Just had this problem.
Turned out to be the battery!
If your battery is perfectly flat ie how it was first installed this probably isn't it for you, but if yours has a 'bubble' on the sides like there's been a little explosion inside, then your battey is probably dead/dying and all you need is a new battery!
Went back to P4U and they said it's a known problem, bad batch, had to wait for them to go wrong so we could find all the bad ones, here have a new one. Problem shouldn't occur again, new supplier.
Good Luck....