There was a thread recently about someone wanting new batteries for their cordless phone, which looked happy but would then die without warning. Mine did that, and I put the old NiMH batteries on an analyser to find out why.
The first discovery is that one of the batteries has a capacity of 7mAh, and the other one 568mAh, for nominally 750mAh AAA batteries. The 7mAh battery would explain the sudden death - when pulled off the charger, cell voltage is good at about 3v for two cells. As a phone call progresses, the voltage declines. When the 7mAh cell gives up, there's a rapid drop to maybe 1.7v (I didn't measure it) and the phone gives up. The second cell is holding up its voltage fine, but it's no good by that point. I suspect the 7mAh cell has died from overcharging or being reverse biased when overdischarging.
The second discovery is Poundland cells are *awful*. I bought some to replace the above, thinking that the phone is only off-cradle for short periods so capacity doesn't matter. They were labelled 350mAh AAAs, but the analyser says their capacity is 9mAh and 12mAh respectively (testing AAAs at
200mA charge/discharge current). I had some other Poundland AAs, labelled
850mAh, which managed about 100mAh each (when measured at 500mA).
My collection of other AAs, including some 2100mAh Vapextech ones that must be at least 10 years old, all were 1500mAh or more (with the exception of one dud at 1100mAh).
So the conclusions are that cordless phones kill batteries over the long term, and there's not much you can do about it. And don't buy rechargeable batteries at Poundland.
Theo