.

 

[caladin]

I was thinking to put a 250 on either end of the long cable that connects the two batteries and a 500 on the battery to winch cable, since I have one already. Seems easy to cut in a collision hanging out the front like that.

I wish they had more compact form factors though, the fuse itself is small, but the holder is huge.

 

[60Works]

I've set up 3 dual battery systems in 60's and run them long enough to find what didn't work.

150 Amp fuse is plenty for the starter or joining the batteries. I've never blown a 150A fuse.

I recommend a 250A for a strong 12V winch.

Toyota didn't use fuses on the starter or on the joining wire on their 24V diesels.

Toyota ran the linking wire across the fan shroud on their 24V diesels. Distance matters on high amp circuits.

Every single digital or diode isolator system I've tried continuously leaked miliamps and drained both batteries. Enough voltage leaks through to power little Bosch relays. If you run a stereo amplifier the capacitors in it will drain the battery through the diodes. They are all crap. Even the Blue Sea.

Use the small Blue Sea manual switch. Or a continuous duty solenoid, though those give problems too. You've got locking hubs and that isn't too inconvenient. Embrace the manual switch and let go of electronic gizmos.

Do not connect any loads to the spare battery. None. Connect all 'house' loads to the main battery. The point of having a spare is to have a spare. Once a month, top charge it or close your manual switch for a drive then open it after. Leave it open unless you need it or are charging.

Connecting 2 batteries, even with a fancy diode isolator, will shorten the life of both.

m-Series - Blue Sea Systems

300 Amps continuous rating for outboards and small gasoline or diesel engines.

www.bluesea.com

 

[OSS]

Toyota diesels had dual 12V batteries to create a 24V system: one battery on each side of the radiator.
The batteries were just connected together without any fuse.

If the battery cable is secured with cushioned wire clamps and protected with split loom— it’s never going to short out in a million years.
Recommending a fuse is standard EE practice, but there comes a time when in certain situations it’s absolute overkill.