charging marine battery off car?

Yes, that works very well with a proper isolator. They make them to handle mis matched batteries so you end up with both batteries fully charged.

You should check with RV shops for the setup and it is easy enough to put some quick connects for welding cables at the marine battery end so you don't need tools to plug in the battery. The trolling motor's cables can also have the quick connects.

I know places like Warn (just because I have a winch) sell the cable kits with the quick connects for putting a winch at the back of the vehicle or plugging in booster cables. Something like that with an isolator instead of a switch would be perfect for having the battery in the back charging up.

That would work OK but nothing is free and there's no 'unused output from the alternator'. It might be small but there's still a cost (in fuel, in this case).

Right, but very, very small. A 10-amp draw, for instance, equals about 140 watts at the battery's charging voltage, and since 746 watts equals one horsepower, that 140 watts requires another 0.19 hp from the engine. The engine will burn no more than 1/10 of a gallon of fuel per horsepower per hour, and prtobably a lot less, so that 140 watts will require at most an extra .019 gallons per hour. Your mileage will go from, say, 25 mpg to 25.019 mpg or a tiny bit more than that if we factor in the efficiency loss of the alternator.

Dan

See that! I made an error. The mileage drops from 25 mpg to

Dan

But the poster wants to charge it while he's driving down the highway in a mis-matched parallel set-up. That is not the best thing to do unless there's an emergency.

This is a common practice to charge a starting battery and deep cycle in parallel.. what would be the problem?

Jim

It is better to use an isolator, so that the fully charged battery is not discharged into the flat one. The electronics to do it are cheap and simple.

Thanks for the replies, but I don't understand why it costs extra fuel. The alternator belt stays connected turns the alternator whether it's charging anything or not. You are just using wasted power to charge the extra battery, are you not?

It causes extra load because the more current the alternator has to deliver, the harder it is to rotate.

No, you are NOT! I wish I knew where this crazy idea came from and why it keeps popping up all the time. The alternator does NOT waste a bunch of power when its not needed!!!

When the battery is full, the alternator is just supplying the regular electrical load of the car. But when it is charging a dead battery, it is putting out a lot more power, and therefore it draws much more power from the engine. In simple language, the more current the alternator has to put out, the harder it is for the engine to turn it. You can hear a lot of small car engines slow down when you turn the headlamps on at idle- the load the alternator puts on the engine is NOT constant!

Get a little permanent magnet motor. Spin it, and feel how loose it is. Now short out the leads. Spin it, and you'll feel that it isn't so loose any more.

The mechanical resistance depends on the electrical load.

--scott