I'm now the proud papa of a used British "Valiant Roamer", courtesy of EBay: https://www.ebay.com/itm/283072424577
Never heard of these until someone asked about one in SERVO's "Mr. Roboto" column. This is the first version AFAIK. http://www.valiant-technology.com/uk/pages/roamer_home.php?cat=1&1
It uses a classic 6V SLA battery, which I'm prepared to replace. These 4A types can be found online cheap, until you add shipping for such a heavy battery. Home Depot has these for $8.69 (and several other SLA types), with free ship to your local store. https://www.homedepot.com/p/UPG-SLA-6-Volt-F1-Terminal-AGM-Battery-UB645/203770472
These batteries aren't high tech, but they are still useful for some robots. Maybe not flying drones.
Chargers can be had for as little as$4... https://www.ebay.com/itm/1-pcs-6V-Volt-500MA-Sealed-Lead-Acid-Rechargeable-Battery-Charger-US-Plug/322939016979