[Ryland]

motorcycles and cars vary quite a bit in electrical loads, with a motorcycle you normally don't have a big radio or dual head lights or heater fan, and if you do on a motor cycle then they tend to have a car style alternator that can vary the output, they can also turn the field winding off once the battery is charged, or while you are doing something like starting the engine, the design that is used on cars also tends to operate under a wider range of speeds, allowing it to charge the battery at an idle and not over charge it while going down the highway.