A lot of cordless phones still use NiCads (Nickel Cadmium) batteries. They suffer from a memory effect so that if they are frequently recharged when 1/2 discharged (for example) they will think they’re depleted at 1/2 discharge. Leaving them on charge perpetually guaranteed the problem. The solution is to either discharge them completely before recharge or take care to give them charges at random levels. Some more modern devices manage this with internal circuitryThe kind of rechargable battery you’d find in older remote-control cars and the like.