This is somewhat off topic, but might help.
Batteries lose their capacity over time. With new batteries, I could get 8+ hours of play before needing another charge. After some years, I might only be able to play for 1-2 hours before needing a charge.
Periodically I disconnect the charger, and see how long the batteries can supply the amp. When this time gets below about 1 hour, I replace the batteries.