12v parallel wiring size and related questions

hazed · Apr 4, 2024

Hello. I have (4) 12v 280ah lifepo4 batteries I am going to run in parallel, along with a 3000w pure sine inverter (6000w surge). I have 8 100w solar panels wired 4s2p into a Renogy 60amp controller via 10g wire. All components are within 2-3 feet of each other, with the exception of the solar panels. All of this is in my motorhome. I believe 10g from the inverter to the fuse panel inside is sufficient.

My questions are:

What is the best wire size for battery to battery parallel wiring?

Wire size from battery to buss bar/inverter?

Wire size from controller to buss bar/battery?

Bonus question....I will be running cable from the motor in the front of the rv where the BIM225 is located to the rear where the batteries are for additional charging from the alternator while running. Any idea on wire size for that 30 foot run???

Thanks!!

sunshine_eggo · Apr 4, 2024

hazed said:
What is the best wire size for battery to battery parallel wiring?

same as below:

hazed said:
Wire size from battery to buss bar/inverter?

Wire rated for 3000W/12.8V/0.85A = 275A OR the value specified in the manual

hazed said:
Wire size from controller to buss bar/battery?

Wire rated for at least 60A.

hazed said:
Bonus question....I will be running cable from the motor in the front of the rv where the BIM225 is located to the rear where the batteries are for additional charging from the alternator while running. Any idea on wire size for that 30 foot run???

Wire suitable for whatever the maximum output of the alternator is.

On all positions, confirm voltage drop is acceptable per:

Voltage Drop Calculator

This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.

www.calculator.net

Estimated resistance tab.

JBertok · Apr 5, 2024

I am not sure connecting the chassis alternator and battery to the LiPO4 batteries without some sort of regulation would be adequate. Victron makes a DC-DC charge controller just for that purpose that would help. Victron is obviously not inexpensive, but it's on a completely different level of engineering quality than the other stuff.

For connecting the batteries to the bus bars you have two choices:
- You can parallel connect the batteries together (safest if each has its own fuse/breaker) and then cable the battery bank to the bus bars. With batteries in parallel you want the positive and negative cables to connect to the battery bank at opposite ends; this helps keeps them close to equal states of charge. For example, attach the positive to battery 1 and the negative to battery 4.
- You can connect each battery to the bus bars with its own set of leads. If you do this keep the leads the same gauge and length to each battery, to help them state close to the same state of charge.

hazed · Apr 5, 2024

JBertok said:
I am not sure connecting the chassis alternator and battery to the LiPO4 batteries without some sort of regulation would be adequate. Victron makes a DC-DC charge controller just for that purpose that would help. Victron is obviously not inexpensive, but it's on a completely different level of engineering quality than the other stuff.

For connecting the batteries to the bus bars you have two choices:
- You can parallel connect the batteries together (safest if each has its own fuse/breaker) and then cable the battery bank to the bus bars. With batteries in parallel you want the positive and negative cables to connect to the battery bank at opposite ends; this helps keeps them close to equal states of charge. For example, attach the positive to battery 1 and the negative to battery 4.
- You can connect each battery to the bus bars with its own set of leads. If you do this keep the leads the same gauge and length to each battery, to help them state close to the same state of charge.

With 4 12v batteries at 280ah each and in parallel, I see some diagrams with 4awg between posts and then 2/0 or 4/0 to bus bars. Do all wires need to be the same gauge? I have 4awg and 4/0. 1 foot between posts and 2 ft to bus bar from each post on opposing ends. Should I try to wire using the halfway method?

JBertok · Apr 6, 2024

Sounds like the batteries are paralleled as a bank, and the bank connects to the bus bars rather than each battery having it own cables.
Cable sizing is more than X amount of current requires X gauge. There is a relationship between the conductor gauge, its length, the material it's made of, temperature, and the current flowing through it. Oversizing conductors favorably results in less voltage loss and therefore less energy wasted to resistive heating. Undersizing causes more voltage drop as current increases, and more energy is wasted as heat caused by the conductor's resistance. Resistance also rises with temperature, so there is a slope to that relationship.

The interconnections between the batteries needs to be capable of carrying the full surge load of the inverter(s) with acceptable voltage loss and not overheating the conductors. Some will say the interconnects can be smaller capacity by a factor of how many batteries you have in parallel - for example 25% less capacity because you have 4 batteries They might even say they're short and voltage drop is negligible. I disagree with that undersizing logic for the reason that any battery with a BMS can be tripped off by several different not-uncommon conditions. The interconnects need to be sized with consideration that a non-contributing battery should not cause overload to any of those interconnections. So, it would be acceptable to use smaller conductors for the interconnections than the feeds to the bus bars if those feeds are intentionally oversized.

I guess the right way to word it would be to say neither the interconnects or the bus bar feeds should be sized any less that what is required to carry their full surge-rated loads. As long as that criteria is met, it doesn't hurt to *oversize* the feeds to the busbars. Each interconnect, however, needs to be sized to safely carry the entire full load of the equipment.

hazed · Apr 7, 2024

So with what I have:
4 12v lifepo4 280ah batteries, a 60 amp Renogy charger, 800w solar and a 3000w inverter on my motorhome. I have 10awg wire into the mppt and 4awg out. I am looking to do the halfway wiring method. I have 4/0 wire to wire between batteries and to the bus bar and inverter.

A couple questions are, do you need equal lengths of wire to from the mppt to the batteries for balance? Or, do the wires from the batteries to the bus bar through fuses and shutoff to the inverter need to be equal? Do I pull the loads of the batteries from the opposing end terminals to the bus bar and equal lengths of wire? Or, mppt to bus bar along with everything else. Just trying to do it the best way. Perhaps any readily available diagrams that you know of?

Yeesh! I built and wired my house and can rebuild an entire cat but for some reason I get hung up on these setups!!
Thank you!!

hazed · Apr 7, 2024

JBertok said:
Sounds like the batteries are paralleled as a bank, and the bank connects to the bus bars rather than each battery having it own cables.
Cable sizing is more than X amount of current requires X gauge. There is a relationship between the conductor gauge, its length, the material it's made of, temperature, and the current flowing through it. Oversizing conductors favorably results in less voltage loss and therefore less energy wasted to resistive heating. Undersizing causes more voltage drop as current increases, and more energy is wasted as heat caused by the conductor's resistance. Resistance also rises with temperature, so there is a slope to that relationship.

The interconnections between the batteries needs to be capable of carrying the full surge load of the inverter(s) with acceptable voltage loss and not overheating the conductors. Some will say the interconnects can be smaller capacity by a factor of how many batteries you have in parallel - for example 25% less capacity because you have 4 batteries They might even say they're short and voltage drop is negligible. I disagree with that undersizing logic for the reason that any battery with a BMS can be tripped off by several different not-uncommon conditions. The interconnects need to be sized with consideration that a non-contributing battery should not cause overload to any of those interconnections. So, it would be acceptable to use smaller conductors for the interconnections than the feeds to the bus bars if those feeds are intentionally oversized.

I guess the right way to word it would be to say neither the interconnects or the bus bar feeds should be sized any less that what is required to carry their full surge-rated loads. As long as that criteria is met, it doesn't hurt to *oversize* the feeds to the busbars. Each interconnect, however, needs to be sized to safely carry the entire full load of the equipment.

That all makes sense. Thank you. Fortunately, I have a good bit of 4/0 cable to make all the connections needed. Are you able to read my other post about equal lengths?

12v parallel wiring size and related questions

hazed

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sunshine_eggo

Happy Breffast!

Voltage Drop Calculator

JBertok

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hazed

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JBertok

Network Engineer

hazed

New Member

hazed

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