RE-Building A123 Packs

[JThiessen]

I have 3 A123 packs that appear to have some bad cells (I have another thread on that). Today I want to take one of them apart and I need to find the bad cell. They are 2S2P packs, and the last charge only took them to 3.3V.
1) How do I check for the bad cell? Do I have to completely disassemble the pack? Or can I use a multimeter to measure each cell somehow (while its still assembled)? I'm fairly certain I can figure out which "side" the bad cell is on, but then how do I isolate which one is bad.
2) Once I figure this out, I want to try to charge that cell. How do I manage this? Should I solder on a temporary plug? Somehow clamp on some wires? Can I do this with the pack assembled?
3) When taking apart packs, I assume I should have my iron on high. I've seen something about de-soldering braid. What is that, and should I use it?
4) Anyone want to volunteer for this???!???!!!

[JThiessen]

Here's one of the packs. The bars dont look like they were soldered on, but it could just be a small amount of solder behind them (there are a few little "punch" holes in them). The cells with 0 volts are on the left.

[lincpimp]

Just use the multimeter on each cell. Most meters do not care if you reverse the polarity. So put the black probe on one end of the cell and the red one on the other end. The good cells will read higher voltage than the bad ones.'

The tabs are resistance welded on, the only ways I have found to seperate these is to slowly wiggle the cell back and forth and the tab will eventually come off. You can solder to the cells, but you have to be careful. I usually leave the tab on the cell, and just cut it off in the middle where it bridges to another cell and put the solder on the tab...

[JThiessen]

Measuring them was easy. Thats how I figured out which were bad and which were good.

Now on to how to fix them. If I can figure out how to charge those cells without taking the whole thing apart, I'd be really happy.

[lincpimp]

Originally Posted by JThiessen

Measuring them was easy. Thats how I figured out which were bad and which were good.

Now on to how to fix them. If I can figure out how to charge those cells without taking the whole thing apart, I'd be really happy.

The paralled cells will have to be seperated at least on one end to charge. Some small alligator clips work well, that is what I use to charge individual lipos.

[JThiessen]

I put one cell on the charger, and it looked like it was going good, got it up to about 1.2V, and then the charger gave that "output voltage too low" error again. Wish i knew what that was all about (the manual doesnt say anything). BTW, had the Duo set on A123, 3.3V, 2300maH, .5amp.

Guess this could be the bad cell.

[JThiessen]

Just to update, I got one of the cells up to 3.3XX volts. I'll be trying the other one tomorrow. With the Hyperion Charger, about every 5 minutes it would give the "output voltage too low" error. I just reset it again and again until after it hit about 3 volts, then it started working fine. Its been sitting on the last one now for about an hour, and seems to have stabilized.

After seeing how this happened, I took one of my other "bad" packs and charged it on Nimh up to about 4 volts. Then I put it on A123 and just kept resetting it. Its now up to 5.2V. There are no hot spots on the pack, so it doesnt appear that I'm overcharging any one cell.....

[tc3_racer_001]

good to hear! one of my a123 racing packs, i added balance tabs on them only reads up to 3.37v... while the other one just bleeds off the extra current/voltage. one good thing i guess rather than overcharing the other pack!! i would suggest adding taps if you havent already to get them balanced.

[JThiessen]

I added Hyperion taps last week.

To update: The pack that I though I was able to rescue may infact still need to be torn down. I've been able to get it up to 6.6V charging without the balancer, then I pull it off, and rehook it back up and balance charge it. The "bad" cells still drop off to about 2 volts before it gets to the charging part of the process. I've gone through this process twice so far - I'll give it a couple more tries before I give up on it.

[JThiessen]

Originally Posted by lincpimp

The tabs are resistance welded on, the only ways I have found to seperate these is to slowly wiggle the cell back and forth and the tab will eventually come off. You can solder to the cells, but you have to be careful. I usually leave the tab on the cell, and just cut it off in the middle where it bridges to another cell and put the solder on the tab...

So have you had any that have come loose? There appears to be four spots where its welded. I had one of them pop loose on what appears to be one of those, possible two.

When you made your packs, were they 2S2P config? I'm asking because the center tab connected all four batteries together. It's now cut in half. I'm uncertain how to approach soldering that back together. I'm not certain what to use to solder the other end either...should/can I just use a normal battery bar? I've seen a thread about someone using desoldering braid for this purpose.

[JThiessen]

I got the pack put back together again (I'll call this one - Humpty). I used braided copper desoldering wire to join the packs together again - worked pretty slick (I read about doing this on a thread here somewhere).
Balance charged it, and its sitting at 3.30 and 3.31V.

So all in all, the first one was a success so far.

I also have to thank those that recommended the Hakko Soldering iron to me in another thread. I never would have accomplished it without a good iron.

[JThiessen]

I pulled one more pack apart last night and found it to be the same end of the pack that was bad. But these cells wont take a charge. They run up to about .13V, the charger gives the low voltage error, and I end up restarting at zero. Attempted several times..guess I'll stop by the LHS and pick up a few cells (they have them for 19 each - not a good price, but.....convenient).

[rcmonkey]

I too bought A123 packs from hopmeup (2x 9.9v 2300mAh) and so far 3 of the cells have died, I have not checked the dates on the cells but will do later. Where is the best place to buy loose cells that havn't been sitting for two years?

Cheers

[Arct1k]

try rclipos.com

[rcmonkey]

Excellent, thanks