Wednesday, 5 January 2011

So what is in a LiPO pack then?

Last night I did some thing I've been waiting to since I got back into racing electric cars, I managed to connect a LiPO pack to my speedo with the polarity reversed. The plugs were connected for a spit second, to be honest the loud crack and bright spark was enough to ensure that! I guess 4600mAh 40C batteries really can deliver a quite a current.

It's a totally dumb thing to do, and happened whilst I was doing some work on the car and was in a rush, all the typical excuses! What I was left with was a speedo that fortunately seems to have escaped harm, but a battery that wouldn't charge as the charger just showed a connection break. With nothing to loose I decided to see if the pack was salvageable.

Warning - delving into LiPO packs can be extremely hazardous and isn't something to be done lighly. If you choose to do this, you do so at your own risk!

Step 1: A dead hard case LiPO pack

First things first, how to get at the guts of the battery? From the picture you can probably seen a thing split around the circumference off the hard case. I used a cut-off wheel in a Dremel to very carefully cut the case in front of the sockets, figuring that I wouldn't cut into the cells that way. With a small cut in the case I inserted a flat blade screw driver and twisted to pop the case in half.

Step 2: The case is now split allowing access to the cells

OK, at this point the bottom is of the pack, but the cells (the silver foil packets in the picture above) are stuck to the top of the case. Judicious use of a smooth plastic tool allowed me to safely lever them out, it turns out that they are stuck to the top of the pack with double-sided foam tape of the very sticky variety.

Step 3: A 2s 1p set of bare cells. Handle with care!

With the cells out I gave them a cursor inspection. Everything looked good, now splits or puffing, but trying to charge them resulted in the same 'connection break' warning. On a whim I flipped the cells over and plugged in from the bottom. Success! Charging started quite happily. Hum, that's got to mean somethings wrong with the pole connector sockets, so lets take a look there...

Step 4: You can just see the connection break (click to enlarge)

Now the problem is obvious. There should be a bridge of solder running from the positive connection across the board and to the cells. Looks like the short caused so much heat in that split second that the bridge blew, rather like a fuse. With luck it's saved the cells, and almost certainly it blowing so fast is what saved my speedo so I'm not going to moan too much.

The fix is pretty simple, I'll re-solder it tomorrow and see what happens when I charge the cells. With luck I'll have got away with it, but needless to say I'll be watching closely as a charge to ensure nothing goes haywire. I'm also going to put more definite markings on all my packs from now on to try and avoid doing this kind of thing in future.

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