Using a 12vdc Alternator to Charge a 24vdc Battery Bank

[Andina Marie]

Since they are 24 volts you must size the batteries so they have sufficient capacity to do all the desired tasks without relying on help from the engine alternators or line charger. Estimate (or measure) average current drawn and multiply by the worst case time they will have to run (in hours) to get an amp-hour rating. For good battery life you should not discharge below 50%, especially in 24 volt systems so double the number to get the minimum needed battery capacity. Any increase over that will more than pay for itself in extended battery life if you have the space and weight is OK.

If you run the genset normally when underway a 2 bank charger will recharage the batteries.

A Trollbridge24 would charge the batteries from a 12 volt line coming from the starting battery but you are correct, you would not be able to run the windlass and the thruster at the same time without overloading it.

I've never used one but I understand there are some 12 to 24 volt voltage doubler converters you can use to charge the 24 volt battery. Since they just double the incoming voltage you don't need a regulator because the 12 volt supply is already being regulated.

[Florida Mike]

Steve, the simple way to handle your 24 V needs for a bow thruster and winless is to install a series parallel switch. It is a starter solenoid type of an arrangement that was designed for tractor-trailers several years ago. It basically keeps one twelve volt battery as a system battery and takes the other 12 V battery in an out of series/parallel as needed. You also need to consider the size of the wire running from the 12 V charging source to those batteries in the front, primarily to limit the amperage flow while you’re using power up there. One way to do this is to put a high current low ohm resistor that would limit current flow to the maximum your can handle during almost a dead short. Also although I don’t particularly care for this circuit it will work; 24 Volts from 12. Just my two cents, Mike.