Q-45: I want to replace an 8-cell NiMH battery pack (nominal 9.6 volts) with a 2-cell Li-Poly battery pack (nominal 7.4 volts). I realize that the voltage will be lower, so I want to know if it is okay to add 2 NiMH from my surplus supply and place them in series with the 2 Li-Poly cells. That ought to get me close to the desired 9.6 volts. Since I’ll be using part NiMH cells in this pack, is it okay for me to use my peak detect charger?
A-45: With all of the current controversy regarding the safety aspects
of Li-Poly battery cells in general, this nontypical question is timely.
Basically, the answer is “No” and “No”!
I was so upset when I read this that I immediately E-mailed the reader with my
concerns and told him to please never try such a mixture of cells. You should
never mix battery types, brands, or capacities in any single pack or grouping of
cells.
In this instance the reader knew his voltage would be lower and thought he could
add a couple of NiMH cells he had lying around his shop to increase the voltage
to what it was originally.
The problem with Li-Poly cells, as we are quickly learning, is that they have a
nominal voltage of roughly 3.7 volts. Two cells at 7.4 volts might not be
enough, but three cells at 11.1 volts might be too much.
The solution to this problem is generally to go to the higher voltage, and then
select a propeller that draws less current or, if using a gear drive, change the
reduction ratio to reduce the current. But never add in other types of
battery cells in an effort to increase the voltage.
In the same regard, if you attempted to charge this “combination” of Li-Poly and
NiMH cells on a peak-detect charger, you would quickly exceed the 4.2 volts
maximum per Li-Poly cell, and with that the cell(s) would quickly be destroyed.
You won’t get an explosion with Li-Poly batteries, but the pack will swell up,
and in some worst cases catch fire.
Again, never add other types of cells to Li-Poly types, and never attempt to
charge Li-Poly cells with anything except a dedicated Li-Poly charger.