| In a phone the most likely cause of failure is long term overcharge. Once | the battery is fully charged most phone chargers switch over to a | maintenance charge program.
In my experience, most cordless phones don't switch over at all -- they just charge at a C/10 rate or so forever. So yes, it's long term overcharge. Maybe the better quality ones do better, but I've never seen one. Obviously, any cordless phone that used lipo batteries would have to shut off the charger when it's full.
With NiCds cells, overcharging for a long time at a low rate leads to voltage depression, which many people mistakenly call `memory'. Discharging the cell down to 1 volt/cell and then recharging will help undo this, at least until it happens again.
| NiMH have a higher self-discharge rate but not everyone sees | this.
The only way somebody wouldn't see it would be to not be looking -- the difference is huge. :)
| They are also less tollerant to long term overcharge.
... and (other forms of) abuse in general.
| Personally I always prefer to buy products that use loose AA size | cells rather than an assembled battery pack. That way if the | manufacture has gone bust you can still replace the battery.
It's very rare that you can't find a battery pack for something, even if the manufacturer has gone bust. Places like Batteries Plus can usually make something if needed.
However, I do prefer to get things that use AA or AAA cells, even if they're in an assembled battery pack. This is because it's usually quite easy to rebuld a pack -- soldering to cells isn't too bad if you can be very quick about it (but there's definately an art to it) -- and with AA or AAA cells, the cells you need are cheap and easy to find. It's very satisfying to fix your battery pack for $2.50 (4 pack AA 700 mAh NiCd cells are $2.50 at Harbor Freight Tools when on sale) rather than the $30 that the manufacturer might want. (Cordless phone packs are usually cheaper, but ham radio packs ... sheesh!)