Balance Lithium Cells or Not? - Page 1
Forum section: electronics / general

author: Paul Bryson
author_rank: Contributor
timestamp: November 22, 2019, 08:56:19 pm
content: I am designing an application similar to a UPS using a lithium battery pack. The main difference from a UPS is that the load will regularly draw from the battery which is always trickle charging.To get longer life from the battery (10+ years) we were planning to refrain from fully charging the battery. It seems to me that drawing from the battery regularly; but, never fully charging or discharging it, would cause the cells to get out of balance. However, our battery vendor insists that we don't need cell balancing. My fear is that they don't really understand our application and are giving generic advice that applies to a typical application where the battery pack gets charged fully.The battery pack is a series stack of 3 pairs of 18650 cells. Not sure which lithium chemistry we will be using yet.The load will pull large currents for short periods of time with long periods in between.My question is do we need to balance the cells? And if not - what is the mechanism by which the cells stay in balance on their own?

author: thm_w
author_rank: Super Contributor
timestamp: November 22, 2019, 11:50:29 pm
content: Generally the experienced recommendation here is balancing is not needed, unless you are dealing with very large loads, temperature differentials, or have unmatched cells.You can find a lot of insightful posts from Siwastaja for example:https://www.eevblog.com/forum/projects/lithium-ion-cell-balancing/However, we are talking about 10+ year life then I feel this is a totally different story, and the few cents or dollars for monitoring/balancing circuitry seems worth it to me. As this sounds like a high end product.Chemistry I can't think of what you'd use other than LiFePO?

author: Paul Bryson
author_rank: Contributor
timestamp: November 23, 2019, 08:50:49 pm
content: With lots of Googling I haven't come across an explanation of why a series stack of lithium cells does not require explicit balancing - but, I think I have figured it out. Correct me if you think I've got it wrong.During constant current (CC) charging the voltage rises quickly until it hits a knee between 3.5V and 3.9V. This rapid change in the slope of the voltage curve helps keep all of the cells close in voltage. I.e. the first cells to reach this voltage slow down and the stragglers catch up. There is also a knee in the "charge capacity" curve. That is, during the constant voltage (CV) section of the charge cycle the rate of charge completion slows down and again allows the stragglers to catch up.So as long the cells are more or less balanced to start, and are CC charged to a voltage above the knee of the voltage curve and then are CV charged to a point above the knee of the charge capacity curve - they should stay more or less balanced.Speculation here: It is also possible that if a cell does get charged to a higher voltage, this may cause an increase in the self-discharge rate which would tend to balance the cells over time.Edit:I should also note that it is necessary to discharge the cells below the knee(s) before recharging to above the knee(s) to maintain a balance.2nd Edit:I see now that the knee of the voltage curve is below 10% "charge capacity" and probably will never occur.Also the knee in the "charge capacity" curve appears to be the inverse of the "current" curve and not at a function of time - so irrelevant.

author: f4eru
author_rank: Super Contributor
timestamp: November 23, 2019, 11:19:55 pm
content: no balancing -> cells drift appart over a few tens of cycles -> quick destruction of the cellsIn old Pb batteries, the balancing is done by very slightly overcharging the cells, who act as zeners, balancing the string out.In Lithium batteries, this does not work.

author: Rerouter
author_rank: Super Contributor
timestamp: November 23, 2019, 11:34:03 pm
content: It is ideal to have at least mitigations, The issues come about when the ESR or capacity is not quite matchedIf a cell has a lower capacity (mAh) it will charge or discharge faster than any of the other cells,If a cell is more worn out than other cells, it's ESR increases, which means it charges effectively slower than other cells as some of that energy is lost as heat.This does maintain a balance to a certain point, but over many cycles it will still become unbalanced, So i would recommend some simple type of balancing circuit, e.g. every few cycles, something measures each cell, if the difference is bigger than X, then balance the problem cells, This should help get your longer life time, the other thing that can help is keeping the load draws much lower than the C rating,

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 23, 2019, 11:41:28 pm
content: You should be able to model your load profile and battery characteristics to determine the likely imbalance over life. You don't mention if you are operating the cells in parallel or series. It makes a huge difference in whether you need to balance. The self balancing characteristic mentioned earlier applies to the parallel configuration. I am not as sure whether it applies to the series situation, but my experience is that something causes them to get unbalanced in that configuration. Perhaps it is differences in self discharge or venting of electrolyte from the cells which are being overcharged as the lesser cells catch up.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 24, 2019, 08:43:39 am
content: Paul, your explanation is incorrect. Unfortunately, there is no self-balancing action in li-ion series stack - or if there is, it's very, very weak. If you put cells with different charge level together and charge them in series, some of them will be full first, overcharge, and get damaged.f4eru's explanation of cells "drifting apart" in tens of cycles is naturally total bullshit. Manufacturers like BOSCH have no issue building series packs without balancing that last for hundreds or thousands cycles just OK. Especially constructing two cells in series for a 7.2V pack without balancing has been industry standard.What makes unbalanced packs possible, is the remarkable Coulombic efficiency (around 99.96%) of li-ion cells, remarkably small leakage (self-discharge) current, and, what finally defines the suitability for non-balanced packs, thedifferencesof these parameters between the series cells.Example on coulombic efficiency, if you charge 2000mAh in a series pack of two cells, cell #1 having coulombic efficiency of 99.970% and cell #2 99.975%, the first one loses 0.6mAh, the second loses 0.5mAh, and thus, imbalance of 0.1mAh is generated. Repeat for 1000 cycles, and 100mAh imbalance is generated.Example on self-discharge, a typical cell loses around 5% of it's capacity per year. Say a 2000mAh cell #1 loses 500mAh during 5 year lifetime. Cell #2 may self-discharge at, say, 5.5%, and lose 550mAh during the same time. 50mAh of imbalance is generated.Balancing is good to have, but the industry has shown it's not mandatory when the packs are physically small, thermally coupled i.e., sit at the same temperature, have low number of cells in series (so that individual voltages still have significant effect on the pack voltage), and the cells are of proper quality.Note that when introducing balancing, you are introducing a circuit that, if not working perfectly at all times, have high chances of actually imbalancing the cells.The self-discharge (aka leakage) goes quickly up when the voltage goes up, so this provides a self-balancing action, but it is so weak and unreliable that I wouldn't count on it.

author: Alfons
author_rank: Regular Contributor
timestamp: November 24, 2019, 09:04:52 am
content: I can only talk about my experiences. I use four 36V Bosch battery packs with about 4800mAh, all without balancer. These are now about 3 years old and since I use them not only in the tools, but also in an e-bike, relatively heavily loaded. There are Sony Konions installed. So far, there have been no problems, the capacity and power delivery is still OK. I think, they will slow down in 2-3 years, but a balancer would not change that either. I think if the batteries fit together well, there are no problems without Balancer.

author: Cerebus
author_rank: Super Contributor
timestamp: November 24, 2019, 10:54:07 am
content: Quote from: CatalinaWOW on November 23, 2019, 11:41:28 pmHe did actually:Quote from: Paul Bryson on November 22, 2019, 08:56:19 pmBut perhaps it might have been slightly more explicit. I read that to mean a series pack of 3 stages, each of 2 cells in parallel.

author: Paul Bryson
author_rank: Contributor
timestamp: November 25, 2019, 12:20:35 am
content: Your arguments make sense, as far they they go. I am mostly persuaded. However, it is hard for me to feel confident in a design which depends upon the near electrical perfection of a can of chemicals.a. "Coulombic efficiency" is not listed in the cell's datasheet.b. My application is very different from most; which makes me hesitant to accept "common practice" as a solution.c. I can't run a 10 year test to see what happens; I don't really even have time to run realistic full cycle tests.Perhaps I am being overly cautious - I will consider what you have written.

author: jetsam
author_rank: Newbie
timestamp: November 25, 2019, 01:17:45 pm
content: You would lose some efficiency, but is it crazy to put all the cells in parallel and then boost convert the voltage to the needed level later on? I see several advantages to doing things this way, but there must be something wrong with the idea... I've not seen it in use much "in the wild."It isn't all that inefficient depending on... efficiency things. Maybe 85% efficient, most boost converters, I think? No balancing, though, means some savings in complexity and even some energy. Most balancing schemes just blow the extra energy they balance away by pushing it through resistors and into heat.

author: thm_w
author_rank: Super Contributor
timestamp: November 25, 2019, 10:40:45 pm
content: Quote from: Paul Bryson on November 25, 2019, 12:20:35 amI don't think he explicitly said it would be a good idea for your scenario.For example his point: "Repeat for 1000 cycles, and 100mAh imbalance is generated."If you are running the cells for 10 years then you might expect several thousand cycles, so imbalance could be 100's of mAh. It would be advisable to have over voltage and under voltage protection, at the very least in this scenario, if you care about high reliability. Perhaps a simple form of over-voltage clamping. You could also set a low clamp voltage, and prevent the cell from exceeding ~85% charge which will increase the life significantly.Its not yet clear if this is a consumer grade or industrial product and what budget there is.Quote from: jetsam on November 25, 2019, 01:17:45 pmThis works but OP mentioned pulling high currents which may either mean a very large boost converter is required or maybe just big enough capacitance.

author: Siwastaja
author_rank: Super Contributor
timestamp: November 26, 2019, 07:46:06 am
content: Also note the difference between cell-level monitoring and balancing.Monitoring is a safety feature, meaning that the charging is prevented when any of the cells hit the full state of charge, and discharging is prevented when any of the cells hit the empty state of charge. The key is taking action. The action is system-level, not cell-level.Balancing is always optional, and only to maximize the energy storage capability of packs near their end-of-life. It has absolutely nothing to do with safety, ever.Balancing, in practice, cannot replace monitoring, but works as an extra "convenience" or value-add feature in a system which already has monitoring.So whenever we talk about "balancing", we kind of assume it comes with the monitoring by default. But this assumption has been shown to be wrong many times, when inexperienced people start designing battery management systems.The key point to ask yourself, is what happens when, say, cell #3 hits 4.21V while others are at, say, 4.0V? You must be pretty damn sure your charger enable signaling works. And then, you need to ask, what happens when cell #57 hits 2.5V while others are at, say, 3.5V? What if you have multiple loads? Are you completely sure all of them are disabled?In practice, adding balancing once you have monitoring is trivial enough that it's typically worth doing. A typical beginner mistake is to go for excessive balancing currents, complicating the design and possibly generating thermal risks.

author: f4eru
author_rank: Super Contributor
timestamp: November 28, 2019, 08:32:56 pm
content: QuoteNot really, I meant of course that the SOC drift appart.The SOC is, put simply, the integration of of the current, into a conainer and, as with every integral, small errors (size of the container, leakage, spillage, etc...) will make two of them that seem identical simply drift appart over time due to minor differences.And once the SOCs have big differences (imbalance), you need either to slash useable capacity, or to rebalance the SOCs.

author: implor
author_rank: Regular Contributor
timestamp: November 29, 2019, 10:19:28 am
content: I have a similar issue.In my design I need to use two 18650 lithium cells connected in series.because of mechanical constraints I can't use a pack of 2s.So I have found BQ25887 from TI. All other IC I have checked do not have built in cell balancing. Do everyone else expect you to use an external IC for cell balancing? Feels weird not having a "charging feature" in the charge.Planing to add a Fuel gauge also to track charge level.How would you solve the problem with 2 unmatched single cells? Am I missing some obvious solution or is this just an unusual setup?FYI expected product lifetime ~4-5 years. Not a consumer product.

author: cgroen
author_rank: Unknown
timestamp: November 29, 2019, 12:54:50 pm
content: Quote from: implor on November 29, 2019, 10:19:28 amHave a look at Maxim MAX1720x devices, balancer and fuelgauge in one device. Works wonderful here!https://www.eevblog.com/forum/projects/using-lifepo4-batteries-on-maxim-max1720x/msg2794628/#msg2794628

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 29, 2019, 01:35:15 pm
content: Quote from: implor on November 29, 2019, 10:19:28 amYou can use a low quiescent charge pump chip to balance the voltages. If minimizing self discharge is important, you can only enable the charge pump while charging.

author: splin
author_rank: Frequent Contributor
timestamp: November 30, 2019, 07:25:38 am
content: The elephant in the room is how well matched the cells are. Bosch, Tesla etc have a great deal of control over the manufacturing process but you? You can reasonably ensure that all the cells in the battery pack are made by the same manfacturer and have the same part number.But can you guarantee they are all from the same batch? Were they all made on the same machine? I very much doubt that you can, so you need statistics on the variance of the relevant parameters (especially self discharge). I very much doubt you will get any supplier to give you guaranteed numbers that would allow you to qualify your batteries for 10 years - that is a is a long time to expect the cells to remain in balance. It would then be up to you to characterise cell matching and control your production appropriately which I would expect to be very difficult.I'd be astonished if you can avoid providing cell monitoring at the least, let alone balancing unless you can very carefully select and match the cells in the battery which would be atypical of normal manufacturing processes.This might be helpful:https://www.nasa.gov/sites/default/files/atoms/files/cap_bal_dur_li-ion_cell_life_testing_sklaus.pdfNote BOL probably means 'Beginning of Life'

author: nctnico
author_rank: Super Contributor
timestamp: November 30, 2019, 11:20:45 am
content: Quote from: splin on November 30, 2019, 07:25:38 amThis is a point of concern indeed. The battery pack manufacturer I know tests each individual cell to tighter specifications to make sure there are no lesser cells in the pack. They use the rejected cells for welding quality control. And these guys buy in bulk from A-brand cell manufacturers.IMHO it is wise to have a balancer inside a Li-ion pack. The discharge current doesn't need to be high because most things in a battery pack are slow processes. Something else to consider is that balancing can only be done when the pack is nearly discharged or nearly fully charged because the delta v/delta charge is the largest in these conditions.

author: implor
author_rank: Regular Contributor
timestamp: December 02, 2019, 06:53:09 am
content: Quote from: cgroen on November 29, 2019, 12:54:50 pmThanks for the suggestion. I'm leaning towards bq28z610 right now. Fuel Gauge, balancer andprotectorin one.

author: cgroen
author_rank: Unknown
timestamp: December 02, 2019, 06:57:03 am
content: Quote from: implor on December 02, 2019, 06:53:09 amAh yes, thats a nice candidate!In my case I needed support for 2S LiFePO4, 3S LiFePO4 and 2S LiIon in 3 different products, I wanted to keep the base design the same hence the Maxim device (it can be programmed to other chemistrys than LiIon etc)And I did not want a protecting circuit, going to draw 30 to 50 amps from the pack in peaks