3-cell power bank balancing

[Tobias89]

Hello,


we are developing power bank with 3S2P configuration. But we noticed there are no simple dedicated cell balancing ICs (for 3 or more cells) widely available on the market. Everything that is available requires MCU, or has multiple functions (besides protection) that we won't use but raises IC price (like fuel gauging).

We teared down a particular 3S2P power bank (and not a cheap one, cca $60 on Amazon) and discovered they don't have cell balancing at all! Just a protection IC from Ablic S-8254AA series.

So we were curious - how come a serious power bank manufacturer doesn't have cell balancing? Do they rely on Li Ion leakage for balancing? Or do they use matched cells that start off with identical SOC and this ensures that cell voltages won't diverge during power bank life? Is cell balancing in this case not mandatory? Or are we missing something?


We appreciate any help!

[Siwastaja]

Many high-profile manufacturers, such as BOSCH, use neither cell balancing nor cell-level voltage measurements at all, up to 6s as I have seen.

Everybody agrees going up to 2s without center tap monitoring or balancing is OK. After that, the opinions vary within the industry.

I have seen countless of products fail due to imbalance; all implemented tap monitoring and balancing: typically laptop batteries. I have never seen a non-monitored, non-balanced solution fail due to imbalance. Even a few completely worn down (to <50% capacity and >400% DCR) packs I have dissected have had perfectly in-balance cells (within 0.01V) when dissected.

Not connecting anything (faulty-by-design (too leaky) measurement/"protection" circuits, for example) to the taps helps prevent accidental imbalancing.

Self-discharge and coulombic efficiency differences between matched high-quality cells, which are environmentally coupled together, is simply negligible.

Some minor self-balancing does happen (i.e., a full cell self-discharges more quickly), but I wouldn't count on it 'cause it's almost negligible as well, but maybe it compensates for whatever negligible imbalance happens over the years anyway.

Balancing/monitoring ICs, like li-ion related ICs generally, are notoriosly broken-by-design, even when they come from brand names such as TI. You need to do a very thorough evaluation and testing before committing to use one. I guess that's one of the reasons they are not universally being used.

[mikerj]

Hello,


we are developing power bank with 3S2P configuration. But we noticed there are no simple dedicated cell balancing ICs (for 3 or more cells) widely available on the market. Everything that is available requires MCU, or has multiple functions (besides protection) that we won't use but raises IC price (like fuel gauging).

We teared down a particular 3S2P power bank (and not a cheap one, cca $60 on Amazon) and discovered they don't have cell balancing at all! Just a protection IC from Ablic S-8254AA series.

So we were curious - how come a serious power bank manufacturer doesn't have cell balancing? Do they rely on Li Ion leakage for balancing? Or do they use matched cells that start off with identical SOC and this ensures that cell voltages won't diverge during power bank life? Is cell balancing in this case not mandatory? Or are we missing something?


We appreciate any help!

None of the laptop packs I've torn apart have had any cell balancing, and they very often fail due to one pair of cells being significantly out of balance with the rest.

Which powerbank uses a 3S2P configuration, do you have a link?

[Siwastaja]

None of the laptop packs I've torn apart have had any cell balancing, and they very often fail due to one pair of cells being significantly out of balance with the rest.

Yes - in most cases, because they implement cell-level measurement using any of the many broken-by-design chips. I have measured these "BMS" devices drawing orders of magnitude too high quiescent currents (hundreds of uA), in itself causing overdischarging of the cells when the user leaves the pack in discharged state for some weeks or months, leading to self-destruction (a li-ion cell itself is OK to be stored fully discharged, even for years, but any external load in this state destroys it easily).

In some cases, instead of complete self-destruction, these laptop BMS boards "just" unbalance the pack due to differences between quiescent currents per cell tap - for example, if one tap consumes 20uA and another 50uA, it's 262 mAh of imbalance (or about 5% of typical 2*2600mAh capacity) in just one year.

It's true that laptop BMS boards do not always provide balancing - balancing would be needed to compensate for the parasitic unbalancer circuit. I have seen many that provide balancing, though. Most balancer circuits, however, have limited operating region where they can do balancing, and "wrong" usage patterns still cause significant imbalance.

The most robust packs I have seen are totally BMS-less power tool packs such as used by Bosch.

Though, some laptops may have an additional imbalancing issue of having a huge local CPU/GPU hotspot, which could heat one cell to significantly different temperature than the others. In these cases, however, capacity loss and ESR rise is often even a bigger issue than imbalance; manually balancing these cells usually remains a partial remedy only.

Li-ion management IC market, lead by the usual big guys, is totally crazy, completely driven by Powerpoint and handwaving instead of scientific study of battery technology. For example, as anybody who has done any analysis knows very well, non-dissipative (redistributive) balancing almost never makes any economical or ecological sense, yet we have a shitload of redistributive balancer ICs, usually using uselessly inefficient topologies such as neighbor-only switched capacitor.

[killcode]

The most robust packs I have seen are totally BMS-less power tool packs such as used by Bosch.

Maybe they are doing some capacity and internal resistance matching prior to pack assembly to get a more stable pack.

Totally agree with you on the state of the BMS market. If redist balancing is providing an efficiency gain then you have some serious problems with your pack.

I've considered rolling my own with a micro, fet switched voltage dividers and low voltage power latch-off etc. But for anything but personal use it seems risky to say the least. Have you come across any good solutions?

[sleemanj]

I think realistically with modern matched cells to start with that any out-of-balance is probably going to be indicative of a major problem anyway (failing cell) so the manufactuers figure the added complexity and cost  for realistically eeking out a few more mAh isn't worth it.

Cell protection  will catch major fails for safety, so balancing just isn't a critical function.  In short, balancing, assuming there is suitable (preferably conservative) protection, is about extending performance  when things start getting worn down, why  would the manufacturer care much about that, better they just sell a new battery!

[Daixiwen]

When I was working at Saft the Li-Ion cells were monitored during one or two weeks just after manufacturing to measure their self discharge (and of course take away those with obvious defects) and then the cells were matched when making battery packs. Most of the batteries weren't using any balancing circuitry, but we would match the cells by self-discharge rate, rather than capacity. We seldom had any problems with unbalanced packs.
Saft is making rather high-end Li-Ion batteries though (military/industrial) with high margins and I'm not sure the manufacturers for mass markets can do the same.

[BravoV]

When I was working at Saft the Li-Ion cells were monitored during one or two weeks just after manufacturing to measure their self discharge (and of course take away those with obvious defects) and then the cells were matched when making battery packs. Most of the batteries weren't using any balancing circuitry, but we would match the cells by self-discharge rate, rather than capacity. We seldom had any problems with unbalanced packs.
Saft is making rather high-end Li-Ion batteries though (military/industrial) with high margins and I'm not sure the manufacturers for mass markets can do the same.

Noted, thanks for the hint, definitely doable on DIY stuff or one off project. 

[Tobias89]

Which powerbank uses a 3S2P configuration, do you have a link?

For example, this one - https://www.amazon.com/20100mah-Portable-Delivery-External-Nintendo/dp/B075NXR2T4?th=1


Then it seems reasonable to choose a suitable protection IC (overcharge, overdischarge, overcurrent..) with reasonable leakage current at every cell (we can look at it as mild passive balancing) and omit the balancer IC. If the cells are valid, they shouldn't diverge in a near future..

[sstepane]

Hi,

What's wrong with using 3 x TP4056 chips? It's "dirty" cheap, has lots of even lower cost clones and different packages (SOT8 5056 version for example). As I understand - it's not an RC application and you don't need exact balance among cells - just proper charge for each cell.

[sleemanj]

Using 3 TP4056 (on 3 series) would require 3 floating supplies.  Typical TP4056 circuit is not isolated.

[ali_asadzadeh]

Using several TP4056 seems a sound Idea! If we could solve the input Power supply cheaply too? using several TPS54331 with different output voltages like 5V,10V,15V and 20V ... is it doable? or using a flyback converter with several isolated outputs! Flybacks are cheap too, using powerinegration parts