Looking for 5 cell LiIon balancer IC, microcontroller-less operation - Page 1
Forum section: electronics / projects

author: Yansi
author_rank: Super Contributor
timestamp: December 01, 2016, 08:11:43 pm
content: Hi!Currently trying to solve reasonably, how to balance a 5 cell LiIon pack. The pack is charged using a programmable CC/CV source.The requirement is, that the pack will be self contained, only two wires poking out. It has to handle the cell balancing itself.The original idea was to use a simple set of jellybean low power comparators/references for each cell (like a low power TL431 like reference as a comparator with some small hysteresis set), with preset thresholds to turn ON or OFF the discharge resistor. No programmable circuits inside the battery, assuming the charging source will be programed to charge with low current only, when certain voltage is reached across the pack terminals, so the charging current will be small enough to be handled by the discharge resistors inside the pack.Do you know any better solution to it, like some microcontroller-less BMS IC for 5 cells, that would handle the cell balancing? Can you recommend an IC you have experience with?Overdischarge protection or overcharge protection might be a good bonus, but is not currently required as itshouldbe handled by an external circuitry.Thank you for tips and hints!Yansi

author: Yansi
author_rank: Super Contributor
timestamp: December 01, 2016, 09:35:11 pm
content: Update: This seems like the ideal IC I need. But I can't find it nowhere to buyhttp://www.e-devices.ricoh.co.jp/en/products/product_power/bmu/r5436/r5436-ea.pdf

author: Yansi
author_rank: Super Contributor
timestamp: December 01, 2016, 11:16:53 pm
content: So maybe I will drop my no-software requiremnt from the design. As it seems, there is no good enough solution to it:Building something using discretes yields much higher consumption, than any integrated solution can provide. Also quite complex circuits with not enough precision.Using off the shelf ICs isn't going to cut it either - there seems to be nothing good enough (or available enough).So fu*ck it, I might just stick with these two: bq76925 (SOP20) and STM32L011F3 (SOP14) small package devices.http://www.ti.com/lit/ds/symlink/bq76925.pdfhttp://www.mouser.com/ds/2/389/stm32l011d4-956266.pdfWill be cheap enough to run a 100 pcs series with that. I think there is no cheaper or better solution to itIf you know about one, please let me know!Still I am a bit concerned about the software reliability. It is a quite bit of responsibility, as the device will be used in aerospace

author: digsys
author_rank: Unknown
timestamp: December 02, 2016, 02:01:54 am
content: One thing I've learned from designing solar / electric vehicle LiIon battery packs is - NEVER put active cell balancing / bypass elements fixed in the pack !!Especially if it is going on an airoplane. If the pack is balanced well at the start, even in our race cars, there is very little drift over long periods.Naturally, have an accurate BMS and an easy way to interrogate it / display any problems, which can then be sorted out at a service interval.If there is a major cell failure, the cell balancer will do bugger all anyyway, andmayadd to the fault.This is a design I make freely available for cell monitoring, you can use parts as required.www.pbase.com/digsys/image/161061592For balancing, there several designs that use a gang of bi-directional DC-DC converters, usually with a 12V battery as the storage inter-mediate. The BMS then instructswhich cell will move energey in / out. Like I say, the actuall balancing should'nt need to be done very often, maybe once a year. IF it requires more, then you've donesomething wrong in the design.

author: Yansi
author_rank: Super Contributor
timestamp: December 02, 2016, 07:03:05 pm
content: Why not to put BMS fixed into a pack? It is the most reasonable solution for any multipack system, sure.This is only to be a single pack though. Has to be compatible with previous NiCd pack, there can be no additional electronics to it - have to be completely self managed only two wires poking out.QuoteThat really is not true at all, at least in our case. Might be true for barely loaded small pack, certainly not for a race car one. The battery pack had to be balanced after say each 10 discharge cycles. Do not remember the voltage imbalance after that. (Note - this pack is forced air cooled, up to about 60kW power being drawn from it peak, about 15kW on average)I am not new to BMS design. Have already done few, including some for a race car. From my experience, I find your BMS to be rather complicated or expensive, so to speak, due to use of the multiple LT1990. Not a great (or at least cheap) idea I think. Dunno how old is your design, but I have designed this right after Linear has come with their new BMS chips. It is a BMS for 12cell in series, self powered from the pack, including temperature measurements from several (44 to be exact) probes. Single twisted pair daisy-chain communication through signal transformers. It is only a BMS slave, so a maser control unit has to be present for the whole chain.Currently I have one another BMS design ongoing at my work, that will also be using the same LTC chips, but for a smaller pack, like about 2kWh. That system on the photo was for about 7kWh 260V nominal LiIon battery pack (72s11p if I am not mistaken).

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 02, 2016, 07:17:12 pm
content: Have you seen the ISL94203? While it can be attached to an MCU, it doesn't have to be, it fully supports stand-alone operation.http://www.intersil.com/content/dam/Intersil/documents/isl9/isl94203.pdf

author: digsys
author_rank: Unknown
timestamp: December 02, 2016, 10:17:52 pm
content: Quote from: YansiThe BM(monitoring)S does live in the pack and the data is accessible via various methods, including wireless. I NEVER put active elements in a pack.Quote from: YansiFair enough, a tough ask.Quote from: YansiIF I run our race packs at extremes, that it quite true. I also do EV packs, where I keep everything below 1/4C (equivalent to 400Km range), normal driving.In thatnormalworld, they rarely require active balancing, and we simply make it a "service stop". Teslas greatest benefit in having huge range, isn't justthe range in itself. It means the pack is always working around 1/5C or less. The cells, for those conditions can expect a full life.Quote from: YansiI appreciate that, and same here.Quote from: YansiStrange comment. BOM costs are very cheap, and in the race car arena ....some teams spend in the $millions. There is nothing expensive / proprietary or unusual.Quote from: YansiIt is my current design, and I have found no problems with it. I've tried ALL the BMS all-in-one etc chips. I wouldn't be a good R+D engineer if I didn't. MEH with issues.Quote from: YansiI wish you well, with no malice at all. Any progress is good for EVs / Solar EVs. We can simply agree to disagree :-)Sorry I couldn't help at all.