Electronics > Power/Renewable Energy/EV's

Is a cell balancer needed for a 2S1P pack ? (cells already balanced at assembly)

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Hydrochloride:
Hi everyone,

I’ve been researching the balancing feature for BMS/PCM design and have come across conflicting information. I’m hoping you could help me reach a conclusion.

Balancing its a performance feature, not a safety one. As long as you ensure that all cells are at the same state of charge when assembling the pack. It’s more of a function to prevent runtime loss due to the cells’ different self-discharge rates, since not all cells are manufactured 100% identically. We’re talking about losing runtime over years, not months.

I understand that the more cells you use, the more justified it is to incorporate this option, as there’s a higher probability of encountering imbalance(manufacturing tolerances, temperature differences within the pack,..).
I’ve seen that there are many PCM (Protection Circuit Module) options—purely analog BMS like ABLIC, Texas Instruments, etc.—for 3 or more cells, but it’s hard to find options for just 2 cells. I’ve found a few options, but I’m always struck by how few there are compared to the rest.

Considering that my battery pack will be assembled with Grade A cells, we’ll ask the manufacturer to supply cells with the same levels of state of charge, internal resistance, and capacity. They’ll be assembled in a compact pack, ensuring both cells experience same thermal/humidity conditions.

This way, the difference in self-discharge should be minimal (low enough that balancing shouldn’t be necessary). From what I’ve read about balancing, I see in some forums that it can do more harm than good if poorly implemented, potentially causing cell imbalance or significantly extending charge times, among other issues.

squadchannel:
use the battery pack rather than a built-in battery, have you considered an off-the-shelf battery pack?

https://www.inspired-energy.com/
https://www.rrc-ps.com/

even leading company, such as Tektronix and keysight etc..., do not design batteries.
i believe that it is difficult to obtain certification for safety standards.

if it were me, i would not design a battery pack.

Hydrochloride:
Unfortunately, I cannot buy a commercial product and connect it to the system due to company restrictions (proprietary products that cannot be remolded). It must be a new product development. So, I am evaluating what features are necessary, and one of them is balancing. From what I’ve seen, to design a battery protection circuit, you can take two paths. One is to buy a PCM, which are standalone BMS (no intelligence, all analog) and adapt your requirements to what is available on the market. The other is to make a BMS, which involves adding a microcontroller and an AFE (analog front end), which I assume will be the protection MOSFETs. Then, you would program the microcontroller to meet the under/over voltage, over/under temperature, overcurrent, and short circuit protection levels. Obviously, this would also include some sensors for current detection (Hall effect or resistively. Common practice is resistive).

Clearly, the simplest option is the PCM, as the options I’ve seen cover everything I want, except for balancing for 2 cells in series. There are some, but then they remove some protections (like the current protection and short circuit, for example)

RoGeorge:

--- Quote from: Hydrochloride on November 08, 2024, 09:04:30 am ---Balancing its a performance feature, not a safety one.

--- End quote ---

No, balancing is about safety in the first place.  It was not added for performance.  Better performance is only a side effect of balancing.

You won't see balancing for other types of batteries, yet you see it for Li-ion.  Why only Li battery packs have balancers, and NiMH or NiCd don't have balancers?  Because Li based batteries require balancing for safety.  Li cells are very prone to catching fire, when compared to batteries of other chemistries.

Or, if you are lucky to not start a fire, at best you'll kill one cell very fast.  In a series pack, the weaker cell becomes reverse polarized.  Li based cells get permanent damage when overdischarge, let alone when reversed polarized.

Find a balancer chip that is right for the size of your batteries, and use it.  You must have balancing.  Or else, buy a ready made battery pack with an internal BMS already included.

voltsandjolts:
Using good quality cells for a 2S Li-ion you don't really need cell balancing. If you want to 'gild the lily' then BQ29209 might work for you.

I would recommend over-current, over/under-volt protection.

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