Reviving an old NiMH battery pack

kuksul08

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I have an old R/C battery - 6 cell 7.2V 900mAh. It is 9 years old. They are pretty cheap, but I wanted to experiment with cycling it to see what kind of capacity it still has.

It had only .42V on the entire pack when I took it out. I first did a .1A charge/discharge, then a .5A, then a .9A. On the final cycle the battery took a full 911mAh and powered my r/c airplane for a solid 5 minutes at full power. It still felt strong when I turned it off.

How is this possible? I thought NiMH cells were supposed to be tossed after ~3 years. It seems like it's performing well considering its age. Anyway I am still going to recycle it because there is white powder built up around the positive end of each cell. It's not worth risking damage for a $9 battery.
 

Etsu

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How is this possible? I thought NiMH cells were supposed to be tossed after ~3 years.

Where did you get that idea? I've got several NiMH cells that I'm still using 14 years after I bought them. They're not as good as they were when new, but they still work well in low-drain devices.

That said, the multi-cell packs tend not to last as long, because 1 or 2 cells will constantly get over-discharged and over-charged. Depends how much you used them. 9 years is getting old, but if it hasn't been used all that time, they should still be recoverable.
 

kuksul08

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Where did you get that idea? I've got several NiMH cells that I'm still using 14 years after I bought them. They're not as good as they were when new, but they still work well in low-drain devices.

That said, the multi-cell packs tend not to last as long, because 1 or 2 cells will constantly get over-discharged and over-charged. Depends how much you used them. 9 years is getting old, but if it hasn't been used all that time, they should still be recoverable.

I've heard that they develop crystallization in the internal structure, high internal resistance, and will eventually not be able to be charged. Especially considering it had been dropped down to such a low voltage for years. Before sitting there, it was only used a handful of times.

Is the white powder a clear indication that they have failed, or is that normal with age? It doesn't look good, that's for sure!
 

SilverFox

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Hello Kuksul08,

The forming of crystals is specific to NiCd chemistry. The white powder indicates that some of the electrolyte has escaped. The most common cause of that, after overcharging, is that the vent seals get brittle and don't seal as well as they did when the cells were new.

I suggest recycling cells when they drop below 80% of their initial capacity. If you battery pack is close to 80% go ahead and use it. Other factors are how fast it self discharges and what voltage the battery holds under load.

Tom
 

inetdog

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White powder is an indication that some of the alkaline electrolyte has left the cell. This could either be a small amount of leakage at the vent, a seal failure or the result of abuse.
 

kuksul08

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Hello Kuksul08,

The forming of crystals is specific to NiCd chemistry. The white powder indicates that some of the electrolyte has escaped. The most common cause of that, after overcharging, is that the vent seals get brittle and don't seal as well as they did when the cells were new.

I suggest recycling cells when they drop below 80% of their initial capacity. If you battery pack is close to 80% go ahead and use it. Other factors are how fast it self discharges and what voltage the battery holds under load.

Tom

Interesting. I was really surprised when the battery was able to be fully charged after just a few cycles.

If NiMH's don't crystallize internally, what causes them to go bad over time - or do they?

Guess I'll run some tests to determine the capacity and maybe they'll still work.
 

SilverFox

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Hello Kuksul08,

In NiMh cells the electrolyte breaks down and dries up and the electrodes and separators break down. This results in an increase in internal resistance and a reduction in capacity.

Tom
 
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