Bipolar power supply from serial lithium-ion batteries - balancing? - Page 1
Forum section: electronics / general

author: cape zoloh
author_rank: Contributor
timestamp: July 07, 2021, 06:01:17 pm
content: I would like to build a +/- 5v power supply from four cells (2S/rail).Should I care about these balancing circuits, if so, do I need two 2S boards or one 4S?I'm looking for assembled ready to go boards on amazon, as these chips seem pretty complex and I rather spend time in other areas.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 08, 2021, 02:14:06 am
content: A so called "active balancer board" will do what you want for possibly unbalanced discharge.For the protection, the only way I can think of to make it work and coexist with the balancer would be individual 1S BMS modules on each parallel group, with the balancer downstream of the BMS modules.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 08, 2021, 08:38:58 am
content: It's a colossally bad idea, just don't tap anything between the cells. Basically all BMS and balancer boards on the market are designed and tested to account for the small differences in self-discharge of the cells and almost always only operate at some operating point (like at 4.2V), they can not correct the massive imbalance you create by tapping into the pack.Additionally, each cell will need monitoring and functional undervoltage and overvoltage cutoffs that turns the thing off. Then balancing is only there to make sure you have near full capacity available and don't slowly lose the functionality so that one cell undervolt cutoffs too soon after another cell cut off due to overvoltage.It can be done but given a product designed for exactly what you are doing, or one designed by you.

author: cape zoloh
author_rank: Contributor
timestamp: July 08, 2021, 04:05:50 pm
content: Siwastaja Kitos, but you make it sound really complicated. Looking all the portable products using lithium cells today, I can't imagine it would be that hard. Maybe I'm wrong. There isn't too much info on this specific subject, too bad..I also can't understand why balancing would be necessary when having them in series, NOT parallel.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 08, 2021, 04:17:27 pm
content: It's complex because it's niche use so there is no simple ready-to-use solution at least I know of, that's simply because no one does that, and that's because other ways of dealing with the actual problem are better, i.e., simpler and lower cost.In other words, none of the portable products using lithium ion cells do +/-5V rails by centertapping into the li-ion pack.Most of the ubiquitous devices you think about are designed to get away completely without negative voltage rails. For example, by AC coupling inputs, using "virtual grounds", etc. For example, so that for any analog signal, 2.5V corresponds for zero, values below it are considered "negative" and values over it "positive".When negative rails are truly required, they are often such low current ones that cheap charge pumps are used, cost is a few cents in masses. If higher currents are needed, inverting buck-boost allows building the negative rail for maybe $0.50.Besides, single cell is easier and cheaper to manage than multiple in series so multiple cells in series are mostly only used when higher voltage is needed to keep current manageable, i.e., in power tools. If you need voltages in excess of some 3.3V, a very typical solution is aboost converterrunning off a single cell. So now all that's left is to either "copy-paste" the boost to provide another inverting buck-boost, design in a small transformer instead of single boost to directly give your bipolar supply, or many other simple and low-cost solutions. I'm sure you can prototype this solution with Ebay modules; I'm pretty sure you can get both boost and inverting buck-boost modules easily, just run them off a so-called "protected cell" which has the small PCB with undervoltage cutoff and overvoltage protection.This is a classicX-Y problem. It would help tremendously if you told us what you are actually building. I'm 99% sure the optimal, smallest and simplest solution does not involve tapping into the series pack center tap.In series pack, the same current runs through all cells, so everybody's voltage rise/fall together. Hence, you typically need to monitor each cell voltage to guarantee none of them go beyond legal values (say between 3.0V to 4.2V). Series cells require balancing in order to maximize energy output. Otherwise, if one cell is at 90% and another at 10%, you can only charge 10% until the 90% cell has become 100% forcing you to stop charging. At that point, the 10% cell has become 20%. You can now only discharge 20% until it's empty, again forcing you to stop discharging. If you apply different load current to different cells, you end up in a situation where some cells are empty, some full and the thing just stops working. Unless you have a balancer which has suitable algorithm for this use case, can balance at any voltage and has enough balancing "beef" to do its job. This is possible but an expensive and complex solution for such simple and commonplace problem (I guess, we don't know about the problem yet).

author: cape zoloh
author_rank: Contributor
timestamp: July 08, 2021, 05:26:08 pm
content: I did some reading and I'm getting an understanding of the issue, little by little.I can't have anything switching circuits (noise). Its a very low noise bipolar power supply for a very high gain/low noise preamp where no switching noises is allowed.I think the best solution is to just use two batteries in series, only in circuit when the power is on.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: July 08, 2021, 11:59:21 pm
content: If you use a commercial battery charger it will do the balancing when you charge. You just need to make sure you don't run the voltage too low when you discharge.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 09, 2021, 10:29:31 am
content: Quote from: ahbushnell on July 08, 2021, 11:59:21 pmPossibly yes, possibly no. Robert Bosch manufactures/manufactured commercial li-ion packs and chargers that do not have balancing or even cell-level monitoring. There is no accessible tap even, but if you open the pack and access the cells you ruin the factory balance they trust (contrary to all internet "experts"!), rendering it dangerous to charge or discharge.In any case, even if it does balance, a commercial battery charger is not designed to correct large imbalances caused by unbalanced external circuitry. Balancing current itself can be as much as some 100mA but it may be limited to do actual work for some seconds only, this happens whenever CV stopping current > balancing current: unless the charger has logic to modulate the charging on/off, it can only bleed charge off from the top cell(s) for some seconds, maybe a minute if you are lucky. This is enough for the intended task, i.e., to correct for accumulated microampere level cell leakage differences. Tapping into the battery is considered battery abuse and not designed around. Yes, with cell-level UV and OV cutoffs it's allsafe, but you can slowly lose all usable capacity.You may get away with it if the actual circuit is designed to have balanced currents between the sides but this may prove almost impossible as the loads are ICs like opamps, not just resistors, so that current consumption from the rails is different but also varies differently with respect to inputs and environmental parameters.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 09, 2021, 10:37:06 am
content: Easiest solution might be to use "protected" loose 18650 cells in holders so you can run the center-tapped thing, after use remove the cells from holders and charge them all separately in dedicated chargers (or a charger with multiple channels). Then they are always balanced at top when removed from charger(s) fully charged.This is simple and all OK as long as the cells and their protection boards are of proper quality and designed to be inserted into holders repeatedly, and their UV/OV cutoffs work as they should.Note though you can't safely connect arbitrary number of "protected" cells in series, but I think 4s should be OK, just don't go any further with this unless the protection boards are specified to handle the increased voltage.

author: cape zoloh
author_rank: Contributor
timestamp: July 13, 2021, 10:27:19 pm
content: The plan was always to recharge them separately, but I did not mentioned this as I had way too little knowledge before I asked the question.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 14, 2021, 10:09:33 pm
content: Quote from: cape zoloh on July 13, 2021, 10:27:19 pmOK, then all that's left it to cut discharging whenanyof the cells goes below undervolt threshold (say e.g. 3.0V). "Protected" cells do this for you.