Q: Can an older Ni-Cd battery charger be used to charge Ni-MH batteries?
A: It depends on the type of Ni-Cd charger. It could undercharge Ni-MH batteries or it may overcharge them. In few cases it may charge Ni-MH batteries really properly. What could happen ? Let us have a look at three cases.
1) Many of the older Ni-Cd chargers are the simple timed type charger which will charge batteries for a fixed amount of time and then shut off. Unfortunately, since Ni-Cd batteries have a much lower capacity than Ni-MH batteries, the timer is likely to shut off long before the Ni-MH batteries are fully charged. This will not harm the batteries but the benefit of the high capacity offered by Ni-MHs will not be used.
2) Also common among older Ni-Cd chargers are the so called "overnight" chargers which charge batteries at a low rate as long as the charger is plugged in. This type of charger can fully charge Ni-MH batteries, but it might take a very long time to do so. It's possible that an old Ni-Cd charger could take as long as 48 hours to fully charge new high capacity Ni-MH batteries. This type of charger is not likely to damage Ni-MH batteries unless the batteries are left in the charger for weeks at a time, but it may not be very convenient to use.
3) The final possibility is that the older Ni-Cd charger is a rapid charger that will charge Ni-MH batteries but will not have the necessary circuitry to stop the charge cycle once the Ni-MH batteries are fully charged. If the Ni-Cd charger is designed to charge batteries in less than two hours it may be this type. In this case the risk is that the older charger will overcharge Ni-MH batteries. This will be apparent if the batteries get very hot during the charge cycle. If the Ni-MH batteries get too hot to handle and stay that way for more than 20 or 30 minutes, then the Ni-Cd charger is most likely overcharging the Ni-MH batteries and may shorten their life.