Q-99: I am in the process of finishing a twin-engine model aircraft
and plan on employing up to 10 Futaba S3003s servos. Do you think a standard
four-cell, 4.8-volt, 600 mAh receiver pack is okay for this?
Futaba sells four-cell packs, but with higher capacities (both 1000 and 1500 mAh),
which I could upgrade to. They also sell a five-cell, 6.0-volt, 600 mAh pack.
Would going to five cells be better for my particular application?
A-99: I'm not sure what the average current draw is on the Futaba S3003s
servos. With the exception of the new digital servos (that draw a lot of
current), most receivers and four servos will average 50 mA at idle.
When you pulse (move) one servo rapidly, you can easily draw 300 mA. If you
pulse or operate two servos at one time, you can get current levels up to
500-600 mA. When using 10 servos, the idle alone could exceed 1000 mA. You won't
realistically be operating all of these servos at one time. However, you might
still see loads upward of 1000 mA (1.0 amp).
That being the case, I feel that the 600 mAh pack is marginal! You would
probably be wise to go to the Futaba 1500 mAh battery pack. I suggest that you
buy a four-cell pack consisting of the newer 2000-2300 mAh NiMH cells. They are
still AA size and weigh almost the same as a 600 mAh pack.
If you do go that route, make sure you charge a 2000 mAh pack overnight at 200
mA. Don't play games using an ordinary RC-system charger, leaving the pack on
charge for three days instead of overnight. The battery chemistry won't be
happy, and the result could be that your 2000 mAh pack gives you much less
capacity. Buy that Ace DDVC charger that I always recommend for charging these
larger-capacity packs at the proper overnight current level.
Regarding the question of four- versus five-cell receiver packs, using higher
voltage, by itself, does not improve your battery capacity. That extra cell
allows your servos to move faster and have a little more output torque. Some
people like this faster servo response time.
But on the less-expensive servos, sometimes more is not better. The servos may
move faster, but they might overshoot their intended position, causing erratic
flying because of the higher transit speed.
It is a compromise! I've been flying digital proportional control since 1968 and
have never used a five-cell (6.0-volt) airborne battery pack.