Sounds like the battery could simply be worn out - they wear out and eventually simply need replacing. You should try a new battery first. of course, maybe nothing is wrong with the battery and the charging circuitry is the problem. So you could also try an external battery charger on the old battery, if they make such things these days (In the old days, you could buy external battery chargers, and several batteries, and always have a charged battery ready to swap out when your power-hungry smart phone was spent).