Q-110: I have a perfectly good Hitec 335 charger that I purchased
several years ago to charge Ni-Cd battery packs. Now I also have several NiMH
battery packs and wonder if I can use this same charger. I understand from what
I’ve found on the Web that NiMH cells have a different peak voltage than Ni-Cd
cells, and that as a result, my charger may have difficulty in detecting it.
A-110: Your popular Hitec 335 field-type peak-detect charger should have no
trouble charging Ni-Cd and NiMH battery cells. The actual peak that occurs with
NiMH cells is slightly less discernable than it is with Ni-Cd cells, but
that difference is usually taken into account in the charger circuitry. Some
chargers (but not many!) allow you to select the peak for either type of cell.
When you mentioned a “different peak voltage,” I think you really meant a
different “nominal voltage.” We usually refer to a Ni-Cd cell's nominal, or
average, voltage as 1.2. That is because in practice this cell can get up to 1.3
volts at the point of full charge, and it can be discharged to 1.0 or 0.9 volt.
The nominal voltage of NiMH cells is slightly less than that of Ni-Cd cells.
What this boils down to is that if you are using a seven-cell Ni-Cd battery pack
and decide to replace it with a NiMH pack, you will likely have to add one cell.
It will take an eight-cell NiMH battery pack to replace a seven-cell Ni-Cd
battery pack. That's not a startling difference!
However, many who make this kind of substitution have found that their aircraft
fly slower when going from Ni-Cd to NiMH (with the same number of cells). But
going one NiMH cell more (higher voltage) produces roughly the same performance.
Of course, if you decide to go for Li-Poly batteries, you end up with a totally
different situation.