Re: How do I make a 7.2v charger that cuts off when the battery is charged?
- --- In Electronics_101@yahoogroups.com, "Shane" <shanepickens@...> wrote:
> Well I don't know what else to do. I can not find a decent charger anywhere online at a decent price. I have a charger but it does not shut off when the battery is charged. I need a simple circut with a LED to add to it that will cut off.
> 7.2 6 cell Ni-MH battery. I want to prevent it from over charging.
I think the simple circuit needs a separate power supply. Is this possible?
Let us assume, the added circuit will be made very simple so that it needs to detect the maximum voltage of the battery pack only. The maximum voltage could be about 8.4V to 8.7V for a fresh pack. When the batteries get older, their internal sum resistance increases and their voltage during the charge will be higher with an amount of:
Rs * I_charge
So with time, the monitoring circuit will cut off the charge before the pack reaches the full charge mainly if the charging current is relatively high since the sensed voltage will be V_bat + Rs*I_charge and not V_bat when fresh (when Rs close to 0).
Also the circuit must be reset manually or a minimum voltage should be chosen so that the charge will be on again when the pack voltage will be lower.