My recollection from reading the various technical documentation is that the battery is limited to a 5kw (?) maximum charge/discharge rate. When run through the motor-generator to provide motion, this translates to a maximum speed of 42 mph on the straight-and-level. This is also why you can only accelerate slowly from a standing stop (on the level) if you work at staying in EV-mode: the discharge cap prevents the battery from providing a higher power level to the electric motor. My recollection (again) is that the charge/discharge rate is a function of the total charge the battery can take, so a deeper battery could have a higher rate (which would require reprogramming the ECU to take advantage of, of course), but that would mean not just higher speed but better acceleration in EV-mode. I don't recall the horsepower of the MG or the wattage equivalent, but I think the MGs could handle in the range of a 50-100% higher discharge rate - 63mph instead of 42. That's why *I* want a bigger battery - not necessarily to run longer, though I do love rolling in EV - but to be able to go faster in EV and run uphill in EV.