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

author: jaxbird
author_rank: Frequent Contributor
timestamp: June 06, 2014, 01:30:26 pm
content: I need something like 4 cells in series, so charging is a challenge to make sure the cells do not get out of balance.I was thinking something like this initially:But then rethinking, would it be reasonable to get away with something much simpler like this:Surely just a few resistors across each cell should balance it out eventually, but I don't want to waste too much of the charge.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 06, 2014, 02:56:38 pm
content: There are chips specifically designed for that task.

author: jaxbird
author_rank: Frequent Contributor
timestamp: June 06, 2014, 03:11:58 pm
content: Quote from: NiHaoMike on June 06, 2014, 02:56:38 pmThanks, yeah, I've looked at like:http://datasheets.maximintegrated.com/en/ds/MAX921-MAX924.pdfI just wonder if they are really worth their value in a configuration where the cells will always be closely in sync?Seems high BOM addition if they can (generally) be replaced by a couple of dirt cheap diodes.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 06, 2014, 06:01:11 pm
content: Those chips take care of overcharge, overdischarge, overcurrent, and balancing.

author: Neilm
author_rank: Super Contributor
timestamp: June 06, 2014, 06:14:31 pm
content: Dedicated chips will not not overly load the cells, preventing over-discharge. If this is a commercial pack design, it has to be correctly qualified to the relevant standards so a chip designed for that purpose is a more certain route to get qualification than a designed circuit. The last thing a company wants is to pay thousands to qualify a pack that fails 15 days into a 20 day test.

author: jaxbird
author_rank: Frequent Contributor
timestamp: June 06, 2014, 06:16:53 pm
content: Quote from: NiHaoMike on June 06, 2014, 06:01:11 pmYeah, I'm not talking about the circuit preventing the battery from going up into flames.I'm interested in balance charging.

author: jaxbird
author_rank: Frequent Contributor
timestamp: June 06, 2014, 08:31:08 pm
content: Thanks for replies.But I'm primarily interested if anyone got experience with Li-ion balancing charging circuits with multiple cells in series.Not relying on potential built in protection, as that is only designed to prevent major disasters.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 06, 2014, 09:38:43 pm
content: i worked on li-ion charger, but was a single cell charger, however am sure i saw some Li-ion protection IC specially designed for multi-cell balancing in texas instrument battery power management catalogue.

author: Afrotechmods
author_rank: Unknown
timestamp: June 07, 2014, 03:11:06 am
content: That BZX884 is not going to be able to handle much power. What kind of charge current are you hoping to achieve for the pack, and how much cell discharge current when balancing? The forward voltage on the SS13 is also going to vary from diode to diode and across the operating temperature/current range. Add that to the voltage variation on the BZX884, and I can easily see a variation of 5+% occurring which would be outside of my comfort zone for Li-ion cell balancing/protection.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 07, 2014, 02:07:53 pm
content: http://www.linear.com/product/LTC4070I actually recommend charging to 4.1V/cell instead of 4.2V/cell. You'll get more service life out of the battery that way.