Li-ion batteries - cell balancing - Page 1
Forum section: electronics / projects

author: danmcb
author_rank: Regular Contributor
timestamp: April 08, 2017, 11:10:26 am
content: There seems to be differing information as to how necessary balancing is, at least in lower power applications. Battery University, for instance, suggest that it is not needed at <24V, provided cells are well matched and of good quality:http://batteryuniversity.com/learn/article/bu_803a_cell_mismatch_balancingTo quote : "A battery expert once said: “I have not seen a cell balancing circuit that works.” For multi-cell packs, he suggested using quality Li-ion cells that have been factory-sorted on capacity and voltage. This works well for Li-ion packs up to 24V; packs above 24V should have balancing."As usual, I prefer having direct measurements to back up such assertions. I have here a prototype pack I purchased, made of 5 Panasonic 3400mAh Li-ions - the individual cells are not wired out, so it must be bulk charged. I am about to set up a cycle test, logging voltage and temperature, to see how this pack handles repeated bulk charging, without balancing.Interested to know if others here have specific knowledge or experience on this subject.

author: anishkgt
author_rank: Frequent Contributor
timestamp: April 08, 2017, 11:46:19 am
content: I have done several packs. The logic behind cell balancing to extend the life of each cell. So that each cell is used to its full potential at all times that is when charging and especially when being discharged. So what happens when not using a BMS ? One cell would be discharged or charged more than the other when this happens the BMS would cut it off when that cell or pack of cells have reached it max charge or discharge voltage.Hope this helps.Sent from my iPhone using Tapatalk

author: digsys
author_rank: Unknown
timestamp: April 08, 2017, 12:17:22 pm
content: Quote from: danmcbHaving spent years designing / testing / destroying LiIon packs for Solar race cars - these are my opinions. refer :www.pbase.com/digsys/image/161061565gallery- I NEVER ever put active balancing on ANY LiIon pack. As mentioned, and in my experience, if a pack is well matched at the start, it can go several months plus beforeit requires re-balancing (depending on severity of charge / discharge). I have seen many fires, or results of, when an active circuit has failed. We also ship our cars onplanes, and I have NO intention of fronting a coroner. :-)What IS important, is an accurate reporting BMS, as per pix. Obviously, doesn't need to be as complex. The BMS needs to keep track of cell voltages to 1mV if possible,run an average, median and mean - at at least 3 load groups, L M H, and inform "someone" when a pack drifts past preset limits. A couple temp sensors and asimple current sensor is also very desirable. It can be that simple.I have a pre-wired connector for individual balancing when required. Just plug it in, and use the data from the monitor BMS.And I do agree with - "rarely seen a balancing system that works".

author: danmcb
author_rank: Regular Contributor
timestamp: April 08, 2017, 12:31:46 pm
content: Quote from: digsys on April 08, 2017, 12:17:22 pmThank you digsys. So, to check I understand you - you are saying that access to individual cells is essential, even though you don't (always) balance charge? and just log what the cells are doing over time, then manually balance when needed?

author: digsys
author_rank: Unknown
timestamp: April 08, 2017, 01:16:18 pm
content: Quote from: danmcbCorrect. Actually, if you do it right, the monitor BMS "talks" directly to the charge modules, which can be as simple as a bi-dir DC-DC switcher. Can even be fixed current.You can use a 6V or 12V SLA or similar to be the transfer storage, as some cells will need to drain a bit and some will need extra charge. A 12V storage gives a niceefficient I/O Voltage ratio. In early days, we just used 5V 1-5A plugpacks (wallwarts) :-) switched them in/out.

author: danmcb
author_rank: Regular Contributor
timestamp: April 08, 2017, 02:55:00 pm
content: OK, thank you. But this is for a vehicle application, and a pretty heavy one by the sound of it.Do you think the requirements for a 5 cell pack, probably discharging at max of about 1A (often less) really has to have that much complexity?What do you mean by "transfer storage"? I guess that in your application, you try not to waste the extra charge in cells that need to drain. but that wouldn't be needed in a simple piece of portable equipment that recharges from the wall.The main thing I have to figure out is whether the extra complexity (=> cost) of individual cell monitoring is worth it, for reasons of, either (a) extending cell life or (b) safety. The company who make this prototype pack up for me didn't seem to think bulk charge would be a problem. Of course, I will have to see how my tests work out, in the last analysis ...

author: Audioguru
author_rank: Super Contributor
timestamp: April 08, 2017, 03:24:51 pm
content: All the Li-PO batteries for my radio controlled model airplanes have only 2 cells but have a connector with 3 wires and chargers that provide balanced-charging. When the batteries get old then their capacity normally drops.

author: tatus1969
author_rank: Super Contributor
timestamp: April 08, 2017, 04:00:37 pm
content: Quote from: danmcb on April 08, 2017, 02:55:00 pmYou need to protect each cell of a LiPo or LiIon pack against over charging, over discharging, and overtemperature. Over/undervoltage leads to permanent cell damage and can result in explosion and fire, although in my experience there is usually a delay inbetween. A damaged cell can even catch fire during storage (I know this for LiPo, not sure for LiIon). Overtemperature will eventually lead to separator failure, internally short-circuiting the cell and, again, bang.Even when the cells are perfectly selected when manufacturing the pack, they a) will likely not reach their end of life at the same time, and b) can have sporadic defects that are not detected during production.If you want to balance them or not could be decided based on cost: does the extended battery lifetime justify the extra cost?BMS is a must in my opinion, in order to be able to control the inherent danger of this type of energy storage.

author: Kilrah
author_rank: Unknown
timestamp: April 08, 2017, 04:21:16 pm
content: A BMS is important in any application where you don't otherwise check cell voltages regularly anyway as in with R/C batteries for example. But it is mainly for checking safe operation limits, balancing itself is not very important indeed. A full and balanced new pack will stay balanced at the top end pretty much forever unless once cell goes VERY bad.Cells that lose some capacity will be empty earlier (the BMS will catch that) but once you recharge they'll be matched again. It takes either a strong increase of cell internal resistance or self-discharge rate to actually cause a lasting imbalance, and when you get to this point (it happens, cells of hobby-level batteries will rarely age in sync and that's if they're well matched initially in the first place) balancing won't help much anyway, time to throw it out or replace the cell.That said while I usually use LiPos for RC and thus with balance plug I've recently built several packs that are meant to be "plug and forget" for my electric kick scooter and have gone with BMS with balancing. Only a couple of $ more, and if anything if I feel the balancing resistors get warm for more than a few minutes after charge I know I should look more closely.

author: jbb
author_rank: Super Contributor
timestamp: April 08, 2017, 09:29:43 pm
content: Quote from: danmcb on April 08, 2017, 02:55:00 pmAt risk of sounding a bit academic, the complexity of the battery protection is due to the complexity of the battery configuration. The power of any balancing system is due to the energy level of the battery.One potential approach is to parallel the cells (Caution! Not trivial!) and use a boost converter to get the voltage you need. This simplifies the battery management at the expense of main circuit complexity and losses.Quote from: digsys on April 08, 2017, 12:17:22 pmYou'll have to evaluate whether or not this is practical; connectors are often the east reliable part of the system. It might be that some basic light-duty resistor balancing (support built in to many battery monitor chips) is safer than exposing more connectors to the outside world.

author: digsys
author_rank: Unknown
timestamp: April 08, 2017, 11:03:36 pm
content: Quote from: danmcbNot at all. Just something that "keeps an eye" on cell voltage variations, and temperatures is perfectly adequate. Actually, the biggest killer is over-charge !Even say 1% over-capacity can have a dramatic effect on life.Quote from: danmcbYes again. It's a "deluxe" version, where you can leave it all unattended, or if you have to do it a lot. For a few cells, and you're nearby, just plug packs in / out.Quote from: danmcbI'm really just providing some ideas. With opinions from others as well, you should have enough to decide what is best in your case. It can be very simple and effective.

author: digsys
author_rank: Unknown
timestamp: April 08, 2017, 11:22:13 pm
content: Quote from: jbbI'd have to disagree with you on this point. There are plenty of extremely reliable connectors for DC power. For these small packs, they abound. IMO, being able to justgo up and plug in a balancing system (or having a break-out point), is a lot safer than messing around with trying to manually connect up to various points.Nearly ALL those cell balancing ICs, top balance only, ie discharge cells that are too high, usually at the higher end of the capacity. We get dramatically better life stabilityand longevity, by balancing at the bottom of the curve ! down past the knee ~ 2.0V+. The characteristics here tell you a lot more about the state of the cells.Up the top, effects like surface charge etc make it very difficult to "rate" the cell. It is also why you should never check voltages without a load !! Even ~1/25C is fine.

author: NW27
author_rank: Regular Contributor
timestamp: April 08, 2017, 11:42:30 pm
content: Quote from: digsys on April 08, 2017, 11:22:13 pmHiI'm very interested in building a battery management system for four serially connected cells and will have a serial interface to an external micro.I would appreciate any further info you could provide in regard to your above bottom cell balancing techniques.Neil.Sent from my SM-N920I using Tapatalk

author: danmcb
author_rank: Regular Contributor
timestamp: April 09, 2017, 02:59:00 am
content: Right - thanks for the advice and clarification. The pack DOES have a protection board against overvoltage and over current built in - that is mandatory I know. I am now making a rig to run charge/discharge cycles with voltage/current/temperature logging, and I will see how the pack holds up. I'll make a decision based on the results of that - maybe with one or two few more packs as well.Again thanks all for the info and suggestions. Always good to be able to bounce ideas around.