My Droid has an unusual power source: lithium ion cordless tool batteries. I wanted to use a lightweight power source, placed low in the body or in the feet to help keep him stable during the 2-3-2 transition. I was inspired by Andrew to put the batteries in the "Battery Boxes" as they are called, which are attached to the outer feet. The boxes small size necessitated a small pack. Since I had a bunch of Ryobi tools, I decided to start with one of my own packs, cracking it open to see what was inside.
As it turns out, Ryobi batteries are made up of 18650 Li-ion cells from Samsung. These are very high quality batteries, and the same size that Tesla uses in their Model S. If they were good enough for Tesla, they were good enough for me. Four packs would give me 16 Amp/hours of power, which I figured was enough.
The Ryobi batteries also had a cool feature that I wanted to keep. A press of a button lights up a "fuel gauge" that lets you know how much power is left in the battery. I decided to reroute this gauge to the outside of the Battery box.
I designed a battery box that would accommodate two of the Ryobi batteries, and still look like the prop part. Getting both of the batteries to fit in there was a challenge. I added wiring to the batteries that would plug into a new "Fuel Gauge" that I had added to the end of the battery box.
Pressing the black button at the top causes the LEDs to light up, letting you know what the battery's state of charge is. Each battery also has a cut off switch and a charge port.
The control panel with the fuel gauge and side opening of the box are concealed by a magnetic covers.
The other end of the Battery Box has some nice aluminum details bolted on as well as the "Battery Hoses" which connect to each foot.
The completed battery boxes are bolted to the feet. In this photo you can just see the wiring passing from the box into the leg.