Hi, I have this question for a long time.
I built a hexapod robot. It uses 18 996R servos. Right now it works great by using the 5V out of a PSU.
Now I need it to be powered by battery. However I do not know how to find a suitable battery for it.
The battery needs to be 5-7 volt but I don't know how to figure out the correct Amp/hour of the battery.
According the specs each 996R the stall current is 2 Amp so how big of a battery I need? And the weight of the battery needs to be under 1 pound and small in size.
Thank you in advance
Well the worst case is that you need 18x2 = 36 Amps for when all the motors stall at once.....
You could put your ammeter in series with the supply though and see what its maximum draw is under practical circumstances. But most meters only go up to 10 Amps.
Thank you for your reply. 
That is my problem, where can I find a battery that has such an high output and light weight at the same time?
Lots of batteries are capable of very big amperages; hundreds of amps is possible, even in small LiPo batteries. NiCd is the absolute best but you don't see those very often these days. I've seen charts for 100A from a single sub-C size NiCd cell.
To work out your AH (amp-hours) you need to know the average power consumption, not the peak. Do you have a way of reading the current consumed from the 5V power supply? That gives you half of the equation: the amps.
Then you need to decide how long you want it to run for. Half an hour? Three hours? Multiply that by the amps and you will have the AH required for the battery. You already know what voltage and that's all you need to go shopping for batteries.
Tenergy make good batteries. I've bought a few of their lithium batteries from Batteries - Chemistry - Li-ion - Page 1 - Tenergy Power. There will certainly be something in that catalog that will work for you.
You can use this
Use three of this to supply 6v to the servo