Does either parallel option work the same?

markhbrown

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Jul 18, 2021
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I have 2 batteries, both 280amp lifepo4 and both are using overkill 120amp BMS. I want to wire them in parallel to keep my system a 12 volt system(all my components are 12 volt). It would be easier to wire each battery directly to my pos and neg on main bus bar. However do I need to wire the 2 batteries together and then run a pos from one of them and a neg from the other one to the bus bar? Will I get 120 amp draw from each for a 240amp total? I dont need that much but curious. Here are the diagrams.
 

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rickst29

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This will be fine, and your two BMS units will allow up to 240A total. But you should take care to make your total length of lead-in wires (the sum of wire length to BMS "P-" and the wire length to reach "+" on the same battery) the same for each of two batteries, or else you will have less resistance on the wire path to reach one battery versus the other.

When they are not the same, the least-resistant battery path will tend to be used first path (in charging AND discharging), because the internal resistance of these LFP batteries is very small. You will reach the BMS limit of the "favored battery" before the "less favored" battery is fully used, and "preferred" battery will have deeper cycles of discharge//recharge over time, shortening its lifespan in comparison to the other.
 

markhbrown

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Jul 18, 2021
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11
This will be fine, and your two BMS units will allow up to 240A total. But you should take care to make your total length of lead-in wires (the sum of wire length to BMS "P-" and the wire length to reach "+" on the same battery) the same for each of two batteries, or else you will have less resistance on the wire path to reach one battery versus the other.

When they are not the same, the least-resistant battery path will tend to be used first path (in charging AND discharging), because the internal resistance of these LFP batteries is very small. You will reach the BMS limit of the "favored battery" before the "less favored" battery is fully used, and "preferred" battery will have deeper cycles of discharge//recharge over time, shortening its lifespan in comparison to the other.
Thank you. That was helpful. I'll make the wire the same as well.
 
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