Recharging Under Sail

When electric power is fitted to a sail boat, the propeller is driven by the water flow when sailing and will regenerate a lot of power which can be used to charge the batteries.

However – there are several theoretical problems.

  1. Charge monitoring
  2. Overcharging.
  3. Current and voltage limiting

Battery charge monitoring

Motor speed controllers are designed to control the speed of the motor. Yes, they can and do recharge the battery during braking – but charging is not monitored or controlled. It is simply an energy dump of braking energy into the battery and the fact that this happens to charge the battery is really serendipitous.

To charge under sail you would then have to monitor the battery state of charge and use a signal from this monitoring circuit this to control the controller.


When the battery becomes fully charged: what do you do then? Switching off the controller might seem an option: but the propeller will still be generating voltage and you must consider what the controller will do with a generating motor and an inactive controller. Disconnecting the battery is one option you might consider – but under sail the motor can be generating a voltage higher than the battery. This could in theory destroy an unprotected controller!

However our controllers have overvoltage limiting that causes the controller to speed up the motor when overvoltage occurs, so the braking effectively ceases (so does drag from the propeller). However this overvoltage is often at the peak handling voltage of components such as the main capacitors and these may well overheat and vent out.

It is therefore necessary to either mechanically brake the propeller so it cannot rotate or (probably easier) electrically disconnect the motor from the controller.

The solution

We have outlined the problems above: clearly there are solutions but the solution is on overall system design thing: it must involve mechanical and electrical components. These must

  • Ensure that the generated motor voltage can never exceed the maximum battery voltage.
  • Mechanically or electrically disconnect or brake the motor when charging is complete or when maximum voltage is reached.

Clearly the solution is not a simple one will need significant intelligence so will require control by a computer/microcontroller. If you are a programmer capable of making and programming such a controller then we may be interested in helping you!

It may well be easier to use the speed controller only when under electrical power and to use a totally separate electronic circuit to control charging.