Step 2: Hardware part - power supply
In the prototype, battery consists of 4 AAA Ni-MH cells as AA Ni-CD cells wouldn't fit.
I use universal charger intended mainly for RC models to charge Ni-MH cells but if you possess only charger for AA Ni-Cd cells then I'd suggest to stick to these.
Raspberry operates safely on 4.8V so 4 AA or AAA 1.2V calls may be used.
You need a micro-usb connector to power raspberry as designed.