Charging Lithium Polymer Cell at < 0.5C - Page 1
Forum section: electronics / projects

author: stoic
author_rank: Newbie
timestamp: June 16, 2014, 07:34:43 pm
content: For a small battery powered project, I’d like to use a high capacity 5000 mAh 3.7 V lithium polymer cell. I have seen some conflicting information regarding the charge current to set in order to safely charge these cells.“Battery University” declares most cells are charged at 0.5C to 1C. I’ve had a hard time finding a charger IC that can supply 2.5A or higher. I found an interesting charging IC with power management that I’d like to use (MCP73871), but the effective charge current would be 0.2C as the IC can supply a maximum of 1 A.I was wondering, are there any long-term negative effects of charging at low currents, other than the long charge times?Thank very much,-S

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 16, 2014, 07:37:48 pm
content: You can charge them as slow as you want to. I recommend using a 4.1V/cell cutoff instead of the more common 4.2V since you'll actually get more service life out of it.

author: stoic
author_rank: Newbie
timestamp: June 16, 2014, 07:47:48 pm
content: Quote from: NiHaoMike on June 16, 2014, 07:37:48 pmThat's fantastic, thank you very much!I'll design around the 4.1 V mode for the increased service life, from quick research it seems to make a pretty significant difference!-S

author: microbug
author_rank: Frequent Contributor
timestamp: June 16, 2014, 09:13:08 pm
content: I actually designed a lithium battery charger based around the MCP73831. It's very easy to implement –here's the link. That chip can only do a max of 500mA, although Microchip does do other battery charging ICs; they might have one that can do higher currents. Generally speaking, the charge rate won't matter a great deal in terms of battery life unless it is near the rated limit. What type of battery are you using – ones designed for RC applications can typically handle very high discharge currents and relatively high charge currents (several C). If it is some mobile phone type battery, it will want a much lower charge current of 0.5C or so.@NiHaoMike, I have never heard that before... mind providing a reference? Although the MCP73831 does come in different types for various regulation voltages, it doesn't come lower than 4.2V. A 4.1V cutoff could be nicely implemented with a transistor on the PROG pin fed by a comparator with a couple of voltage dividers, as the MCP73831 will properly terminate its charge cycle when the PROG pin is disconnected.

author: stoic
author_rank: Newbie
timestamp: June 16, 2014, 09:40:56 pm
content: Quote from: microbug on June 16, 2014, 09:13:08 pmMicrobug:Thanks for the reply. I'm in fact designing around the MCP738_7_1 as it contains some interesting features for power ORing. It also allows for selectable charge voltage as hinted by NiHaoMike.I'll take a crack at providing a reference:http://batteryuniversity.com/learn/article/charging_lithium_ion_batteriesSpecifically:Quote-S

author: microbug
author_rank: Frequent Contributor
timestamp: June 17, 2014, 06:34:34 am
content: Oops, missed the part number! That charging info is quite interesting. Since most of the battery's charge is not in the last 100mV, you wouldn't lose a great deal of life.