Cliveshep
Well-Known Member
Don't know if this will help: Years ago when wiring the car towing sockets for caravan I was concerned that the leisure battery in the 'van be charged by the car but not flattened by starting the car. A Lucas tech tipped me off to a connection method that was simple yet foolproof. It's this (assuming I still remember it correctly):
Take the negative supply from the starter solenoid connection between the solenoid and the starter button/switch, take the positive from the charging side of the alternator, or battery positive. With engine running the solenoid is not energised and therefore negative via the solenoid coil to ground. However, during starting the solenoid feed becomes positive when the solenoid is energised by the starter switch/button, the two positives cancel each other out. That should effectively provide a charge to battery 2 without imposing engine start loads onto it. (I hope!) No doubt someone here will improve on this or scorn it altogether. A separate switch can then be installed in one of the two charging cables to the leisure battery and used at night to isolate the engine battery from any leisure loads. It means that the alternator is not at risk yet effectively charges both batteries when the engine is running. I found that the system worked well on several cars and I see no reason why it shouldn't work equally as well on a boat.
Take the negative supply from the starter solenoid connection between the solenoid and the starter button/switch, take the positive from the charging side of the alternator, or battery positive. With engine running the solenoid is not energised and therefore negative via the solenoid coil to ground. However, during starting the solenoid feed becomes positive when the solenoid is energised by the starter switch/button, the two positives cancel each other out. That should effectively provide a charge to battery 2 without imposing engine start loads onto it. (I hope!) No doubt someone here will improve on this or scorn it altogether. A separate switch can then be installed in one of the two charging cables to the leisure battery and used at night to isolate the engine battery from any leisure loads. It means that the alternator is not at risk yet effectively charges both batteries when the engine is running. I found that the system worked well on several cars and I see no reason why it shouldn't work equally as well on a boat.
