Where I work, the condo complex is full of rich old people. Hence, they often shouldn't be driving anymore in the first place, then to make it worse, they sneak up on ya in a prius. Seriously, it can be a hazard that the car is so silent. There are lots other cars that though audible, you can't hear till they are about to brush you.
Not sure what the answer is, but a back up beeper is sure not the solution. Nor annoying music. People would haul ass to shut off the noise if it stopped above 15 mph. Something in a simulated engine sound would be my choice, and have it continue at all speeds. Maybe allow a selection, Cadilac sound for some, porche sound for another.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.