The robot has a space at the back to hold a battery of the size of a standard PP3 9 volt battery (approximately 50 x 17 x 25mm). Having said that, a standard PP3 zinc or alkaline battery is not a good choice as it is unable to deliver the high current required for the motor, and its voltage drops quickly down towards 6 volts.
A rechargeable battery of PP3 form or size and associated charger is a much better option.
These are the main options available
- NiMh rechargeable PP3 format. Typically 200mAh capacity and weighing 36 grams. Available from DIY stores such as B&Q or online for about £5 but needing a separate charger.
- Lithium thionyl chloride (Li-SOCl2) rechargeable PP3 format weighing 29 grams. A 3 cell lithium battery giving around 10.8 volts and a capacity of around 1200mAh, available from Farnell for around £10. This has a much higher capacity than the other ones and is lighter as well, but requires a specialist Lithium battery charger.
- Lithium ion high volume 9v battery. Typically of around 600mAH capacity and weight of 26 grams for the one from EBL. Often sold in a pack of 4 for £18 but needs a separate specialist charger.
- As above but available in 280mAH capacity in a pack of 5 with a charger for around £25.
- Lithium ion USB charged 9v battery. Typically of around 400 to 500mAH capacity this battery has internal circuitry that delivers a steady 9 volt output but can be recharged from a standard USB 5 volt mobile phone charger. These are available online for around £10 each
Any of the above are likely to be suitable and have adequate capacity, but the option 5 is a good choice because of the steady voltage produced and the ease of charging.
It is useful to have two batteries at least so that one can be being charged while one is being used