The regenerative braking system that is installed in your Hybrid is designed to recapture the friction that is normally lost during the braking and use that friction to generate a charge for your battery.During its normal use you should never have to worry about the battery getting low enough to warrant any type of concern.If you are driving the car like a teenager that stole it then you may have the battery level deplete to less than 50 % but again nothing to worry about.This system is designed to recharge the battery during braking as well as during normal cruising when there is little demand on the battery assist mode.I notice that you mention San Francisco and if you use the vehicle normally during your up and down hills travels you should have no problem with charging.If the battery capacity meter shows less than 50% no matter what you do then take the car to the dealership on Van Ness and ask them to do a system inspection to make sure a relay has not gone bad in the charging system.