Somebody on here is bound to be a battery expert.
I’ve got a few dozen rechargeable NiMH AA and AAA batteries I use around the house for various purposes. I use two chargers, both of which are “intelligent” units with little status LCD screens.
About a month ago, I started using my programmable electrical outlets to charge batteries for 1 hour each day. In other words, I keep the chargers loaded and turn the power on for a short period to keep them topped off. I notice that the charge indicators will show that they are at 50-75% and then they charge completely in about 30 or 40 minutes.
I mentioned this to a friend and he told me that this was bad for the batteries as they have a limited number of charge cycles and I’m wasting them. He says I should either leave the units on all the time, in which case the charger senses that they are charged and only tries to charge them if they really need it, or plug the chargers in and charge them fully when I need to use the batteries.
Frankly, all three scenarios (mine and his) seem to have pros and cons. Can anyone give me advice and the reasoning behind it?
Thanks!
Your friend is (partially) correct. With nickel metal hydride (NiMH) batteries repeatedly top-charging them is creating a ‘memory’ in the battery chemistry that is making them appear to be partially depleted within a day, and repeatedly charging them to the maximum is doing progressive damage that will limit the life of the cells. A ‘smart’ (not ‘fast’) charger will follow a programmed charge cycle where it lowers the current at ~80% charge to reduce this damage but it happens regardless. With NiMH (and NiCad, if you still have any) it is strongly recommended to not just leave them in a charger for more than a couple of days because of the danger of overcharging doing damage. I’ve found that most NiMH batteries when new will retain >70% charge after a year; the Panasonic Eneloop Pro seem to be the best, and I’ve seen them hold more than 80% after two or three years in storage, but these are flat voltage output with a more generally limited cycle life, intended for electronics with high minimum voltage requirements and high constant power drain, i.e. GPS units.
To fix your cells put them in an application and drain them down to ~50% (a little lower is fine but don’t discharge the below 20%), then charge them back up to above 90%; you may need to do this a few times and it is likely they’ve become somewhat degraded by your current practice. Fortunately, NiMH batteries are fairly cheap and the materials can be economically recycled (though few people actually bother to do so). In generally, to get maximum life you don’t want to “deep cycle” the batteries, nor keep them topped off; you want to top them off just before use if possible, and then use them down to about 30% to 40% charge then swap. You should be able to get five or six hundred ‘full’ cycles (about 90% to 30%) out of a battery cycle provided you limit the discharge. Most high voltage applications like electronics will start behaving erratically or will shut own below this anyway.
A good source of information for most common questions about batteries:
Stranger