"Main problem is it requires you to unscrew two tiny screws and then wrestle with 6 tiny batteries. You might want to buy a new charger if yours charges 4 at a time. I fixed mine by replacing the batteries with a 9v li-ion battery with a micro-usb charging socket on the side. Then cut a small hole in the side of the battery box casing so you can directly charge it without unscrewing anything (see pictures)."