How to diagnose a dead cell

[chuck101]

Ok so I have the Stampede and a junky bundled charger from Tower.
Went to the LHS and picked up an EPIC 3000 battery. Took it home and slapped it on my tower hobbies charger not realizing that I was about to screw up. I guessed at the charging time and kept an eye on the pack while it charged.
Now I have learned that I screwed up and got a Duratrax intellipeak charger.
My Epic pack still works, but seems that I have killed one or maybe 2 cells in this fiasco. Is it feasable to take it apart and try to diagnose and replace the bad cells or should I call it a wash and start from scratch? How would I diagnose a dead cell?
I realize any cell replacement may not match, but I'd like to see if I can salvage this pack to atleast be a good enough for some backyard bashing.
thx.

[Red01MustangGT]

What do you mean? How did you screw up? Did you overheat the pack?

[chuck101]

I think I nuked it charging it at 5 amps with the standard tower hobbies timed charger. http://www2.towerhobbies.com/cgi-bin...?&I=LXJC34&P=M
Its symptoms are:
Decent run times - longer than my shark 1500, which isn't saying much.
Crummy power - The shark delivers a ton more power and my friend's 3k pack runs like a scalded dog in my truck.

I have a multi meter, but don't know if it is possible or feasable to test or replace cells in this pack.

[Red01MustangGT]

5 amps shouldn't be too much, however a timed charger is meant for NiCDs, does the charger say it charges NiMH? I just got a couple Epic 3000s myself and I was really disapointed at first (still am, but getting better). I just keep cycling them and they are getting better. Deffinatly not going to have as much punch as higher quality cells, but... you get what you pay for.

Try peak charging it a few more times to see if the perfomance improves.

[chuck101]

Ya that Tower charger was not meant for the NiMh batteries and it took me almost melting the charger to figure that out.
I've cycled that pack about 8 times since and it doesn't seem to be getting better.
The duratrax charger does seem to be putting some juice to it. It takes a while to charge as I would expect of a healthy 3k pack, and the pack gets warm while charging, just the power output is crummy.

[imPEDEd]

"the pack gets warm"... How warm is warm? To properly charge a NiMH pack it should reach 120-130 degrees! That is actually quite a bit above "warm". I have had packs that I thought were charged but when I changed the setting on my charger (to a higher delta V cutoff) I got way more power and runtime.

How is it you think you killed that pack? Did it get blistering hot? That WILL kill a pack. 5 amps is fine- it is when it quits that is tricky.

I am guessing this Epic pack is a stick pack. Those are much harder to check without trashing the outer plastic skin (which can be replaced easily with stuff from Tower). With the ends of the cells exposed, you can check the voltages of each cell. If they are good, you can expect a single cell voltage in the 1.5V range. If it is toast you will know it. The full 6 cell pack voltage will range from 8-11 volts. The higher end is generally what you see with older lousy packs that still have all the cells working.

You can replace cells but they wont be matched to the existing one. I have killed unmatched 3300 cells and simply soldered in replacements and had good results.

Hope that helps.

[Red01MustangGT]

I think the power output is going to stay crummy. The Epic 3000 packs are quite noticably slower than other quality NiMH packs. Also your 1500 pack is NiCd. NiCds typically have more punch than NiMh. If punch is what your after your best bet is to go with either another NiCd (you can go up to 2400 mAh, or a GP3300 pack, they are a little more spendy, but quite a difference in performance. The other option is to go to 7 cell packs.

[dbrown17]

I had a cheap stick pack go out on me. Symptoms were low power and long (but slow) run times. Would get extremely hot when charging. I used a discharger and brought the pack down to 5.5 (ish) volts, but it back on the charger and within 5 minutes the plastic shrink wrapping started to melt. I took the pack apart separating each cell and found 1 cell reading -.3 volts - the other 5 were 1.25. I'm planning on getting a deans jig and putting at least 4 of the good ones back together - may add it to a 6cell pack to run 10 and see what I can destroy!

[SS Pede]

If one area of the pack gets much warmer than the rest while charging, that likely indicates a reversed cell in that spot.

[chuck101]

Thanks for the very helpful posts! Maybe I will take it apart and see what's up with the voltages.