making a pack from lipo cells that already have protection boards - Page 1
Forum section: electronics / beginners

author: robinb
author_rank: Newbie
timestamp: January 12, 2016, 02:08:48 pm
content: Hi Folks,I'm looking at making a battery pack from six of these cells:http://docs-europe.electrocomponents.com/webdocs/141b/0900766b8141bf6b.pdfI need either 2 x (3 in series) in parallel or 3 x (2 in parallel) in series , to give a nominal 11.1V 4Ah.If they were unprotected cells then I understand I'd need to be using protection / balance boards like this:http://www.all-battery.com/pcb-11-1V-13A-32051.aspxHowever these already have an overcharge / undercharge protection circuit built in (details are in data sheet above) and I'm unclear about how they might interact. Can i just wire up the packs (after a topping up charge of all cells to balance them)? If so, Which topology of (3 Serial )x 2 Parallel or (2 Parallel ) x 3 Series is preferable?All info welcome!Cheers,Robin.

author: Audioguru
author_rank: Super Contributor
timestamp: January 12, 2016, 06:56:39 pm
content: A protection circuit is not a charger circuit. You need a balanced lithium battery cells charger circuit.

author: KL27x
author_rank: Super Contributor
timestamp: January 12, 2016, 09:49:15 pm
content: QuoteIMO, yes this works absolutely fine. As long as you avoid overdischarge, the cells will remain in balance for a long time. Yes, you still need a charger. No, you don'tneeda balancing charger, IMO. It would be nice, but once balanced the pack will be fine for a long damn time. If the pack gets out of balance, the internal protection on each cell would kick in and disable the cell or do w/e it is going to do. And if it goes out of balance, it's because one or more of the cells are dying/damaged. Balancing can extend the life a little, but the end is already nigh.For example, to charge a 3S pack youcoulduse a CV charge of 12.6V, current limited to maximum charge rate for the cells. There is nothing wrong with this, but when you get into bigger battery packs you have to take into consideration the greater potential for a fire if you may have gotten faulty or abused cells. Seems like there will be thousands of expert witnesses who think balance charging is defacto necessary.There are numerous examples of non-balanced charging in consumer devices made by large manufacturers.

author: marshallh
author_rank: Unknown
timestamp: January 13, 2016, 02:48:13 am
content: You can string them together and get away with not balancing in most applications.For high-current discharge applications (like drones, R/C flying and cars) its very necessary to balance charge to prevent this type of "avalanche" failure - the lipos are already getting superhot, no reason to make it worse.

author: Chalcogenide
author_rank: Regular Contributor
timestamp: January 13, 2016, 12:25:04 pm
content: You could always DIY a simple balancer based on TL431 ICs. Super simple and cheap, yet still better than nothing. I converted a couple of cordless drill to lithium and used such a circuit with success.

author: corrado33
author_rank: Frequent Contributor
timestamp: January 14, 2016, 04:27:35 pm
content: I tend to charge my homemade Lipo packs in a fireproof bag, just... you know... in case.

author: Audioguru
author_rank: Super Contributor
timestamp: January 14, 2016, 07:25:14 pm
content: I have a fairly old two-cells Li-Po that suddenly died while being discharged a little too low. One cell is fine at 4.1V but the other cell has gone past 0V and was charged a little in reverse during the discharge. This battery was always charged with a balanced charger then suddenly became very unbalanced.