DC16's have a common issue with batteries. Before ordering a new battery, try cleaning the metal contacts of the battery and the contacts in the vacuum. I would say something like a cotton bud is best. It might look clean but there might be anything like rust or dirt insulating the circuit
Best of luck
yeah was a pain in the arse they can be my uncle had one but took a men-ti
with it and that was that. it ran for 3 or 4 seconds and switched off, when he visited me i told him to Reset the battery, nope he forgot and broke it in anger!, what a silly bugger! ah well.