I'm using 2 X 18650 3200mAh batteries with my DIY Nybble. Mine are Tanuoyi brand purchased on Amazon. They work fine and seem to last ok but not great. When I start having weird behaviors I know it's time to swap them out and recharge them 😉 (go to Amazon and search for "TANUOYI 4 Pack Battery for 3.7v 20A 3200mah")