A question about NiMH batteries

Hello everyone, my first post here (request to moderators: if m posting in the wrong section, please lemme know where a thread like this should be posted - if possible. thank you).

Recently, I've done something stupid. I have a wireless mouse that lasts nearly 8 months on a single Duracell AA alkaline battery, it's a Logitech M235. While purchasing it, I saw some "Made in Japan" Sony "CycleEnergy" 2500mAh NiMH batteries (LSD type, pack says "Maintains 80% capacity after 500 recharge cycles, retains 75% charge after one year storage at room temperature, PreCharged- ready to use"). Got tempted by the "Made in Japan" logo, and purchased 4 of them. This is the first dumb thing. The second dumb thing I did – the salesman talked me into purchasing a super slow (150mA) "Dumb" charger for these batteries. He said the "Smart" chargers are not reliable as they might cut of arbitrarily- either undercharging or overcharging. It's better to have full manual control. Set an alarm in your phone to disconnect the charger after 20 hours (that's right, he said it'd take 20 hours for this charger to recharge these batteries).


As you can see, it's probably been a worthless investment. I don't have a single "high drain" device that'll accept AA batteries. My camera, cell phone, laptop, etc all have in-built Li-poly batteries. I can expect a single recharge on a single battery to last about 6 months in the mouse, if not 8 months like it does with a Duracell Alkaline. It'll take nearly 2 years for the mouse to use up a single charge cycle on all my 4 batteries.
To make matters worse, the charger doesn't charge a single battery, it charges in pairs- (2 or 4). And the mouse uses a single battery. So when the first battery runs out, I cannot immediately put it back in the charger- I must wait for the second battery to run out too, and this means the first battery sits "discharged" for nearly 6 whole months before I get an opportunity to recharge it.
Will this usage pattern (leaving a discharged LSD NiMH battery for 6 months before recharging it 20 hours in an ultra-slow "dumb" charger) destroy my batteries? I don't know what to do now, short of purchasing stuff which will actually use these batteries in a better way- a solution in search of a problem. PLEASE ADVICE.


If needed, here's a picture of the battery pack I got. I got two of these, 4 batteries in total.
https://s31.postimg.org/sw7cbgdor/WIN_20160615_134947.jpg

NiCd and NiMh are remarkably tolerant of overcharging as long as the charge rate remains below C/10. Since these are 2500MaH cells, C/10 would be 250ma. You charger operates at 150ma. Overcharging does slowly convert the water in the electrolyte to hydrogen which is vented, but it takes a lot of amp hours to do that. The reality is your mouse is likely to die of old age long before the batteries are damaged by overcharging. LSD batteries are typically good for several hundred cycles. Even if you halve that, every 6months is 50 years!! In short, don't worry about it.

Thanks Mathew,
Is it okay to store them in discharged state? Like I said, the charger charges batteries in pairs (either two or four, cannot charge a single battery alone) and my mouse uses just one. Once a battery is used, I cannot put it back on the charger right away- I must wait for the second battery to get discharged and then charge both of them together. The second battery will probably take another 6 months to be discharged, hence the first battery must sit in a discharged state for 6 months in a drawer outside any gadgets. Will this damage it?

To be honest, I'd charge them before storing them. Older nickel batteries (NiCd and earlier NiMH) self-discharge like crazy (sometimes to the tune of several percent per day), and storing them in a charged state is an exercise in futility. However, low self-discharge batteries (such as yours) will still have the majority of their charge after a year, and some advertise significant remaining charge after 3 years.

All of this suggests that their intended use is to be fully charged before being stored.

I can't find much info for the Sony batteries, but Energizer has a datasheet(pdf) that, in the "Storage" section on page 12, states "Store in a charged condition (except for large bulk volumes)." For other manufacturers, I managed to dig up some FAQ from Panasonic that answers the question "Where's the best place to store my eneloop batteries?" with "If stored fully charged, it's possible to use them within about five years. However, we recommend giving stored batteries a charge once a year for optimal performance.", which also suggests they expect you to charge before storing.

That charger sounds truly awful, worse than the trickle charger we had when I was a kid mumblemumble years ago. NiCd AA's were on the order of 800mAh back then, and it was an overnight job to charge them, with no notion of whether the charge cycle was actually complete. We had a 4-slot charger that could physically handle everything up to a D cell, and we had AA's and fake C and D cells (effectively an AA prepackaged in a plastic tube to make it as wide as a C or D) for our torches and bike lights. At least that thing's slots were independent.

For your use case, though...yeah, rechargeables are overkill. Some of Logitech's mice are ludicrously low-powered, especially if you're careful about switching them off each time your turn your PC off. The cost of one (or perhaps two) alkaline AA's a year isn't really something you'd ever recoup your investment on. For the sake of your sanity you might want to consider either grabbing a cheap charger that charges your Sony batteries in a reasonable time, but if you don't really have much of a use for the rechargeables, that's really just throwing more money at a problem that you probably don't need.

Alternatively, just flog the Sonys on ebay or whatever and go back to alkalines? Give them away to a family member/friend that has a use for them?

Either that, or this might be a good opportunity to start a flashlight addiction?

This dinosaur recommends that you accept your losses and buy a better charger that charges cells individually. Additionally, I'd suggest buying a single AA powered flashlight or headlamp so you can have a use for the other three cells.