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I want to make a 12V range battery array with 2P 3S 18650 cells. The cells will be charged in an external charger NOT in the pack. They are removable in holders So I can pluck them out and charge externally. I have the basic plan below.

Some criteria: The amp draw is relatively low at around 1A. I used the protected cells so one or more cells can't discharge below recommended volt limit. The protection also prevents a good cell from trying to charge and dump too much current into a bum cell that may be in parallel with it. That would be rare unless there is a mistake or a bad cell, it is a precaution. That is the theory.

The question is do I need BMS and if so how and where would I put it? Would I use a regular 3S BMS and treat each two battery cluster as one cell? Would a BMS conflict with the protection chips in the batteries?

Any suggestions? Thanks.

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The generally accepted method for grouping Lithium Ion cells in a pack is to start with well matched cells (same brand and model and new ideally, and factories will go as far as to buy a massive batch and group similar cells from that in a process called binning) and bring them to the same voltage before ganging them. in addition, balance parallel cells by temporarily connecting them in parallel with a resistor ahead of time to limit current before welding them in parallel.

Weld them with bus strips preferably over using battery holders and strongly over soldering. Welding is preferable as once the batteries are matched they should be kept as a group until end of life.

You have a series arrangement, which means you should indeed use a balancer board. It won't conflict with the protection in the batteries, but if your current was close to the limits of a single 18650 (it's not), you would want to take into account that if one battery dies and removes itself from the circuit, it will double the current through the other battery, increasing it's charge and discharge rate. It's output voltage will change faster than the other sets in the series, so the BMS will have to work harder to balance voltage. The balancer works by wasting the extra power causing voltage mismatch, so efficiency of the entire arrangement will drop, pessimistically similar to if you had changed to a 3S1P arrangement, although it should continue working.

So with those 3 pieces of information: Matching should be done, connections should be permanent and a balancer board is necessary, you should strongly consider buying and already made pack. You may well get better performance for your cost and superior protection, although you may be giving up individually protected cells and the redundancy you're creating.

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you do not need a BMS. (the BMS would be required if you wanted to charge the cells in place, which you expressly do not.)

the caveats:

  1. during discharge, one cell will trigger its low V cutoff first, and its parallel companion will suddenly bare all the current. but you expressly said the current was low, so this is not an issue for you.

  2. it is conceivable that one of the cells may develop a loose connection. its pair will then bare all the load and discharge faster. then some vibration may reconnect the loose cell and a high current discharge through the pair cell might happen (charging it at high current). this current may exceed allowed charge/discharge currents and present a fire risk. but if the protection circuits work, then they should trip before an accident happen, which means IMHO there is low risk.

  3. while using any non-soldered lithium-ion, a bad connection could develop and its resistance might cause a localized spike in temperature, which again would be a fire risk. but i have not heard of this being an issue in real life, so again IMHO there is low risk.

notes:

matching cells is not required. but if i were to match them, i would try to make pairs of cells of similar total capacity (ie, matching the two highest capacity cells together is NOT what you want).

but given that the cells will be removed for charging, just rotate them around randomly and you will be totally fine.

IMPORTANT:

each time you load the battery on the target device, the cells MUST ALL BE FULLY CHARGED. otherwise a high current loop between paired cells of different state of charge might ensue. (yes, you have the cell protections anyway, but those are to handle failure cases which should hopefully never happen.)

during initial connection, current loops among pairs will happen. but i would expect the currents to be absolutely negligible, because cells have higher impedance when fully charged. you can check the current anyway by paring a fully charged test pair through a multimeter set to measure current.

to minimize these current loops, try loading the cells into the device immediately after charging them. although self-discharge in lithiums is low, some cells might self-discharge more than others causing a larger current loop when installed if left in storage for long after charging.

all in all, the safety restrictions (1. fully charge all cells before installing them in the load, 2. install them ASAP after charging) mean this arrangement is not safe enough for consumer products, but it is certainly safe enough for a responsible person.

but if you want it to be fool-proof:

just place a low forward voltage diode (eg, schottky) in series with each cell stopping it from charging to break those current loops. a 1N5817 will drop around 0.4V at 1A forward current, dissipating 0.4W (so keep them away from the cells!) and zapping around 11% of the total energy of the pack.

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  • \$\begingroup\$ Please start each sentence with capital letter. \$\endgroup\$
    – winny
    Sep 22, 2023 at 8:45

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