Step 3: Prepare the Battery
There are 2 sets of 4 batteries in the stock R2, 4 x AA for running the controllers and 4 x D for driving the motors. The IOIO takes 5-15V and 4 rechargeable D cells would give me 4.8V, and I wanted to make sure that the voltage will be able to power an Android phone off of the IOIO USB port eventually, so I opted for an 8 x AA battery holder, for 9.6V that fits perfectly into the compartment.
I drilled a 3/8" hole through to the back of the compartment and threaded the female barrel connector through.
With 5V-15V in through the JST connector, the IOIO delivers either 5V or 3.3V out through the pins and 5V out through USB to power a smartphone device. Although the IOIO can theoretically supply 1.5A at 5V, my brother-in-law fried an IOIO running fairly small motors directly off an IOIO, so I decided to splice into my battery output and run a direct 9.2V wire to the Motor Driver.
Note: The increased voltage overdrives R2's motors a bit, so he's faster, but it could eventually burn them out - more testing required.