If you puta second battery in and wire up in parallel instead of serial this should double the drive time, you’ll also need to charge it for longer. Connect the positive of the internal battery to the positive of the external battery, and negative to negative too.
Apparently we aren’t using the batteries to their max capacity, someone (forget who, sorry) reportedly peak charged a single battery, took 30 minutes and the car ran for near 40 minutes.
One custom mod I’d love to do is make a limo and fit it with two AAA’s (extend the wheelbase of the car somewhatlike the 3 battery mod), that way you can take them out and charge them in a normal charger, you could also wire them inparallel for drive time (I think you’d get bored after an hour…) or in series for speed.