Does a 3S 18650 battery pack need a balanced charge circuit?

Question

I am looking at building a custom powerbank with 3 or 4 cells in series to generate 11-15V. I am looking at some of the TI Li-Ion charger IC's BQ257x3 and BQ241xx. These are really nice IC's but don't charge or balance the cells individually. Of course, they do implement the proper Li-ion charge cycle with constant current first and then a constant voltage until the current drops. They also have over-voltage protection, temperature monitoring, and input current limitation, and that kind of stuff so do overlap in functionality with the battery pack protection boards that usually monitor individual cells.

So assuming there is a protection board on the pack that monitors at least the minimum/maximum voltage per cell and that the individual cells are all new and from the same brand/model and likely even the same production batch, would there still be a need to charge/balance all cells independently for the battery pack? Or can I just treat the 3S pack as a single battery and assume they all charge/discharge at the same rates?

The other question I have is if I use one of those charge IC's, which already monitor for over-voltage, (dis)charge current, and temperature on the battery pack, and turn off the output if the total battery pack voltage gets too low, is there still even a need for the protection board on the battery pack (as long as they are new and identical cells)? Or can I just hook up 3 cells in series directly to the charger without the risk of batteries over/undercharging and thus breaking or worse (provided the rest of the circuit matches safe charge currents at around 1/2C and voltage for those cells of course).

Answer 1

Does a 3S 18650 battery pack need a balanced charge circuit?

Balancing a string of cells in series is optional because it is not a safety feature.

However, balancing is highly desirable as it maximizes the effective capacity of a string of cells over time. That is why it is almost universally implemented.

Just about every BMS IC does implement balancing.

BQ25723 and BQ241xx.

Those are not BMS ICs. Those are charger ICs. A charger is not a BMS. You need both: a charger AND and a BMS.

Answer 2

The probability if mismatched Capacity in the equivalent circuit will increase with the number of series cells. There will also be a tolerance on Each cell ESR.

The charging voltage under CC mode will be ;

Vcell= Time Integral { (Ichg * dt) / C [Farads]} + Ichg * ESR ... for each cell

Thus when it changes from CC mode to CV mode and cuts off at say 5% Ichg, will the voltage and energy stored on each cell be the same? No but the % of State of charge or overcharge determines how fast the acceleration occurs on each charge cycle in amplifying this difference.

? Possibly for a couple hundred charge cycles as long as they don't get deeply discharged then aging occurs more rapidly on the weakest cell.

So it all depends how well match your cells are .

Consider how hard it is to make electrolytic caps < 1 % tolerance. This is significant and likewise similar for electrolytic batteries.

in short , you don't need to worry if you dont care how many cycles you get, but if the cells don't come from the the same batch process, you can always extend the life of the pack by using a balancer. The time spent above 4V and below 3V, significantly accelerates aging, so it totally depends on your reliability expectations. manufacturers can expect tighter tolerances with process controls, but end-users cannot know which batch parts come from.

Answer 3

The other question I have is if I use one of those charge IC's, which already monitor for over-voltage, (dis)charge current, and temperature on the battery pack, and turn off the output if the total battery pack voltage gets too low,

NO. THEY. DON'T.

Those ICs are chargers, not protectors. Those ICs do not protect your battery.

is there still even a need for the protection board on the battery pack (as long as they are new and identical cells)

You MUST have a protector BMS in your battery. A protector BMS is absolutely required, and a charger is absolutely required. Both are absolutely required. They are two different devices. Each is absolutely required.