Can an old Ni-Cd battery charger be used for Ni-MH batteries?

[seanspotatobusiness]

I have an old Vidor model RX PC battery charger which is for charging Ni-Cd batteries. It charges at 150 mA for AA, 50 mA for AAA and 10 mA for 9V/PP3. Can it be used to charge Ni-MH batteries? If not, can it potentially be modified to do that? It just seems a waste to discard it if it can still be modified. Als, I like slow chargers because I read the batteries last more cycles when charged slowly.

 

[(*steve*)]

No. It will not detect the end of charge.

 

[seanspotatobusiness]

No. It will not detect the end of charge.

I have a socket timer that I can set to cut power after the required charge time.

 

[(*steve*)]

That may work if you discharge the batteries completely and know their storage capacity with some accuracy. Going for an 80% charge is probably safe. If the cells get hot you've exceeded 100%

 

[Audioguru]

I have two old Ni-Cad chargers and I have used them to charge my Ni-MH cells for years with no problems.
These chargers simply limit the current and keep charging forever. I did not measure them but I suspect the current drops as the voltage of the battery rises.
I have another "smart" stupid charger that has a dumb timer that overcharges a cell that already has some charge or when the power fails then comes back then it resets the timer and it starts over again.
My new Duracell Fastest Smart charger has a mind of its own that keeps changing and Duracell customer service knows nothing about it.

A modern AA Ni-MH cell capacity is 2300mAh or 2500mAh so charging a dead one overnight (8
hours) at 150mA will half charge it. Remove it after charging for 18 hours because the trickle charge current might be more than the maximum allowed of 58mA.

150mA at 1.4V is heating of only 0.21W that makes an AA cell warm. 1A at 1.5V will make it hot.

 

[seanspotatobusiness]

Say you wanted to charge 1500 mAh batteries (to make the calculation easier) with 150 mA, wouldn't you expect it to take longer than 10 hours due to the energy loses? 1500 mAh is ideally what you get out of the battery, right? So it must take more current to charge fully, right?

 

[(*steve*)]

Assuming they were flat, yes, you would require more than 10 hours. The rule of thumb for NiCd was 40% more, but they are far more tolerant of overcharge.

You will get a far better indication of you note the temperature of the cells and remove then when the temperature spikes.

If you're not going to monitor them, just charge them for 10 hours.

 

[seanspotatobusiness]

Okay, thanks. Have you any idea what it only charges if two batteries are charged at the same time? It's a little inconvenient when some devices use an odd number of batteries and I wonder why they would design the unit that way. I guess it was just cheaper than providing for less frequent loads of one or three batteries.

 

[Audioguru]

I have four chargers for AA and AAA cells. They charge 2 cells as a pair in series then they cost a little less.

 

[Harald Kapp]

You could recycle the old charger's case, mechanics and power supply and replace the simple current limiting electronics (possibly only a resistor) by a modern charger IC designed for NiMh batteries, e.g. LTC4060 or any other suitable chip.
You may also be able to re-wire the electro-mechanic contact arrangement to be abel to charge the batteries independently (using 2 chips, of course).