ORIGINAL: ahicks
ORIGINAL: chuckk2
NiCads do have a more significant voltage falloff with load and charge state.
NMIH's have a flatter falloff curve, as do many of the other newer battery types.
After some testing - -
NMIH batteries rated at 2000mah or so may not be able to supply the current and the voltage needed
for 6 standard or digital servos, based on Sanyo's charts.
I did some testing, and found out that some of the common digital servos can draw up to about 1-1/2A when exercised
with a servo tester at a high sweep rate.
Wish this information (which duplicates my experiences using anything bigger than ''standard'' servos) was more commonly available. You darn sure aren't going to see it browsing through a Tower Hobby's catalog....
In their defense, you can run 2 of these packs in parallel without issue. Add a second switch, and you have a redundant system that's pretty bullet proof.
His observation is correct on all counts, however NiCD's and NiMH packs tolerate the drain well and do not exhibit instant voltage drops which help avert brownouts.
I found this out the hard way when I attempted to use an 8A regulator with 3S LiPo packs on my 50cc Ultimate.
The plane uses nine servos, of which 7 are high torque standard sized analog servos that have 1.8A ( I tested ) stall current.
Since the servos should NEVER approach full stall current, and normal operating current under load was measured to be around 800mA, I thought that 8A was enough.
I started connecting servos one at a time, then turning on the plane after each connection. Everything was great until I got to the fifth or six servo. After that the servos would budge slightly when I turned on the plane and everything would freeze.
I double checked the vendor's specs on the regulator. I discovered that the 8A rating was really for max draw and that the regulator was really rated for 5A continuous... but even then it would seem that would be enough right?
Well it turns out that during power up or during simultaneous arm movement to extremes, the servos can draw almost full current for a brief fraction of a second. That is enough drain the small on board capacitors and trip the switching transistors in the regulators causing them to STOP SWITCHING, e.g. producing no more voltage!
I've used an equal number of identical servos with 3200mAh NiMH packs and/or 3000mAh NiCD packs without problems. I can see the momentary drain on a meter particularly during power up, but the older technology packs fare much better than the regulators.
Due to the availability of cheap LiFe packs ( $12.00 for 2100mAH 30C+ packs!! ) I switched the plane over to redundant LiFe's. This is lighter and cheaper than the NiCD/NiMH packs while still providing more than enough current e.g. 4200mA @ 30c+.