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

[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]

Balancing its a performance feature, not a safety one.

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.

[Siwastaja]

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

No, balancing is primarily a performance question - maximization of usable pack capacity during the product lifetime. It does little for safety, although if done right, can offer a extra safety layer when the primary safety layers fail.

You are confusing balancing with monitoring and individual cell-level cutoffs. Safety is brought by monitoring cell voltages individually and cutting charging when any cell of the series string exceeds maximum voltage; and cutting discharging when any cell goes under the minimum voltage.

If you think about it for a while, you will understand how balancing cannot do that: balancing currents are usually much smaller than charge, let alone discharge currents. They cannot fully bypass a cell and prevent it from overcharging or overdischarging.

But don't worry, you are not the only one who confuses these two. It's not surprising, because most of the engineering difficulty is properly measuring cell voltages (level shifting, with minimized quiescent current differences between cells, while maintaining good accuracy). Once you solve that, it is a small addition to add balancing. Nearly every IC which does monitoring also does balancing. And balancing is a catchy word, so people kind of assume that balancing implies having monitoring, cell-level HVC and cell-level LVC.

But once you start saying that it's the balancing which is the important (for safety) part, you have taken that misunderstanding too far, and it needs to be rectified; no, safety comes from cell-level monitoring, and balancing is the obvious extra nearly everyone implements.

Now, the "maybe extra safety layer" I mention is this: if you had a truly redundant way to reliably balance the cells, then if the monitoring system failed, a pack-level HVC (at least in the form of CV mode + stop timer implemented by the charger) and pack-level LVC, especially the former one, have better chances of preventing individual cells from going beyond limits, if those cells are balanced at top, compared to the situation where they are unbalanced. If you think about this deeper though, it is possible that the balancer fails too and unbalances the pack, making the situation worse. And in reality, balancing and monitoring is combined usually so that they would fail together.


Now, if you think that balancing alone, without monitoring of cell voltages - this would mean a distributed balancer with no level-shifting the voltages out to any central decision making - and only use pack-level LVC/HVC, would be a safe enough solution, I would be worried. I mean, balancing needs to be done at either top or bottom, the other end will be unbalanced* because of capacity differences which develop during lifetime. And now you would be adding new sources of error, namely the balancer itself. Every balancer acts as unbalancer because of unmatched quiescent draw. If you combine this with iffy balancing algorithm - e.g. the typical "only balance above certain voltage" - and lack of cell-level monitoring, you create edge cases where unbalance increases and is never rectified and never noticed. For such cheap-assiness, I recommend going completely BMS-less instead. Good initial balancing, smallish pack, good cell quality, good initial cell matching and initial factory balancing. No connections to taps at all, so external unbalancing is impossible. Big names did that for many years without much problems.

* (ignoring very powerful balancers, but that's mostly academic)

[RoGeorge]

The "balancer" I was talking was the loose/popular term for BMS (Battery Management System).  My fault about that.  It can be a BMS that only does cells protection and joule counting, without balancing.

The idea was to not use the bulk cells in series, without a BMS.  In the sense that "my batteries are 100% factory matched" doesn't mean they can be used without protection.

From what I've seen, with very few exceptions (e.g. battery packs for modelism), most other Li based series packs in consumer goods always have an internal BMS.  Even the single cells have internal protection board.

[Siwastaja]

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.

Very nice logical thinking, but arrived at wrong results. First, not all li-ion packs have balancing. BOSCH for example produced li-ion power tools without balancing for many years. This instantly proves the idea that balancing is mandatory for safety wrong.

Then the actual reason why NiCd and NiMH packs did not use balancers - these cells have internal chemical shunting mechanisms (just like lead acid), where excess charge is converter into heat with no or little permanent damage. They act like "zeners" over the cell. You balance them by applying high enough voltage.

Lithium ion does not have such mechanism; at the point where it starts to consume more current and heat up, serious internal permanent damage is done.

But if you don't try to push overvoltage into li-ion cell, then it's safe. Li-ion cell does not magically know the voltage of the neighboring cell. Cell getting into overvoltage is prevented by measuring the voltage and stopping charging, because the option of injecting overvoltage without damage, as in LA or NiXX, is not possible.

Note though that modern day low self discharge NiMH cells might not shunt very well, and instead be damaged from overvoltage. Note, NiXX packs (permanent multi-cell installations) are very rare nowadays! NiMH sold today are single cells which are charged separately so the whole charge time balancing issue does not exist. Separate parallel charging is ultimate form of balancing, so you could say that NiMH is balance charged.

[Siwastaja]

From what I've seen, with very few exceptions (e.g. battery packs for modelism), most other Li based series packs I've seen in consumer goods always have an internal BMS.  Even the single cells have internal protection board.

You are right this is very common today, as designers are confident with the reliability, and management happy with the cost of BMS. But this is a relatively new situation. From just 10 years ago you can find many designs completely without cell-level BMS of any sort, like BOSCH power tools (4s and 6s, e.g using Samsung INR18650-13Q cells), Minolta camera battery packs (2s) and whatnot. E.g. the BOSCH pack that I eventually took apart (for new cells) due to capacity drop and DC ESR rise was still in near perfect balance despite years of use.

During that time, there was very strong consensus on the internets that li-ion packs would instantly explode without a BMS and it was always funny to point out Bosch power tools which do not have it and still do not explode. Most people would just not believe there is no BMS. But it is easy to see from the absence of wires of any sort to the cell connections.

Today I would recommend what industry has transitioned into: always have cell-level BMS. But do it carefully and do it right. Just slapping something random has possibility of making situation worse than no BMS. Especially since Digikey carries 4736 items on the battery management IC category, each coming with appnote written by a summer trainee. A lot of catches from young players.

[nctnico]

From what I've seen, with very few exceptions (e.g. battery packs for modelism), most other Li based series packs I've seen in consumer goods always have an internal BMS.  Even the single cells have internal protection board.

You are right this is very common today, as designers are confident with the reliability, and management happy with the cost of BMS. But this is a relatively new situation. From just 10 years ago you can find many designs completely without cell-level BMS of any sort, like BOSCH power tools (4s and 6s, e.g using Samsung INR18650-13Q cells), Minolta camera battery packs (2s) and whatnot. E.g. the BOSCH pack that I eventually took apart (for new cells) due to capacity drop and DC ESR rise was still in near perfect balance despite years of use.

During that time, there was very strong consensus on the internets that li-ion packs would instantly explode without a BMS and it was always funny to point out Bosch power tools which do not have it and still do not explode. Most people would just not believe there is no BMS. But it is easy to see from the absence of wires of any sort to the cell connections.

There has to be some protection in the pack (or in the cells). How else would it pass UN38.3 testing?

[Siwastaja]

There has to be some protection in the pack (or in the cells). How else would it pass UN38.3 testing?

I don't think there is anything in UN38.3 which would drive specific cells out of balance and beyond limits. If the pack is initially made of cells of same capacity and balanced at manufacture time, pack-level HVC and LVC will prevent the cells from overcharging or overdischarging and the tests would pass. It's a different question if the cells would be overdischarged or overcharged later due to different capacity fade or whatever, but I can see how such hidden problem would still pass through testing.

Cell-level HVC/LVC is considered pretty much obvious today but I don't see anything in standardization which absolute requires this, if manufacturer is confident that pack level protections will do.

[voltsandjolts]

It's perhaps questionable if HVC LVC alone would suffice to pass the short circuit test. Although, simple fuse would fix that.

[nctnico]

It's perhaps questionable if HVC LVC alone would suffice to pass the short circuit test. Although, simple fuse would fix that.

My assumption is that the Bosch packs Siwastaja mentions use cells with some kind of internal protection. I looked up a teardown of -what looks like-  a Bosch Li-ion battery pack and there is no BMS inside which can switch off the connection between the cells and the external terminals. Not even a fuse. AFAIK there is no way such a pack can pass UN38.3 with regular, unprotected cells.
If the cells have internal protection, then external monitoring of cell voltage / balancing is not necessary at all.

[Siwastaja]

My assumption is that the Bosch packs Siwastaja mentions use cells with some kind of internal protection.

They are bog standard 18650, so they do have the PTC caps (and CIDs that permanently disable the cell on overpressure). This level of protection is also known as "unprotected cell" in the hobby scene.

Whether this is "enough protection" is questionable, but clearly at least back then it was acceptable. I'm sure Bosch at least qualified with Samsung (and other suppliers if they used others) that these PTCs and CIDs can take the total pack voltage from a 6s pack.

I always add a good old normal fuse in my designs because they are not expensive and are very reliable. Active MOSFET switch possibly fails as short circuit.

But yeah, IIRC the Bosch pack was just bare cells in series and a temperature sensor. The charger took care of high voltage cutoff, and the drill itself took care of low voltage cutoff. Both pack level, not cell level.

[ejeffrey]

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.

Keysight and tektronix don't because batteries are a low margin component and they make low volume / high margin products with expensive engineering costs.  They also don't have tight integration requirements like a lot of consumer electronics.  It's definitely easier to use an off the shelf pack if you can  but loads of companies large and small design their own packs when it makes sense.