To power the 1st generation of quadruped robot cat Nybble, you need two 14500 Li-ion rechargeable 3.7V batteries and a smart charger. Two batteries can provide 45 mins of playing time.
Be careful with fake batteries! Don't trust batteries that claim a capacity larger than 850mAh.
We have been using EBL Li-ion batteries and they work well. If you are in the US, you can buy the EBL batteries or the battery + charger combo from Walmart.
Note that we no longer offer Nybble V1 for sale. So you don't need to buy any battery if your Nybble is newer.