ORIGINAL: chuckk2

My two cents.
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.
A ''quality'' NMIH 2000Mah battery can provide about 2A when it's fully charged, but at say 50% charge, the voltage drop may not be acceptable.

That said, 5A current capability seems to be adequate for six digital servos in ''normal'' operation.
Just make sure that they don't ''stall'' due to linkage, control surface travel limits, or servo stop limits.

The old 600ma NiCad receiver packs common just a few years ago on 72mhz systems with analog servos don't get the job done these days!
I'm beginning to think that 6 to 10A switching BEC's, powered by 7.4 to 14V or so are really a better way to go.