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Archive for the ‘Boat’ Category


Solar wiring for boat

Saturday, January 24th, 2009

Hi there.

I have two solar panels 80 watt each on my boat wired in series to make 24 volts 160 watt. I have two sets of two batteries also wired in series 24 volt. Start and house batteries. I also have a Morning star 24 volt 10 amp controller.

When I leave the boat I would like to charge both sets of batteries. When using the boat I would like to charge the house batteries only as they will be running a fridge etc and the start batteries will be charged via engine Alternator unless it fails at sea. So I will need a switch for off, battery 1, battery 2, or both.

Is there anyone that can help me with a circuit diagram required for this project?


Simon Brock


As noted on the web site, we cannot provide specific wiring diagrams and other detailed design assistance since we do not have all of the information about your specific application, and this is a free site so time is very limited for each reply.  However, I can give you some basic guidelines.  You have to be careful because if one battery gets really discharged, and the other is fully charged and you close a switch between them, there can be a huge in-rush of current from one to the other that could easily melt any smaller wires between them.  There are 2 ways to charge these batteries separately without tying them together and have a potential melt down.

You can buy a battery isolator at the RV store which has a connection for a charger or alternator input, and two separate outputs to connect the 2 batteries.  This device has diodes that will allow charging current to flow from the single solar charger or alternator out to 2 separate batteries, but will not allow one battery to flow back to the other.  Although intended to connect one alternator to 2 batteries, you can use the solar charge controller as the charging source.  Make sure the isolator has a common negative and one in and 2 outs.  Just let both batteries stay connected to the solar charger.  If one battery starts to get charged separately from the engine, when it raises the voltage of that one battery, the higher voltage will stop the flow in from the solar charger so all of the solar charging will divert to the battery not being charged by the engine because its voltage will be lower.  You would not need any switches to do this, but your connection should be on the battery posts so the existing 1-2-both engine switch will not isolate the charger from the batteries.

The alternative is to use two identical solar charge controllers with one to each battery, and both connected to the same solar modules.  The battery with the lowest voltage will receive most of the charge until they are close, then the charge controllers will be charging equally and sharing the same solar modules.

Be sure to install a DC rated fuse in each charger output connection to each battery.

Good luck and ahoy!

Jeff Yago



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