Most HSD systems have batteries that are sized for maximal boost during a single acceleration from zero to the top speed of the vehicle; if there is more demand, the battery can be completely exhausted, so that this extra torque boost is not available. Then the system reverts to just the power available from the engine. This results in a large decline in performance under certain conditions: an early-model Prius can achieve over on a 6 degree upward slope, but after about of altitude climb the battery is exhausted and the car can achieve only 55–60 mph on the same slope. (until the battery is recharged by driving under less demanding circumstances)