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esteban avatar image
esteban asked

Wiring/Fusing for battery bank

I have a drop-in Lithium (100 amp BattleBorn) under my Van's driver's seat along with 75/15 MPPT, distribution fusing, BMV 702 etc, all packed in there.

I am now adding a second battery which will go under the passenger seat and so I will have to run cables under the carpet to connect the two batteries in parallel.

I know that I need to reduce the resistance between batteries as much as possible, I am looking into running multiple #4 cables to keep the bulk under the carpet acceptable.

I also believe that due to the routing, that I should fuse the positive on both ends that connect the "remote" second battery to avoid any potential danger due to shorts that may occur in accidents or whatever. If the two batteries were adjacent, I would not consider fusing.

Does this sound correct? I'm thinking of using an 80 amp terminal mount fuse off of the new battery and then adding a similar rated breaker where the linking cable connects to the "old" battery under driver's seat. This will also allow me to disconnect the two for any maintenance work and to give me a chance to charge each separately on ocasion to make sure that both are topped off and get their cells balanced correctly.

The rest of my system is fused already between the "old" battery and the house wiring charging etc.

thanks,

Steve

fuses
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4 Answers
boekel avatar image
boekel answered ·

sounds about right. it's never good to have batteries far apart, but you might not have other options. I'd use the lowest rated fuse possible (what loads are you expecting?) and please make sure the cables are protected against sharp edges etc.

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dazey77 answered ·

just be aware that if you end up with a difference between state of charge in the two batteries there will be much higher currents flowing between the link cables until they balance. For example when you add the new battery, if its not charged and the existing is you will pop your fuses (and cook the cable if its not high enough rated).

In operation it wont be an issue but if one fuse pops you can get out of balance again.

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esteban avatar image esteban commented ·

Thanks. Gotcha. I imagine that it would have to have a pretty big voltage differential between the two batteries to pull 50 amps, but something to be aware of. I'll top off each lithium separately at 14.4 for 30 min before connecting them as a bank.

From what I can tell, a 50 amp fuse will pop long before the 2 M of 25mm (round trip) heats up though.

I looked up the DC voltage calcs too. Looks like 4ga (25mm2) will see a voltage drop of .03 V over 2 M. So when the main battery reaches 14.4, the remote will be at 14.37. That seems like a reasonable differential for charge management so maybe I don't need to double up the 4 ga? That would be easier.


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esteban avatar image
esteban answered ·

Thanks for the confirmation. No options for getting both adjacent.

I'm now thinking to put a 50 amp on each end of the positive cable connecting the 50 amp battery. I doubt I will ever pass more than 25-30 amps through it, but seems wise to give it headroom. And I'm gonna look for a good way to add protection to the cables. Where there's room, the usual slit flex conduit. Below the carpet, maybe some thick plastic laid under and then over. I figure two 4ga pos and neg, four total will sit better and hopefully be low enough resistance to keep them charging well.

Mainly I'm thinking that the issue with any voltage drop from using 1M long jumpers will be undercharging the "remote" (to the bmv) battery and not getting it's BMS up to top balancing voltage. Kinda doubt it will be an issue, but I'll have a way to charge from "shore power" directly to the isolated remote battery once in a while to make sure.

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esteban avatar image
esteban answered ·

I thought I would throw in a photo of my underseat installation (VW T4) just for fun. As you can see, space is tight. The other seat base has a Phoenix inverter already but the second battery will fit in there.


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1552340130463.png (7.4 MiB)
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