The windlass motor really likes to see Voltages as high as possible. If you use it a lot, then it's better to have the engine on and putting out enough juice to keep the voltage at the windlass (under load) above 12.7 volts. It's not that you can't raise the anchor under battery power alone -- it's that it's not very good for the "starter motor" that is driving the windlass. Forgeting the Watts = Amps x Volts issue.