The kids lego EV3 robot takes SIX AA batteries, and those last for all of an hour if you’re lucky. Lego sells an appropriately priced rechargeable battery pack, for ONE HUNDRED dollars. Further investigation shows the lego battery pack is just two 18650 cells and a charging circuit. I have 18650s, and a charger. Bit of trial and error, some pop can strips, bit of wire, and some lego, and we have a serviceable battery pack.
