I have a Duracell NIMH battery charger (11DO11 stamped on back); holds 4 AA batteries; has folding electric prongs; 2 red lights on front which light up when it is plugged in. I know the batteries need a charge and red lights came on right away and never went out. Do they mean anything? How long should I leave it plugged in? I finnally unplugged it when it felt warm.