reviving dead NiMH/joining good cells?
 

I realise that there have probably been countless previous threads on this topic but I wasn't able to find much information on my specific case.

I have 3 stick packs: Xcell 4300mAh 7,2V.

They were used in a 1:10 buggy with a 19T brushed motor and did work just fine (never got much warmer than 40°C after being used in it).
The charger always reported 4200-4400mAh discharge capacity.
Then they went into storrage for two years, probably completely charged.

The last two weeks the charger was reviving them at 500mA (stops when the capacity stays the same between cycles, but does at least 5).
One got back to 3800mAh and drives just fine but gets really warm, probably due to internal resistance having risen.
The other two keep 2400mAh but can only deliver very little current. Not even enough to move the rc car.

So what should I do with them? I did order two replacement Melasta 5000mAh packs (I expect them to do 4200mAh but that would be fine) but they will take a month to arrive.

I have read of "zapping" the (bad/highes resistance) cells with a car battery charger at 12V/20A.
Doing that on our terrace from inside the house wouldn't be a problem, no need to explode right in my face :lol:
But that apparently isn't a permanent solution.
Also, maybe just use the "good" cells to make two 7.2V packs, but how do I go about that?
How do I package them up again? Is there anything special about soldering batteries?
Should I discharge the packs before disassembling them?
Will I be able to tell the bad cells by resistance (-> multimeter) if I discharge them?

Or is there something entirely else I should do with them?

Throw the bad away or better yet take them to a place that will recycle them.

Why chance having one/more blowing up on you. Recycle them and start fresh. There's better stuff out there now than what was 2 yrs. ago.

This battery revive set-up worked on a Trinity Reference 3600mah 6-cell Battery Pack.

You can try to revive the battery-pack by first charging it at 0.5Amps and 5Mv.
-waited 1-hour
-then I tried a charge at 1-amp. It took the charge for over 30-minutes
-waited another hour
-then I gave the battery-pack 30-minute test charges with higher and higher amps; until it took a full charge at 3.6amps - 5mv

thank you very much for your answers!

so I hooked up a car brake light bulb to the batteries and measured the voltage of the individual cells (sewing needles though the heatshrink tube) under the 2.5A load.
Some of the cells keep delivering 1.15V just fine, others went as low as 0.8V under load. With the packs coming fresh of the charger.
So right now, I hooked my chargers up to the weakest cell on each battery and let them do a "revive".
We'll see if that works... at least for me it has in the past with other seemingly dead cells being back to almost full capacity after a few days.
But you are right, current LiPo tech is much better (capacity per weight, maintenance is automatic, no need to discharge first) but also a bit more expensive for the same capacity and I'd have to get a LiPo charger first.