Balancing current for 2 cell Li Ion battery(Need help please) - Page 1
Forum section: electronics / projects

author: Sai teja
author_rank: Regular Contributor
timestamp: September 07, 2017, 04:24:30 am
content: Hello,I am designing a charger circuit for 2 cell 7.4 V Li Po battery. I am using MCP73213 charging IC for this purpose. Unfortunately this doesn't provide balance charging and also it does not provide overcharging and over discharging protection. I would like to know amount of balance current required for balance charging.And also is it mandatory to balance charging ? Please through light on this. Any help would be greatly appreciated.

author: jbb
author_rank: Super Contributor
timestamp: September 07, 2017, 06:26:09 am
content: Ah, you're venturing into troubled waters here.You will definitely need the following:Overcurrent protectionOver-voltage protection for each cellUnder-voltage protection (while this should technically be per-cell, you might be OK with just per-pack...)Maybe someone knows a 2-cell protection chip? I'm sure there are a few around.Your charger can use the standard LiPo charging method (first constant Ichg, then constant Vchg, stop when I falls to < Ichg/10), but you might have balance problems. These would show up where 1 cell is fully charged (and the over-voltage protection goes off) while the other cell isn't full yet. If you need a balancer, you'll might only need a little bit of balancing current (like < 0.05C) to keep things in line (once cells are balanced they only need a little nudge to keep them that way).It might be easier overall to use a single LiPo and appropriate boost or buck-boost converters to get the voltages you need.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 08, 2017, 06:27:07 pm
content: Quote from: jbb on September 07, 2017, 06:26:09 amFunny that big industry players such as BOSCH, Fuji, etc. do not agree with this "definite" need for such things.Almost the whole industry agrees that properly constructed 2s does not require cell-level anything: 2-cell cell level ICs are rare for this reason. BOSCH goes as far as 6s in their power tools.But forum posters always seem to "know" what is "definitely" required, without experience on said subject.So, to OP, balancing is most likely not required, and it's very probable that any balancing will cause more reliability issues than it will ever solve.But definitely use genuine quality brand cells. By quality brands, I mean Panasonic, Sony, Samsung SDI, LG Chem, and so on.

author: plazma
author_rank: Frequent Contributor
timestamp: September 08, 2017, 06:44:56 pm
content: Balancing protection PCB can be bought for cheap. Look for a version which has a common port for both discharge and recharge.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: September 08, 2017, 07:03:24 pm
content: Quote from: Siwastaja on September 08, 2017, 06:27:07 pmI disagree completely. They don't skip out on balancers because they're not needed, they skip out on them because it reduces price, AND when the cells fail prematurely because of the lack of balancing, they get to sell a new battery pack.

author: sleemanj
author_rank: Super Contributor
timestamp: September 08, 2017, 10:16:19 pm
content: For 2 cells, I would not care about balancing, as long as the cells are the same type and matched for actual capacity when you series them it's unlikely they will get very unbalanced.Use protected cells, or incorporate (over, under voltage, over/short discharge current) protection for each - the usual chinese go-to of dw01 protection ic and 8205 dual fet is a good place to start - that way if they do somehow get very unbalanced it's not really a problem other than overall capacity reduction.

author: Siwastaja
author_rank: Super Contributor
timestamp: September 09, 2017, 08:06:57 am
content: Quote from: suicidaleggroll on September 08, 2017, 07:03:24 pmHaving dissected such devices and analyzed the packs, my findings are that 2s products without balancing have almost always been in good balance, if high quality cells are used. On the other hand, it has been notorious that BMS circuits have failed. Some have had too big quiescent currents (and differences thereof) even when in-spec, causing imbalance, and worse, overdischarge after full discharge. Sure, it is the optimum solution, if done perfectly right.In any case, this is not a hardrequirementbased on safety.In the end, I prefer simplicity over complication, and when doing a (possibly low-cost) 2s battery pack, especially with limited design skills and limited lab resources, the path of least risk will be obtaining genuine high-quality cells backed up with vast industry experience, and do a thermally coupled, fixed series connection of the cells and handle it as a 7.2V cell. This is the industry practice. Actively balanced 2s constructions are quite rare.

author: Sai teja
author_rank: Regular Contributor
timestamp: September 13, 2017, 06:37:21 am
content: BQ29209 IC from Texas instruments is providing balance charging (upto 15 milli amps). It is secondary protection IC. Incase if I assume that balance charging is required for reliability, can I use this IC for that purpose? \I am designing charging circuit which can charge at 1 ampere during constant charge cycle. Is 10 to 15 milli amp balancing current sufficient in this case?Thank you in advance

author: Siwastaja
author_rank: Super Contributor
timestamp: September 13, 2017, 01:51:58 pm
content: 10..15 mA is surely enough for small battery packs (< 10 Ah maybe). Considerably higher currents would necessitate thermal design, fusing etc. for safety. This particular chip is not limited in time it uses for balancing, it balances as long as the imbalance exists, which means it can remove quite a big imbalance - i.e., it can remove imbalance caused by 10..15 mA of difference in the leakage current between the two cells! Since the leakage of a good cell is some µAs, anything over, say, 1 mA, means a totally busted cell anyways.