I am building a dual robot arm for a school project. I have bought everything but the batteries so far. I will need 2 different constant voltage lines, 5V and 12V. One of the rules states I can't is Lead Acid batteries. Each arm will be separate, so I need 1 battery for each that will be enough for one arm. From my calculations, One arm will draw around 3500mA(does that value seem reasonable?) at peak I believe. (6 servos each and a stepper, being controlled by arduino mega). The entire event takes about 4mins to complete. I was wondering what battery route I should take and how I can get the 2 different voltages with efficiency?
That's a bummer you can't use lead acid... would have been easy.
In any case, I suggest getting a 12V battery and using a switch-mode converter to get the 5V as they are much more efficient than using a linear regulator like an LM7805.
As far as what kind of batteries to use... I would suggest visiting a hobby electronics (Remote Control) store and looking at their selection of batteries, or perhaps sources some batteries commonly used for portable power tools.
The selection will include mostly NiCd batteries on the low end, and Li-ion or LiPo on the higher end with small and large packs ranging from 6V to 14.4 being quite common.
Hopefully the clerk at the hobby store is familiar enough to help you think up different combinations of batteries or packs. Use your imagination.
You can quite easily get a pair of 6V battery packs and use them in series to get your 12V. You 'could' put a wire in the middle of the two packs to get 6V which would be easier and cheaper to regulate to 5V than dropping 12V all the way down. (Depending on the accuracy of the 5V you need, you could use one or two diodes which would give you 5.3V or 4.6V if you put them in series with a 6V battery pack.
The size of the pack in mAh will determine how long your equipment will run between recharges. I can tell you right now, that my little 6V battery pack in my childhood R/C car lasted longer than 15 min. and most certainly had a draw higher than 3Amps xD . It used a 'Tyco 6.0V Jet Turbo NiCd' battery... but I can't for the life of me remember the capacity... it was only a pack of 4 AA sized cells inside the over-priced plastic case.
