Using a NiMH 5 cell charger with a 4 cell pack.

Martin_H

I have a wall wart NiMH charger for a five cell pack that looks like the following:



I'm curious what would happen if I were to use it with a four cell pack? The voltage is higher, so obviously you would need to unplug the pack before it was fully charged. But would it work or ruin the 4 cell pack even if care was taken? Note that I already have a four cell charger in which you take the cells out of the robot, so this is more of an academic question. I'm curious about what would happen without ruining a set of NiMH cells.

john_s

For anything that touches batteries and chargers you might try to ask folks at candlepowerforums.com .

erco

Note that your non-smart charger is labeled AC adapter, not charger. I would guess that its non-regulated maximum voltage is matched to your 5-cell pack so that a safe minimum flows near full charge. With only 4 cells, your charge current might get high, some possibility of damage to charger and batteries. You could throw in a diode or two to drop the voltage, and basically make a low-current or very low current (trickle) charger. And/or add a voltage regulator or current regulator, whatever floats your boat.

Martin_H

Thanks for the suggestions. I like Erco's idea of adding a diode or two to drop the voltage. I bet that would do the trick and if I feel brave I may try it.

erco

You can mix & match silicon diodes (~0.7V drop each) with Schottky diodes (~0.3V each) for slightly finer control over the total voltage drop. I'd recommend monitoring voltage & current carefully for a fully discharged battery and a fully charged battery for your two worst-case scenarios.