NiMH cell death

I had occasion recently to say goodbye to a set of NiMH AA cells which had served me faithfully for ~ 5 years. Their demise was not due to the usual severe reduction in capacity (they would still take and hold charge), but rather to an increase in their internal resistance which made them unsuitable for driving their intended load. Is this a common failure mode? (Google doesn't throw up anything to suggest it is).

Yes, same for Nicads and even lead-acid. You can actually predict the remaining cycles on a battery by measuring the internal resistance of fully charged battery and comparing it to a known value.

I built a thing to test gel-cells this way back in the eighties - used power resistors, relays, timers, integrators and boardfull of other crap because I was too dumb to realize you could do it with AC and an op-amp.

The electrolyte dries out in areas, effectively reducing mA-hr capacity and increasing internal impedance.

NiMH are garbage, wthout the heavy metal cadmium to stabilise the cell reaction they have a poor self-discharge rate, crappy peak current output and even worse short life.

Cadmium is good. People that outlaw heavy metals are morons.

Mr RB said:

NiMH are garbage, wthout the heavy metal cadmium to stabilise the cell reaction they have a poor self-discharge rate, crappy peak current output and even worse short life.(

Click to expand...

Commercial NI-MH was developed for sale in late 80's and early 90's by companies such Gates Energy Corp. At the time, they believed NI-CD cells would be universally outlawed due to cadmium toxicity, so they believed NI-MH would inherit the entire market...... they were wrong about that obviously.

Then they figured they would sell NI-MH as the "premium battery" due to it's 30% greater capacity than NI-CD....... and two things happened: NI-CD makers immediately found ways to greatly increase the capacity of their cells, and Sony brought out Lithium rechargeables which were so far superior to any NI technology that Li batteries became the unquestioned market leader as the highest performance selection and remain so to this day.

NI-MH may go down in history as the biggest marketing blunder in battery technology. I have no idea how much money companies lost developing it, but it sure didn't turn out to be the great new technology they thought it was at the time. It's basically a slightly higher capacity NI-CD that is a lot more difficult to detect end of charge on so it's a lot riskier to recharge. It also doesn't tolerate sustained charging (trickle charge) like NI-CDs will. It is what it is.

The electrolyte dries out in areas, effectively reducing mA-hr capacity and increasing internal impedance.

Click to expand...

"Dry out" is a bit simplistic but I guess good enough; what's actually occurring is a build up of impurities from the charge/discharge chemical process to the point where it becomes inert, so long as the cell isn't over pressurized so that it's forced to leak all the same ingredients are still in there that were originally so it's not really 'drying' out they're just due to cycling chemically inert.

NiMH are garbage, wthout the heavy metal cadmium to stabilise the cell reaction they have a poor self-discharge rate, crappy peak current output and even worse short life.

Click to expand...

I actually don't think NiMh's are garbage, modern version of them use new materials which help with the self-discharge rate (drastically so) their lifespan is fine but they do suffer from heat instability and peak current output still. It's all about design considerations for a specific application. Considering NimH use in consumer electronics and the fact that almost no one recycled their batteries I think it's a better consumer rechargeable because of the reduced amount of Cadmium going into landfills.

I agree with you 100% on the ban of heavy metals. Which I find incredible stupid coming from the perspective of a metal finisher. We used to plate parts with Cadmium because of it's useful friction characteristics as well as corrosion resistance. The European EOL and ROHS standards have virtually eliminated Cadmium plating as acceptable because the declaration of how a 'homogeneous substance' is measured includes the plate as a separate entity. So even though there's only .0002" of Cadmium covering a part that has 2 square inches it's considered out of specs even if the part it's plated on weighs 1 pound. Totally irrational; even worse things like decorative Chrome are exempted and that's far thicker and over a greater area of the vehicle.

Thanks for the info on the cadmium plate, that is interesting.