Automating split charging
QI run a 12v/55amp alternator connected to a 90Ah starter battery, and via a simple manual switch (which I turn on only when running the engine) to two 110Ah domestic batteries. Before this, there was a black relay box type thing with a small bolt-on terminal connected to the starter battery live terminal. Another wire ran from another small bolt-on terminal on to the domestic live terminal, both cables 5mm. The box had two smaller spade connections. Have you seen this type of unit and where are the smaller spades connected? STEVE BRADSHAW, via email
ATONY REPLIES… It is a split charge relay that automates your manual switching. The small wires are connected to the relay coil. One of the small wires is connected to any suitable negative. The other is not so clear cut because it depends upon the relay coil data. It may be connected to the alternator warning lamp terminal ( D+) or to the warning lamp wire on that terminal. It might be fed from the alternator rev counter ( W) terminal or it could be fed from the ‘ignition on’ (aux) terminal on the ignition switch. If it’s connected to the W terminal it might hum to itself when charging. The cable seems undersized for a 55-amp alternator so would tend to extend your charging time a bit. The way you describe the wiring, it would burn out any such relay. The alternator lead should run to the domestic battery bank, with the relay used to connect that to the engine bank. Finally, nowadays I would fit a bi-directional voltage-sensitive relay that probably only requires one negative connection. It switches automatically so will allow solar panels to charge both batteries when you are away from the boat.