Just been talking with a manufacturer of racing batteries for my competition cars .As an aside I learned that they could custom make deep cycle lithium batteries with battery monitor systems , economic quantity 50 sets, at circa £200/set. They would need to know the running time desired, ie range, and the level of interest, to see if its a worthwhile project for them and us.
As an example, a battery with half hour constant running time would weigh only between 1 and 2 kilos,. NB lithium baterries deliver the full voltage until they run out of curent, but their batteries do not experience thermal runaway.They will also endure very many cycles.