Lithium Ion Cell Balancing

[SenSeS]

I’m making a Lithium Ion charger circuit and successfully used a Texas Instruments bq2057c to charge 2 Lithium Ion cells in parallel (4.2V). Now I want to do the same with the cells in series (8.4V). I read that when charging multiple cells in series you have to do cell balancing to take into account the difference in charge speed between the 2 cells to avoid gradually reducing the battery’s capacity.

I bought the bq2057c in the first place because in the datasheet it said that it could charge 1 or 2 cells in series. Indeed, you can set the threshold level to 8.4V and the IC will charge the cells to 8.4V. The IC doesn’t balance the cells though so it might very well be that one cell is at 4.1V and the other is at 4.3V which in the long run will ruin the battery’s capacity. I don’t even know why anyone would want to charge 2 cells and not do cell balancing and therefore why this feature of charging to 8.4V is there. Maybe there is something I’m missing. Is it common to use multiple cells in series and not use cell balancing?

I could use an IC that has cell balancing, or I could use 2 single cell ICs and use them to charge my cells individually. Which one is best, and what is the most common? What I don’t like about the cell balancing ICs (e.g. bq29330) is that they always come in packages with a high pin count. I can’t seem to find one with low pin count. Do they exist?

[KL27x]

If you try it you might find balancing is very rarely a significant benefit. IME it usually occurs to a significant degree only after I have over discharged a battery to the point of damage.

[nctnico]

You still need balancing in order not to overcharge cells. With 2 cells in series it is not much of a problem but with more cells in series you need at least protection against an overvoltage situation. Overcharging a Li-ion cell can go horribly wrong (call the fire department).

[KL27x]

IME, up to 4S, no not at all. Charge all cells fully before assembly. Put fly leads on all cells for testing and balanc8ng. And years later find out it is really not necessary. That is my experience.

[eas]

IME, up to 4S, no not at all. Charge all cells fully before assembly. Put fly leads on all cells for testing and balanc8ng. And years later find out it is really not necessary. That is my experience.

What is your sample size?

If this is just a one-off and the OP is happy to check the cells and, if necessary, rebalance them manually from time to time, then doing as you say is probably fine.

Cell balancing is one of those things that isn't that important when everything is going right, but its cheap insurance against the possibility things get out of wack, because once they get a little out, they'll probably get worse. Most likely, you'll have the pack die prematurely, but you might end up with a YouTube worthy event.

Actually, we should be ask the OP: which lithium-ion chemistry, specifically?

[SenSeS]

Thanks for all the answers! Am I right in assuming then that for hobbyist projects it's probably fine not to do load balancing but that for real applications always cell balancing is used? Because I can't imagine a company running the risk of burning someone's house down, even if it happens only in 1 out of 10000 products.

@eas: I'm not sure what kind of batteries I have at the moment. I found them in a USB battery that I took apart and they have "HLD 18650 220" written on them but I don't think this tells us something about the chemistry. I'll probably be using a Lithium Polymer in my final product because it allows for a variety of shapes. But again those can be made in various chemistries right? Is there a preferred/most common battery chemistry and what is the difference with regards to charging? I'm using an IC so I'm guessing not much, except for getting the appropriate IC?

[IanB]

You should balance the cells manually by charging each of them to exactly the same voltage before joining them into a series configuration. Lithium ion cells have an almost 100% perfect charge retention so once balanced they will tend to stay balanced as long as they remain in series and both cells pass exactly the same current at all times. Both cells should of course be very closely matched by type, lot number and a capacity test before being joined up in this way.

There are a few kinds of lithium ion chemistry and some of them are less energetic in failure than others. However, the common kind found in cell phones and laptop batteries is not of the "safe" kind and can catch fire quite energetically if abused.

[eas]

So, first off, I want to make a caveat that despite the interest in LiIon battery re-use expressed implied by my sig, I don't consider myself anything more than an unevenly informed layman on the topic of LiIon batteries.

I asked about the chemistry because some are indeed safer than others. LiFEPO4 has 1/2 the energy density of the LiION cells typically used in consumer electronics, but is much less prone to thermal runaway from being discharged too quickly (something that can end up happening due to internal damage due to over charge/discharge). Historically consumer electronics batteries have typically been Lithium Cobalt Oxide, which have high energy density but also are prone to spectacular failure. Power tools have used Lithium Manganese Oxide, which is safer than Lithium-Cobalt, and supports high discharge rates, but has rather low energy-density. More recently, some consumer electronics have been using Lithium Nickle Cobalt Aluminum Oxide for highest energy density.

Yours are probably Lithium Cobalt Oxide, and your choices for one-off purchase of pouch cells ("LiPoly") will probably be limited to the same + LiFePo4.

There are two major risks when using cells in series.

The first is that any imbalance in capacity, state of charge or internal resistance between cells can lead to a cell dropping below a safe internal voltage, which can lead to damage. That damage can reduce the capacity of that cell and increase internal resistance, increasing the likelihood of further damage in the future. It can also result in formation of bits of metallic copper which can lead to spontaneous overdischarge, thermal runaway and spectacular failure.

The second is that differences in capacity and state of charge can result in overcharge of a cell. An overcharged cells can suffer from reduced capacity and increased internal resistance, which, again, can lead to further damage during future charge/discharge cycles.

Over time, this can result in some really f'ed up situations. I took apart a bunch of Dell packs where a majority of the cells had reversed polarities, with others at nearly full charge. There were also packs that seemed to be at a safe discharge voltage, but actually contained two series cells near full charge, and one at a reversed polarity. Now, these packs had cell balancing circuits, but something was clearly defective. They were also intact, but it may that I just got the ones that didn't burn up

To avoid these problems, designers use a combination of approaches. To start they use cells with closely matched capacities and internal resistance. To that is added top-balancing cells during charging, cutting off discharge if the discharge rate is too high, and cutting off discharge if one of the cells drops below threshold voltage.

In your case, getting a pouch cell pack that already includes two matched cells in a series is a good start. Confirm that they are balanced, and adjust them if they aren't. Then, limit your discharge to a carefully chose threshold voltage. To choose the threshold, find the manufacturers minimum discharge voltage and internal resistance specification and tolerance. Then figure out your maximum current draw and calculate the internal resistive loss at your max load, and at the limits of the IR spec tolerances. Use that to figure out your possible error in predicting the voltage of an individual cell based on the voltage of the series under load if internal resistance. Add your own safety margin. Adjust your discharge threshold upward to account for this.

Doing so should start you out in the best possible position, and keep you safely away from the conditions that are likely to result in a death-spiral.

Someone with actual experience probably has better advice though, but at the very least, I hope I've provided more context for understanding the tradeoffs.

[mikerj]

Is it common to use multiple cells in series and not use cell balancing?

Yes, very common indeed.  Pretty much every laptop/notebook has three or four LiIon cells in series (usually 3S2P or 4S2P) and no cell balancing is used.

You still need balancing in order not to overcharge cells.

No you don't.  You simply stop charging as soon as any cell reaches the termination voltage (and stop discharging when any cell reaches the lower voltage threshold), which of course means your battery slowly gets unbalanced.  But that's exactly how notebook charging works and you can get through quite a good number of cycles if your cells are well matched to start with.

[eas]

Is it common to use multiple cells in series and not use cell balancing?

Yes, very common indeed.  Pretty much every laptop/notebook has three or four LiIon cells in series (usually 3S2P or 4S2P) and no cell balancing is used.

You still need balancing in order not to overcharge cells.

No you don't.  You simply stop charging as soon as any cell reaches the termination voltage (and stop discharging when any cell reaches the lower voltage threshold), which of course means your battery slowly gets unbalanced.  But that's exactly how notebook charging works and you can get through quite a good number of cycles if your cells are well matched to start with.

I don't think so.

I have in front of me a 1 quart ziplock bag filled with battery management PCBs pulled out of old laptop batteries. I'll pick one at random...

It looks like it is from a Dell pack and the PCB has a copyright date of 2005. It has various chips on it, including two with markings that start "bq." BQ stands for Benchmarq Microelectronics, a company that made, among other things, battery management ICs. Those products have long been part of TI's portfolio though. Even now, TIs latest battery management ICs have part numbers starting with "bq"

One of the chips is a "fuel gauge," the other is a bq29311 battery protection IC. Click the link. Look at the buletted feature list. What's the fourth item?  What to make of that information?

While you are thinking about that, I'll pick another board. This one isn't a random pick, because i looked for one of more recent vintage. In this case, the silkscreen is 2009, but my notes, while not as good as I'd like, suggest that, according the the pack's battery management system, it was manufactured in the middle of 2010. This one also has two "bq" ICs. One of them is a bq29312, like the bq29311 I found on the first PCB I looked at, it is a battery protection IC, and not surprisingly, it has a very similar feature set.

For those not willing to look for themselves, one of the features is, wait for it... "integrated cell balancing control!!" Now you might argue that the mere presence of that feature is not guarantee that the packs actually make use of it, and you'd have a point. I'm not going to bother to trace out to see if the necessary pins are connected to control a bypass element in order to provide more evidence that laptop packs do in fact use cell balancing, but I'll suggest a look at TIs offerings. An awful lot of them, including some new introductions, support cell balancing, suggesting that it is an important feature, even in cost-sensitive mass-market electronics. But I think it is more telling that some of them don't, and yet cost sensitive manufacturers chose chips, or more likely, packaging that went to the extra expense of including the extra pins needed to control cell balancing.

Do you have other information we should be considering?

[Stuart Coyle]

I have used this series of chips for charging two Li cells and it works fine. I think that you won't want the bq2057c but the bq2057t or w variants to charge to 8.2V or 8.4V.

Oh and I found out the magic smoke way that when they say the supply voltage max is 18V they really mean it.

I guess that it really depends on your use case wether you want charge balancing of the cells.

[KL27x]

What is your sample size?

Eas, I was playing with Li ion cells back when qualified EEs thought it was too dangerous to even attempt. Ask the question "how do I charge a little ion cell" on a forum like this (since this one wasn't born yet) and the only response you would get is "don't do it. You'll burn your house down."

Just because laptop cells can go out of balance doesn't mean all li ion batteries need balancing. Some laptops are poorly managed and/or foolishly (cunningly) use batteries with too low a discharge rate to begin with and/or push too close to the empty line. This is intentional. If you use enough battery for the job and protect from over discharge, and you use matched and fully charged (initially balanced) cells, you might find this out for yourself. Laptop batteries that die prematurely are usually not from a failure of proper engneering so much as they are lasting exactly as long as the designers have chosen.

[TerraHertz]

Laptop batteries that die prematurely are usually not from a failure of proper engineering so much as they are lasting exactly as long as the designers have chosen.

Which without charge balancing, will not be as long as the batteries *could* have lasted. This is called planned obsolescence. Also there will be outliers, in which greater initial cell miss-matching will result in some packs entering death-spirals, possibly terminating in fires or at least pack failure much sooner than expected.

I get the feeling that because *you* don't use charge balancing in series-connected packs (which you get away with because you can monitor and manually balance when needed) you are arguing that no one should include balancing in commercial designs.

Seems like faulty reasoning to me.

A three cell series design I'm starting definitely will include cell balancing as well as over-under charge/discharge protection. And the circuitry will be in the device, not the battery pack. It's ridiculous the way that amount of circuitry is included in throwaway battery packs, imo.

[KL27x]

I don't get away with anything. Monitoring the manual header of battery packs has shown me that I don't need to balance cells. Under the conditions I have stated. I am satisfied to where I no longer wire this header in to muti cell packs. Unbalance is mainly the direct result of cell damage. If you want balanced cells and longevity you will avoid the damage in the first place. Do that and the packs stay remarkably balanced.

Planned obsolescence is a dirty word. The preferred term is product life cycle, and there is nothing morally wrong with it. But to achieve desired results in this case takes more than to omit balancing. That won't cause significant premature failure unless the previously stated conditions are intentionally abused. Or if a smart controller simply shuts things down arbitrarily.

This is my opinion, based on real world testing and observation. Your opinion may be more popular. And it is backed up with information that is easily searched on the web. But do you know 5he ultimate source and context of that information? Ask yourself. Am I simply repeating something I have heard 50 other people say, or am I citing fact?

[Siwastaja]

I’m making a Lithium Ion charger circuit and successfully used a Texas Instruments bq2057c to charge 2 Lithium Ion cells in parallel (4.2V). Now I want to do the same with the cells in series (8.4V). I read that when charging multiple cells in series you have to do cell balancing to take into account the difference in charge speed between the 2 cells to avoid gradually reducing the battery’s capacity.

This is the wrong explanation.

In series, all cells charge at exactly the same current.

Percentual charging speed may differ due to the capacity difference, but balancing can't help in that, unless the balancing current is HUGE, balancing is redistributive (non-dissipative) and happens during both charge and discharge. This is mostly subject of academic discussion and not of practical benefit, as the complexity of this approach exceeds any benefits by at least an order of magnitude.

Only thing that balancing does is to prevent slow drift caused by differences in self-discharge rates, which would otherwise reduce the pack capacity (with cell-level monitoring) or cause safety problems (without cell-level monitoring). However, proper quality li-ion cells have very small self-discharge rates to begin with, and differences thereof are even smaller.

It is industry standard to put 2 identical high-quality cells (from proper manufacturers) in series, balance them once (i.e., they are both fully charged separately first), and then treat the pack as a single 7.2V cell. This practice is, or has been used by many many large electronic companies such as digital camera / camcorder manufacturers, power tool manufacturers etc. BOSCH goes as far as putting 6 cells in series without tap monitoring or balancing, but usually this practice stops at 2 cells.

I disassembled a used BOSCH power tool 4s battery pack constructed from Samsung INR18650-13Q cells. Battery had increased its internal resistance too much for power tool use, and capacity seemed also smaller. The pack was 5 years old. All four cells were still in very good balance (measured to +/- 0.01V).

Balancing is really unnecessary if the cells are good. But monitoring is highly suggested, or necessary. Monitoring means that you measure every cell voltage separately, and stop charge/discharge according to single cell voltage limits. If you do this, balancing is only an optional extra which only prevents very, very gradual loss of pack capacity, which might never even happen. Balancing is then a "nice-to-have" feature which is relatively easy to implement when you already have access to all cell voltages and can measure them. That's why it's almost always implemented in commercial battery management solutions. It's not completely unnecessary, after all; it can help retain a bit more capacity in some cases.

The really important caveat in both monitoring and balancing is that you don't want to introduce an extra source for leakage to the cells, so that your monitoring circuit would actually unbalance the cells. Be careful when designing.

[mikerj]

Is it common to use multiple cells in series and not use cell balancing?

Yes, very common indeed.  Pretty much every laptop/notebook has three or four LiIon cells in series (usually 3S2P or 4S2P) and no cell balancing is used.

You still need balancing in order not to overcharge cells.

No you don't.  You simply stop charging as soon as any cell reaches the termination voltage (and stop discharging when any cell reaches the lower voltage threshold), which of course means your battery slowly gets unbalanced.  But that's exactly how notebook charging works and you can get through quite a good number of cycles if your cells are well matched to start with.

I don't think so.

I know so.  I have taken apart literally dozens of notebook battery packs and almost all of them failed by having one cell pair wildly out of balance with the rest (e.g. 3 cell pairs at 4.2v, one pair at 3.4v).  Provided the safety fuse hasn't been blown, balancing the cells will restore most of the lost capacity.

It's possible newer packs have balancing built in, but the old ones absolutely 100% do not.

[amyk]

That is more likely to be the outcome of a brief internal short or other defect that lead to one cell suddenly going out of balance, and the balancing circuit can only correct small imbalances.

[Rick Law]

Is it common to use multiple cells in series and not use cell balancing?

Yes, very common indeed.  Pretty much every laptop/notebook has three or four LiIon cells in series (usually 3S2P or 4S2P) and no cell balancing is used.

You still need balancing in order not to overcharge cells.

No you don't.  You simply stop charging as soon as any cell reaches the termination voltage (and stop discharging when any cell reaches the lower voltage threshold), which of course means your battery slowly gets unbalanced.  But that's exactly how notebook charging works and you can get through quite a good number of cycles if your cells are well matched to start with.

I don't think so.

I know so.  I have taken apart literally dozens of notebook battery packs and almost all of them failed by having one cell pair wildly out of balance with the rest (e.g. 3 cell pairs at 4.2v, one pair at 3.4v).  Provided the safety fuse hasn't been blown, balancing the cells will restore most of the lost capacity.

It's possible newer packs have balancing built in, but the old ones absolutely 100% do not.

Help me understand.  Wouldn't the pack go wildly out of balance again in a short time?  The same reason that caused one of the three pairs to discharge more than the other two pairs would be right there still.

[TerraHertz]

Interesting thread. Let's see if I've grasped the main points:
* Monitoring of LiIon series pack individual cell voltages is essential, to allow stopping charge or discharge when the worst cell reaches its limit.
* LiIon cells have extremely low self-discharge when good.
* Allowing a LiIon cell to go outside voltage limits damages the cell, including increasing self-discharge and reduced charge recovery.
* Balancing in LiIon is really only coping with significant cell differences in self-discharge. Which of course accumulate over time regardless of charge/discharge cycles when no balancing is present.
* Poorly designed balancers can make LiIon balance worse, by mis-matched quiescent current.
* Commercial LiIon series battery packs typically omit balancing, and most 'failed' battery packs are due to cells being greatly out of balance. Re-balancing them mostly restores such packs.

Anything wrong or missing there?

I think if designing a battery LiIon management system based on spare functionality of a micro used for other things, I'd still try to include balancing. For one thing it would give you tracking of the adjustments and their effectiveness. And that would give you advance warning of a cell starting to degrade.

[CatalinaWOW]

I will add my personal observations, which apply only to pouch type LiIon batteries used in radio control model airplanes.  This is a demanding application with very high current discharges (typically 20-50 C).  Batteries are often used to a fairly significant discharge with end state cell voltages of 3.9 or even 3.7 volts.  There are wide variations in price which might be reflections of initial build quality, but my observations reflect all points on the price scale.

1. Cells can, and do, become unbalanced.  Usually they will last a few dozen charge cycles without balancing, but this does vary.  A careful rebalancing will then last for more charge cycles, but always less than the first time around, and less each time.

2.  Chargers sold into this market typically do what is called a balancing charge.  This alternates a few seconds of charging with a few second interval where high voltage cells are discharged through a load.  This has the effect of letting higher capacity cells "catch up" with lower capacity cells.  The discharge rate is often only 50 mA, so it cannot cope with a badly out of balance pack of cells.  Balancing devices are available which do this selective discharge process for hours to balance more severely out of whack packs.

3.  Letting the magic smoke out is spectacular, and something to be avoided.  Usually it occurs due to physical damage in a crash.  Once it has happened to me for an undiagnosed reason during charging.  Rather than take any chances I retired the charger.  The event is common enough that a. Products to contain the effects of an unintended fire are widely marketed and used in the RC community.  b.  Several people in the community have lost a home, or garage or other structure due to battery incidents.

Lithium technology batteries are not evils that should be avoided at all costs, but they do require respect.

[akis]

I will add my personal observations, which apply only to pouch type LiIon batteries used in radio control model airplanes.  This is a demanding application with very high current discharges (typically 20-50 C).  Batteries are often used to a fairly significant discharge with end state cell voltages of 3.9 or even 3.7 volts.  There are wide variations in price which might be reflections of initial build quality, but my observations reflect all points on the price scale.

1. Cells can, and do, become unbalanced.  Usually they will last a few dozen charge cycles without balancing, but this does vary.  A careful rebalancing will then last for more charge cycles, but always less than the first time around, and less each time.

2.  Chargers sold into this market typically do what is called a balancing charge.  This alternates a few seconds of charging with a few second interval where high voltage cells are discharged through a load.  This has the effect of letting higher capacity cells "catch up" with lower capacity cells.  The discharge rate is often only 50 mA, so it cannot cope with a badly out of balance pack of cells.  Balancing devices are available which do this selective discharge process for hours to balance more severely out of whack packs.

3.  Letting the magic smoke out is spectacular, and something to be avoided.  Usually it occurs due to physical damage in a crash.  Once it has happened to me for an undiagnosed reason during charging.  Rather than take any chances I retired the charger.  The event is common enough that a. Products to contain the effects of an unintended fire are widely marketed and used in the RC community.  b.  Several people in the community have lost a home, or garage or other structure due to battery incidents.

Lithium technology batteries are not evils that should be avoided at all costs, but they do require respect.

Fully agree with everything you said.

balancing is tricky with all the extra wires you need, unless you have some sort of enclosure with multi contact connectors and no moving parts. Otherwise you have thin wires directly connected to the cells no fuses no protection nothing, recipe for disaster.

[KL27x]

My oldest li ion batteries are 18650 cells that were salvaged out of my personal 1995ish Dell laptop. I dont recall finding any problem with the cells, but it wouldnt power the laptop even for a second. The ESR may have been increased too high for their original purpose. Who knows. But they ran a 3S laptop for a full 45 minutes with wifi on and playing a DVD. Later, 4 of these cells made it into one half of a 4S pack I used to run a cordless drill. I paired these up with another set of 4 matched cells to make the 8 cell battery. This drill was used for maybe 6 years before I eventually upgraded to a real li ion drill. There was a header 9nl the pack for monitoring and balancing, but it never needed it. Then I dismantled that pack to recombine into anew 8 cell 2S pack (2 of each cell in each stage to run my Owen oscilloscope, which is where they currently reside.

These particular cells have never been balanced. And they have never been balanced charged. Plugged into a CC/CV psu set to the sum total voltage, only. And they are still fine. Some of the earliest common knowledge i could find about li ion batteries came from a website called battery university. One nugget said that li ion batteries were doomed for death the day they were made with a typical life of about 3 years. The suggestion was that there was a shelf life, which is now largely known to be false information. Probably patent misinformation (or blind regurgitation of misinformation) intended to promote/prolong consumer acceptance of shortened product life cycle.

The only time I have ever personally seen a battery go out of balance is when overdrawn or when acvidentally incorrectly charged. I would think in a laptop, a cell that is out of balance is already the beginning of the end and that it should be replaced. If you are eking out maximum cell capacity and using near max discharge/C of your cells, then balancing would be very important. You would be staving off issues from the beginning. If you are concerned about l9ngevity, you should preferentially aim for plenty extra C to avoid ESR/decay issues and avoid deep discharge. This would be much more important than cell balancing, IMO. That is my working hypothesis. In an app like RC aircraft, balancing cells should be more important due to the requirements.

[amyk]

Another possibility is that 20 years ago, 18650s were built to much better standards than those today.

Some of the earliest common knowledge i could find about li ion batteries came from a website called battery university. One nugget said that li ion batteries were doomed for death the day they were made with a typical life of about 3 years. The suggestion was that there was a shelf life, which is now largely known to be false information. Probably patent misinformation (or blind regurgitation of misinformation) intended to promote/prolong consumer acceptance of shortened product life cycle.

There's some very interesting discussion on that here, including the quote about shelf life:

https://en.wikipedia.org/wiki/Talk%3ALithium-ion_battery
https://en.wikipedia.org/wiki/Talk:Lithium-ion_battery/Archive_1

...and this, which just leaves me speechless.

Indeed myself and some work colleagues planted one such 'fact' for Buchmann to find and incorporate into his website. Indeed, he did just that, but although we deliberately chose something relatively harmless, an unintended consequence was that at least one major manufacturer built features into a line of products to protect them from the 'disadvantage' that we had conjured up.

Could that be about balancing?

[richcj10]

You really should ballance cells. Circuits ( w/ uP) that do this are really easy to put together.

Sent from my SM-N910T using Tapatalk

[KL27x]

I dunno why people repeat this like a parrot without adding any new information.

Just browsed the wiki for the first time. Noticed it makes the declaration that balancing is not necessary after a battery is balanced. I did not find a source.