Charging a Varta 7.4v Li-Ion - Page 1
Forum section: electronics / projects

author: bingo600
author_rank: Super Contributor
timestamp: November 26, 2014, 02:02:15 pm
content: GuyzzI just bought 2 of these Varta LiPo 7.4v packs cheap (4$)I was wondering how to charge them correctly, as i don't need a visit from the Fire Department ?I had a look @ *Bay , and found a lot like this one :http://www.ebay.com/itm/171355102976But they all want access to 3 points on the 2 cells:+and-, and the combined+/-, i suppose thats enabling the charger to see each separate cell.But that's not what i have out of the "pack" , i only have + and -If i'm reading the DS (attached) it seems like there's some intelligence in the pack , is that a charger circuit ?Could i just give it 8.4v and the electronics inside will automatic limit current and stop when it have had enough ?I'd like to build a simple circiut for charging them maybe w. a lm317 or would that be possible (practical) ??If i needed to charge them now i'd prob just connect them to a HP 6632B , set it to 8.4v@1A (and get killed by the wife for the fan noise)Does anyone know those packs , or how to "decode" whats in the DS ?TIA/Bingo

author: HKJ
author_rank: Super Contributor
timestamp: November 26, 2014, 05:51:03 pm
content: What you link to is a LiIon safety/protection circuit. The datasheet for you battery shows that this is already included in the battery pack.

author: BennVenn
author_rank: Regular Contributor
timestamp: December 05, 2014, 01:17:25 am
content: Hi!The protection circuit is there to disconnect the cells from the load/source if the cell voltage falls out of a set value, or current draw is exceeded. It doesn't regulate charge current and usually the UV and OV are right on the edge of cell damage.A couple of LM317's set to CC/CV will be fine to charge, the cells are assumed ballanced when they are packaged and to age at the same rate.

author: IanB
author_rank: Super Contributor
timestamp: December 05, 2014, 01:45:36 am
content: Quote from: bingo600 on November 26, 2014, 02:02:15 pmIf you have a laboratory power supply, you can charge lithium ion batteries by setting the power supply to the correct CC/CV settings. For a 2S 18650 pack of 2200 mAh a setting of 8.4 V and 1.0 A should be fine. No need for a special charger unless you need "plug and play" charging.The lack of a balancing tap is troubling, but perhaps the two cell pack contains a balancing circuit internally.(Edit: forgot to mention, stop charging when the charge current drops below 50 mA or so. Don't leave a battery on charge indefinitely. When you take the battery off charge the cell voltage will naturally drop a bit, down towards the 8.2 V mentioned in the post below.)

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 05, 2014, 04:42:56 am
content: Charge them up to 4.1V/cell (8.2V in your case). You will get more service life out of them. It's also OK to hold them at 4.1V/cell, unlike the more common 4.2V/cell that degrades them rather quickly.

author: bingo600
author_rank: Super Contributor
timestamp: December 05, 2014, 08:09:27 am
content: Thanx guyzzI'll start out setting my HP 6632B to 8.1V@1A , for testing.Then i'll have a look for a diy circuit ... (Any pointers for a LM317 circuit would be appreciated)I mean i can make one with just a few 1% resistors , but i don't know anything about those batts , and wouldn't like to start a fire.Even though i suppose the internals of the Batt would prevent it./Bingo

author: carveone
author_rank: Contributor
timestamp: December 05, 2014, 10:41:35 am
content: The simple charging circuits I've seen that use an LM317 just use a transistor to pull down the Adj pin. The sense resistor is between the B and E. Like this one:http://shdesigns.org/pdf/lionchg2.pdf

author: IanB
author_rank: Super Contributor
timestamp: December 05, 2014, 03:21:07 pm
content: Quote from: bingo600 on December 05, 2014, 08:09:27 amA DIY circuit is the wrong idea. Use a lithium ion charging IC. It contains all the right algorithms and features. There are many such ICs available.

author: mikerj
author_rank: Super Contributor
timestamp: December 05, 2014, 03:40:12 pm
content: Quote from: IanB on December 05, 2014, 01:45:36 amThis is so unlikely it can be pretty much discounted. LiPo protection circuits simply prevent over-charging and over-discharge. This means that as the cells (inevitably) go out of balance after a number of charge/discharge cycles, the effective capacity of the pack drops off.This is the primary reason that the capacity of laptop battery packs degrades to the point of uselessness. The cells themselves degrade a little with every charge/discharge cycle, but cell imbalance can dwarf this effect. Cracking open dying laptop battery packs and balancing them can often restore most of their original performance.

author: amyk
author_rank: Super Contributor
timestamp: December 05, 2014, 06:55:02 pm
content: All you need is a well-regulated 4.2V supply and current-limiting resistor to charge one. It won't exactly be CC so it will take longer, but the important part is to keep the cell within the specified voltage and charging current. The former is a range (usually ~2.8-4.2V), the latter is an upper limit. Just don't exceed the limits and it'll be safe.

author: IanB
author_rank: Super Contributor
timestamp: December 05, 2014, 08:13:16 pm
content: Quote from: amyk on December 05, 2014, 06:55:02 pmAnd note that one of the limits is a time limit. Don't leave the battery on endless trickle charge, make sure to disconnect the charge circuit after 3-4 hours once the battery is charged.