What happens if you leave the lights on and the battery dies?

   Most of us have left car lights or other car loads on on occasion, and if it goes unnoticed, eventually the battery runs out of juice and goes dead. Most of us know that even a new lead-acid battery is 'old' after this happens to it even once. Naturally, one wonders what happens to nickel-metal hydride batteries under this circumstance. It would be a bigger investment lost if they were to die like that. The literature warns not to totally discharge them as "irreversible chemical changes occur". (This would probably be corrosion of the lanthanum in the metal alloy, reducing its ability to act as a hydride to store protons. One of the big challenges to designing a metal hydride is to have it not corrode. Some have used something that corrodes even faster on the outside surface, like neodymium, that then (as I understand it) somehow keeps the water from getting at the lanthanum.)

   Although my NiMH experiments might perhaps be considered incomplete without a total discharge test, I've been reluctant to try it on the pricey cells. (Okay, so a few AA cells wouldn't have cost much!) But I left one of the 30 amp-hour, 12 volt batteries in the truck, which has now sat unused since mid August. Evidently something in the truck gradually uses up the battery. I came out to run the truck engine and it was utterly dead - no dash lights, nothing. (Even if it never goes anywhere again before it becomes electric, it still needs to drive over to the 'carport' so the conversion can be done under a roof, so I don't want to let the engine seize up!)
   I put the Canadian Tire 'NiMH' charger on it and let it charge overnight. The next day the truck started. I decided this was the time to give the battery a load test and see how long it ran, so I brought it into the house and hooked it to the pentagon headlight panel. This would give an indication of the amount of deterioration resulting from a complete discharge. Since it's probably been dead for weeks, it's a more severe test than just an overnight "Oops, left the car lights on!"
   I was surprised: the battery had lost about 3/5 of its amp-hours, with only about 4 left out of its original 10. It also seemed to barely start the truck. If that impression was correct, that means it also lost current capacity. Cycling it didn't help - results were about the same with each discharge. On the 31st I tried the "sister battery" made at about the same time but not abused.

   The battery in my car has never been run into the ground and still works fine.

   I think I'll take ten AA cells as a 12 volt battery and run them down overnight, and see how much degradation there is when NiMH cells sit discharged for hours, but not weeks.