Run a servo Nimh battery vs Alcaline batteries?

Question

I was now about to power a microcontroller (3.3V) and a servo (HS-755 HB) externally via batteries. The target is to have the max. life time. The servo will work with 4.8V or 6v as the max. Torque changes respectively. After researching about batteries I have identified two feasible options (considering price, mobility and space) disregarding the mAh value of the batteries.

Option 1: 4x 1.2V AA Nimh batteries --> 4.8 V

Option 2: 4 x 1.5V AA Alcaline batteries --> 6V

Currently the advantage I see at the Nimh batteries is that they supply an average 1.2V throughout most of their discharging life time whilst the voltage of alacaline batteries drops quickly. As I have followed this discussion (see link below) it is said that the Nimh batteries will go below 1.1V after 70% usage whilst the alcaline will go below 1.2V even after 50%. I guess my servo will not work if the Voltage drops <4.8V.
Is my choice of Nimh batteries the most beneficial one considering maximum time of usage for me as long as the torque provided with 4.8V is sufficient? Is there any mistake in my assumption? Replacing 6 series AA alkaline batteries with NiMH batteries – how do I compare possible arrangements to optimise usable capacity?

dathasheet of the servo: http://cdn.sparkfun.com/datasheets/Robotics/33755.pdf

Answer 1

Some of your numbers are a bit off.

A NiMH battery on average has a median voltage of 1.2V, but will happily charge to 1.4V or more and should be considered empty at about 1V.

An Alkaline Battery, on average (many different brands and build qualities) has a median voltage of 1.5V (which incidently does lead to the 6V you mention for 1.4V?), but can be 1.6V fresh from the pack and should be considered empty at 0.8V.

Now, alkaline's have the awesome advantage that you don't have to care about empty, because you can't charge them, so nobody in the world cares if you discharge them to 0.3V.

They have the equally awesome disadvantage of a higher internal resistance compared to NiMH batteries. The internal resistance will limit the peak current your servo can draw, or else your uC will reboot because the voltage drops too much.

Traditionally Servo driven RC applications have used NiCd and NiMH for as long as I can remember. Always consider the original source of whatever you are using, it will help you weigh the options.

Because what is 4.8V, the most traditional RC voltage? It is exactly 4 times the median NiMH voltage. That's not a coincidence.

As such I fully expect your servo to still work at 3VDC, as any old-timey servo would as well, to allow for fully using up most of the energy in whatever battery you use. The 6V allowed maximum, then, is just to allow you to also use Alkaline batteries (since because of their total internal resistance, even if they are 1.6V out of the pack, once in the device they will drop to 1.4V very quickly).

So, for highest torque and lowest losses you should use NiMH, but if you then want to use NiMH for as long as possible, you have to make sure something in your system checks the main voltage stays above 4V and warns you or shuts down if it drops below that. Because discharging a NiMH to 0.5V will severely limit the number of times you can charge it.