I didn't know about lithium AA/AAA rechargeable batteries. Interesting. I see that most are rated at 1200-1500mAh (AA) and 400-500mAh (AAA). These are only a little over 1/2 the capacity of the NiMH that I normally use. I guess the charging circuitry takes up room in the battery.
And yeah, the lithium AA/AAAs were news to me too. I learned about them when I got a headlamp that could either take 3 AAA batteries or a same-sized lithium battery with +/- contacts in the right places. I think the single-cell ones include a buck converter along with the charging circuit, which probably cuts down on the space they can devote to the battery itself.
By the way, this situation happens mostly on my motion-activated staircase LED lights which take 4 x AA. That thing is amazing because it works until 3 x AA are essentially dead and only 1 x AA is giving out power. For most of my other devices, the device stops working before the battery voltage becomes that low.
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Repeated overcharging can cause changing voltage curves that are (apparently) also referred to as memory effect, but NiMH cells are also prone to this.
Most chargers will just assume that you've discharged them fully between uses, and perform a "full" charge cycle when one is plugged in, maybe with a temperature sensor to stop if they get too hot. You can't really use resting voltage as a gauge, and it's tricky to find a good charger which can reliably detect the small and brief "dip" in voltage which occurs around the time that the batteries are fully charged.
They're great batteries, and I also prefer them for applications where the batteries get fully drained before being plugged in, but they can still be iffy for applications where you only discharge them briefly and want to keep them "topped up".
For those applications, these days you can buy 1.5V lithium cells in AA/AAA form factors with built-in microUSB charging ports. But those are lithium-based, so you have to be careful about thermal runaway.
Life is a balancing act full of compromises...