36V 15Ah ebay battery info

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bobcourtney

1 µW
Joined
May 2, 2011
Messages
3
Location
Dartmouth, Nova Scotia, Canada
I had to take apart a 36V 15Ah battery that I recently bought on ebay from daoji888 (http://cgi.ebay.com/36V-15AH-LiFePO4-Li-ion-Battery-Electric-Scooter-/290444225724?pt=LH_DefaultDomain_0&hash=item439fd32cbc#ht_3949wt_982) since I dropped it and sparks flew.........

Anyway, for reference the battery used in the pack is a China Bak Battery (http://www.bak.com.cn/pro_xiangxi.asp?id=794&bigclass=High-power%20battery%20cells) 26650MP-Fe .
It's a 2700 mAh battery with a rated 10A (3.7C) maximum continuous discharge current (see http://www.accu-by-seidel.de/WebRoot/Store15/Shops/61957196/4B05/6CA7/452C/7EB8/945C/C0A8/28BD/B355/26650MP-Fe-2700mAh.pdf).

Total of 72 cells wired in 6 cell parallel.

6P gives 6*(2.6-2.7Ah) = 15.6 to 16.2 Ah with a potential 60 A discharge rate. The BMS and wiring are pretty minimal with a 15 A nominal rating.

One of the cells got damaged by the drop and burned off the covering through a wiring short - the voltage on the damaged cell after the burn was 3.4 V so I guess it's ok.

The welded battery connectors were fried off the damaged cell as well which is hard to repair so I decided to leave the damaged cell unconnected.

The pack still comes in at 39.9 V after the fall.

A question to those in the know:

Am I going to run into any problems running the pack a cell down in one of the parallel series?

I suppose the 5p cell will discharge quicker but the BMS should cutout before it drops too much. Will I now have effectively a 5x2.7 Ah = 13.5Ah pack?
 
One group of cells will drain at a greater C rate than the rest of the pack. It will shorten their life and reduce your power slightly.

I'd replace the cell if you can find one.
 
You get what you pay for. Some Chinese battery packs are just poorly made.
 
there are no pictures, no real info about what failed or what the current arrangement is. did you separate the pack into two pieces when the connection came loose, did it melt or burn up? what is wrong with just showing a picture of your problem rather than a long description or a canned picture?
 
Had I been thinking I would have taken a few pics, but I didn't and I've re-taped the pack back up.

The pack is configured in two layers of 6x6 cells stacked one on top of the other. It's a duct-tape battery pack with an inner layer of cardboard-like paper wrapped in duct tape.

The cells in each layer are hot-glued together.

When I dropped the pack onto my driveway, one corner of the pack impacted the ground, denting the corner-most cell. The plastic covering of that cell was damaged and the welded interconnection strips shorted on the exposed side of the cell, heating up that cell considerably, causing the entire plastic covering of the cell to melt off.

If I decide to reconnect the damaged cell, or if I get a new one with tabs from the supplier to replace the dented cell, I'll take some pics and post them.

I'm also considering changing to a one layer configuration, 12 x 6, as it's a bit of a square brick at the moment (156 mm x 156 mm x 130 mm). It would fit my rack bag much better as a 12x6 pack. I'll take some pics when I get around to this project.
 
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