Just to clarify, letting your battery get that low on a regular basis is truly bad for it.
If the battery is starting to act up, the best way to get it back on track is to let it shut off on its own, then charge it fully and power it on after. i have never had to do this more than two times to get a battery back to normal, but i only let it go below 75% when it takes to depleting at a quick pace.
If the battery is old (or regular full discharge has weakened it), it may have reached its EOL, and replacing it is going to be the solution. Although it is a relatively "easy" sealed battery to change, you will have to remove some double-sided adhesive foam when you replace the back cover.
BlackBerry Classic Q20 - ETrade Supply
Grade
S+ are OE components.