A couple of years ago I bought some "F" size NiMH cells for a particular application.A stroke of madness, cost me a fortune! (Leave me alone it is, or was, my money).Any way, I wanted to ensure that the battery pack would last a long time and not be damaged by overcharging.I read up on the best way to charge NiMH and found that when the voltage started to fall, it was fully charged.To test this I ran a constant current charge and monitored the voltage over the period and graphed it out in Excel.What I got was this:As you can see there is a distinct hump at about 12 hours, I let it carry on for a few hours before switching off the charger.Up to the hump the battery pack remains cool. The charge energy is going into the battery.After the hump it starts to get hot. The charge energy is dissipated as heat.The battery pack also started to smell bad.So I made a voltage monitor using a PIC 16F887, which measured the voltage and stored the reading in memory.A second or so later it measured the voltage, compared the new voltage with the last stored value, if the new voltage was higher then the new value was stored.And so it goes on until the new voltage is something like 20mV (from memory) less that the highest voltage, and then the charger switches to a trickle charge.I also added an LCD display which shows the voltage and whether the battery is "Charging" or "Charged".The battery gets little use, so it has not been through many charge/discharge cycles. But at the end of each charge, the battery pack is just slightly warm, never hot.JimB
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What's the best way to tell if your rechargeable AA and AAA batteries should be replaced? They tend to discharge on their own if I leave them sitting for a month. I know that's typical of rechargeable batteries in general.
But I've had some of them for a few years now, and it seems to get even worse as they age. The problem is, they're still not technically dead. You know, not dead dead. They can still be recharged and used. But then if I leave them in that little flashlight, or remote, or whatever for a month, they're dead the next time I go to use them. So I'm thinking that maybe they should be replaced, but it's a little hard to tell since they all discharge on their own anyway.
So it there a good tester or something that I could buy that would tell me for sure if they should be replaced? Or should I just wait until they don't hold a charge at all? That does happen eventually, but it seems to take about 5 years.
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