So the HiHy factory NiMH traction battery puts out a nominal 288v and about 1.9kWh. What would happen if a guy were to wire in a second battery with a dc/dc converter to, say a couple Chevrolet Volt battery cells (48v and 2kWh each) with a plug in charger? Could add 4kWh quite easily. The Volt battery cells are really quite compact, and I could build a wood enclosure and cover in carpet, just like a subwoofer box, and place it behind the third row seat... theoretically... I think it could drastically improve the mileage of the HiHy in low speed driving. There’s a company that makes such a unit for the Prius, but the Prius uses a 201v battery pack, so I highly doubt their off-the-shelf unit could work in a HiHy. Am I nuts for wanting to build something similar myself or does it seem feasible?

Could easily also add a pair of 48v solar cells to the (relatively flat) roof of the HiHy to charge the two Volt cells for ‘free’ while parked during work or something...

Doesn’t necessarily need to be a GM Volt battery... Nissan Leaf cells are an option as well, can get 3kWh and 48v quite easily/cheap, and even more compact than the Volt pack.