2 cell in series battery charging/protection circuit design - Page 1
Forum section: electronics / projects

author: MiniEE
author_rank: Newbie
timestamp: September 28, 2024, 03:02:07 pm
content: I'm building device which needs 2 li-ion cells in series. I'm not sure how many I will use in paralleel, i'm thinking maybe 2S2P configuration. I'm not sure yet how many cells I want to paralleel. Maybe just 2 cells in series. However I'm overwhelmed with the charging/protection circuit selection. I have been searching TI for a while and I don't know what to choose any more. I would like to have some sort of secondary protection as well since I'm planning to build battery pack myself from unprotected cells. I was looking also monolithicpower and I found their MP2672 is very nice for charging. But ideally I would need secondary protection as well. From TI I have been looking many IC-s like BQ25887 and so on. I would love to have parameters configured over I2C. Also charging IC-s which have internal balancing seem to have quite weak balancing. Not sure will it fit for 2S2P battery pack. I'm wondering what is the go-to solution nowdays? Which chips/configurations are most popular?For input I was thinking for a while using 5V USB input. And yes I know current from 5V will be quite high. But I could buy 12V power supply and have step down charging no problem. I haven't eniterly decided what to do. I'm just overwhelmed with all the IC-s and hoping to hear which solutions others have used.

author: Neilm
author_rank: Super Contributor
timestamp: September 28, 2024, 06:35:27 pm
content: If this is a device you are planning to put on the market, then your battery pack will need to be qualified to the relevent standards, especially the transport of hazardous equipment standard (UN38.3). There are also regulations on how much energy the pack can have, the markings of shipped items etc. Getting it wrong can be a nightmare - the company I work for had a shipment repacked by an unqualified person who didn't know the regulations. The shipment was destroyed.TLDR - get the pack designed properly or find one that is already qualified. The qualification process isn't cheapThat being said, if this is a home brew project, I would suggest buying 18650 cells that already have protection circuits in them. Holders for these cells are available and you can put them together as you like. For a 2S2P I wouldn't bother with balencing the cells during charge. I have tended to use a qualified pack with a protection circuit and then just use a charger and not worry about any form of balencing.

author: Peabody
author_rank: Super Contributor
timestamp: September 28, 2024, 08:52:00 pm
content: I think cells in parallel should not have protection built in. The protection should be included in the circuit. The reason is that you don't want the parallel cells to ever differ in voltage else when reconnected large currents could flow. If the cells have built-in protection, the protection on one could trigger for some reason, such as over discharge, at a different point than on the other, and the voltages of the two cells will begin to diverge. But I have to say that I don't know how this is done in commercial products.You can get little protection chips now for the circuit that are like four pins and include the dual mosfets inside. So it doesn't have to be complicated like the DW01. XB7608A for example, as well as FM5056, both available in the Far East.On the original question, it takes an act of Congress to get me to do anything but 1S with lithium rechargeables. Everything is straighforward, and like the OP, I don't know what to use for 2S and higher. And you know, charging 2S from 5V power, and then balancing, is just asking for trouble. And no, there are no standard solutions that I've found. On the other hand, boost converters work very well.

author: MiniEE
author_rank: Newbie
timestamp: September 29, 2024, 02:36:10 pm
content: Quote from: Neilm on September 28, 2024, 06:35:27 pmThis is for my own device, not commercial. And I want to make my own protection circuit. I already have several cells from good brands and they are unprotected.

author: tooki
author_rank: Super Contributor
timestamp: September 29, 2024, 02:38:40 pm
content: Quote from: MiniEE on September 29, 2024, 02:36:10 pmWithin a pack, you should only use closely-matched cells of the same model, ideally from the same production lot. Don't even think of mixing and matching random cells.

author: MiniEE
author_rank: Newbie
timestamp: September 29, 2024, 02:45:05 pm
content: Quote from: tooki on September 29, 2024, 02:38:40 pmYes I know. I do have cells from same patch.

author: MiniEE
author_rank: Newbie
timestamp: September 29, 2024, 02:46:51 pm
content: Quote from: Peabody on September 28, 2024, 08:52:00 pmYes I have used many times for 1 cell and everyone knows TP4056 is classic solution. It's indeed really simple for 1 cell. I have never used anything more than 1 cells so I'm so overwhelmed by selections. I found really hard to find overcurrent protection for 2 cells or more. Although overcurrent may not be necessary since adding fuse in series would help with that. I have been looking BQ29200 and similar but can I use them with mosfets in series? Datasheet example shows switching heated fuse which I don't plan to use. I have no experience with any of the chargers/protection when it comes 2 cells or more.I thought about going with 1 cell and putting boosting circuit in front. But It seems too unnecessary having one boosting circuit to make like 8V or whatever and then have secondary boosting circuits after that. My board needs several different voltages. Anyway I decided to go with 2 cells in series. Also hoping to learn more about more cells in series.Why you think balancing would suck when having 5V input(boosting charger)? So you would suggest going for 12V and buck type charger? It would be no problem either.

author: Peabody
author_rank: Super Contributor
timestamp: September 29, 2024, 03:12:58 pm
content: If you have 12V input power, then the TP5100 might be a good choice for 2S:https://www.codrey.com/electronic-circuits/tp5100-1s-2s-li-ion-battery-charger-quick-review/I think it gives you charging and protection, but not balancing. There are cheap modules available.I don't know about the BQ29200. I tend to shy away from hobbyist-unfriendly packaging.

author: daisizhou
author_rank: Super Contributor
timestamp: September 29, 2024, 10:29:01 pm
content: Quote from: MiniEE on September 28, 2024, 03:02:07 pmIf the current is small, you can consider using a boost solution