Q-107: I have noticed some advertisements and product reviews that
claim that you can fly a small, electric-powered model aircraft on a single
battery cell. My question: Is a single cell a practical source of power? Are
there any benefits to this approach?
A-107: First, when referring to single-cell electric power, the assumption
is that the single cell is a Li-Poly which has a nominal voltage of 3.7 volts. I
doubt very much if you are going to fly anything successfully on a single Ni-Cd
or NiMH cell, except maybe with a tiny, indoor FF model!
Flying on a single Li-Poly cell is practical because many RC receivers and
servos can easily function down to roughly three volts. You might get slightly
less range out of the receiver and your servos may tend to move more slowly, but
the RC system should work. A number of indoor RC electrics work successfully on
a single cell.
You must select your motor carefully or else your aircraft might never get off
the ground. The very small KP-OO-geared motors will power models up to roughly
1.5 ounces total weight. I can see single-cell Li-Poly power being practical up
to approximately 9- to 12-ounce model weights, using some of the new specialty
brushless motors such as the Hobby Lobby Uberall Nippy Black (model 0803/199),
but electric power, everything is a trade-off.
A two-cell, 10-ounce model can be flown at 4 amps motor current producing 30
watts of power. That works out to a respectable 3.0 watts/oz. and the aircraft
will certainly fly well. Take the same 10-ounce model on a single cell. You
still need 30 watts of power to fly the airplane. With only a single cell (3.7
volts nominal), you will need a motor current of 8 amps to get the 30 watts you
need.
So far, this is both logical and possible, but here is the problem: the two-cell
battery—if it were 640-mAh capacity—could power the motor for approximately 10
minutes. A single 640-mAh cell (running at twice the current), would only
provide a five-minute motor run. You save weight with a single cell, but the
penalty is a shorter motor run time.
If you increased the capacity of the single cell to a 1200-mAh Li-Poly cell, you
probably would have doubled the weight of that cell, while now achieving a 9–10
minute motor-run time. Everything being equal, there is no benefit in using a
single cell but it will work.
With single-cell operation, you must have a suitable ESC that cuts-off at 2.8-3.0 volts. Many ESCs won’t go down to that level. That means your motor might cut off prematurely. There are considerations, but with the right application—especially with the small-sized model aircraft—single cell operation can be a practical and interesting alternative!