ORIGINAL: Phaedrus-MMVI
ORIGINAL: XJet
ORIGINAL: Phaedrus-MMVI
It is exactly what it says. Many NiMH cells have a relatively high internal impedance. This limits their ability to deliver satisfactory voltage under load which reduces the performance of the servo.
For instance, a 2700 A cell has an internal impedance of 20 milliohms while a 3300 Sub-C is 5 milliohms. The difference is that under a 1 amp load the A cell will see a voltage drop of nearly a full volts (0.85 volts) while the sub-C will drop only 0.24 volts. This is a huge difference. It is critical that you make sure when using cells other than Nicad that they can perform as desired.
How do you calculate those voltage drops?
According to my Ohms Law cacluations:
1A x 0.025Ohms = 0-.025V of drop within the cell for the A and
1A x 0.005Ohms = 0.005V of drop within the Sub-C
V= I * R, it's the law :-)
I didn't calculate them. I measured them. These are real life numbers as measured on packs that I have here. And I am not certain that you can use the internal impedance of the cell, which some call internal resistance, in Ohm's law.
At any rate, as I said, my numbers are actual measurements and not theoretically calculated values. Also, these measurements are for a 5 cell packs.
Regardless, the point remains that the lower the internal impedance the lower the voltage drop under load.
I also did some measurements when I was setting up my 33% Extra and did end up with comparable results. The cells were 2300 Mah AA at 6V. I saw more than 1 volt after an extension at 2 amps. These cells were of 25 miliohms resistance. Performed the same test with heavier less capacity and more expensive pack to see that the voltage drop was MUCH lower (internal resistance of 4.5 miliohms only). Actually, those cells deliver more Mah under load than the 2300. The 2300 cells don't deliver more than 1900 at 2 amps and the 1950 4/5FAUP are litterally loafing at 2 amps. I can get close to 2000 Mah even at 2 amps. There are some guys that use these cells in cars and pull close to 40 amps continuous (that's more than 20C).
And yes those are Nimh. New cells are coming out just about every week and some are IMO very useless for us but some are very good. I can't see the point of using Li-po's IMO as those cheaper Nimh do the job nicely and are safe to boot.