While this isn’t enough information to definitively diagnose the problem, My own DC 59 experienced this exact symptom. Li-ion batteries have a very predictable lifetime, following very closely the expected minutes of operation for a charge over recharge cycles. I don’t remember exactly, but expected useful lifecycle is wither 500 or 1000 recharges. After this number of recharges the battery very quickly drops from whatever it is when new, 20 minutes lets say, towards zero. Once you get to around the expected ‘limit’, whether 500 or 1000, you would drop from 15 minutes between recharging very quickly to 10, one or two charges later you would be at 5, etc. Replacement batteries can be found with 2-3 times the capacity of the original for half the price, though research and care must be taken to avoid poorly made copies. I replaced my battery, now my DC 59 can be run for 30 minutes straight on MAX setting.