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For anyone who still has devices with old AA, AAA, C, D, or 9V type batteries in it I highly recommend you move over to the USB chargable Li-ion type of the same format.
Most of those devices dont work with traditional rechargable batteries like NiMH because the voltage is a bit lower. Even if they do work they tend to last only a short period of time because as they discharge the voltage drops even further. So your device will often die even with 70% of charge still in those batteries.
With the newer USB rechargeable Li-ion type batteries of the same form factor, however, you have a drop-in replacement that doesnt have those issues. They basically are a LI-ion battery with a built in charger but more importantly with a DC-Dc converter to keep the voltage fixed at the desired voltage (1.5V or 9V) for the entire life of the battery. So they work in any device that they can fit in and lasts a long time as they deliver 100% of their power before cutting out (at which point it drops from 1.5V directly to about 0V).
The only downside is if your device reports a battery percentage then it will report 100% battery right up until it hits 0.
Anyway I replaced all my devices with batteries like this and they last longer than regular batteries in some cases (especially with 9V)... so I am sold!
@freemo Another issue with this advice is about devices that require certain voltage to operate.
Many so called 1.5v lion packs don't push more than 1.2v. While it is fine for many devices, it might be not sufficient for others, as they expect alcaline bats to start from 1.6v.
This is a consumer note, though, there are lion bats that fit the voltage.
@nm0i every liion AA and AAA battery I have seen is built to delivery exactly 1.5V, they use DC-DC converters. A liion cell by itself is 3.7V so any liion AA or AAA **must** used a DC-DC. I cant imagine why any 1.2V liion would even exist, if they are using a DC-DC they would of course set it at the optimal voltage.
My guess is that you are confusing Liion with NiMH which has a cell voltage of 1.2V and does have the problem you suggest. But as far as I know, no this isnt a problem with liion.