BMS choices for 12V 4P system


New Member
Nov 21, 2023
I have been given 20 100ah cells. These are from random 12v batteries that have failed BMS's. I want to make a 400-500ah 12V system.
What BMS to run for a 4P4S or 5P4S system that can monitor each cell, do I need a balancer to keep everything inline?
Do I just set them all up in 100ah 4S systems and parallel together? What BMS would handle this, plus communication?
I want to make a 400-500ah 12V system.
Can you elaborate? In particular...
a) what is the maximum current you will want to draw?
b) is this for a fixed or mobile solution?
c) is weight of concern - i.e. whether best to have one big battery of 4 paralleled ones

You will need a BMS that will handle balancing. An active balancer would be best for a large Ah configuration.
I have built 2 4S LiFePO4 batteries. I used two brands of identical BMS: OverKill Solar and RadioB. They have a passive balancer. After being idle and offline for a month they went out of balance and when a cell was high they stopped charging. It took 5 daily cycles to restore balance.
One battery is 200AH and the other is 280AH. They play well together. When one approaches full charge it's voltage rises rapidly and more current goes to the lower voltage one.
Both have temperature sensors. Both have Bluetooth. Overkill Solar web site has a better phone app that works for both. Both have an external disable switch option, but only one had a 2 pin connector installed. The APP can also shut off Discharge or Charge.
Both can supply 100A and can be set for 125A. That and the balancing makes a good case for separate batteries, especially when powering an inverter.
My Overkill BMS has 3x 8awg 12" wires in and 3x 8awg 12" wires out, that's close to 24" of 3awg (0.427 milliohms). I suggest the heavier wire gauge option if available. they also suggest a minimum of 4awg for the positive wire in the case.
Get a good battery Shunt that measures current both ways. I use Victron Smart Shunt in my system. The BMS is not an accurate current measurement. The State of Charge is voltage based. I edited my settings to be more realist than the defaults. Cell full: 3.4v, 80% 3.275v, 60% 3.25v, 40% 3.2v, 20% 3.1v
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Have you considered a 24v system? It's much better for inverters over 2000 watts.