Splitting the charge
QMy engine has two alternators: one for the engine start battery, the other feeds a Sterling split-charge diode supplying the domestic battery bank and the fridge battery. This alternator is energised by a 12v 30A four-pin relay.
One morning, the second alternator’s warning light would not go off. A passing boater said he could stop the problem and moved the two separate outputs (on the pair of split-diode studs) onto just one. However, I would now like to return the system to its old configuration by replacing the defective relay and returning the cables to their original positions.
Can you confirm 12v 30A is adequate, recommend a suitable relay, and clarify what should be connected to each of the four connectors on that replacement relay? GAVIN PADGET, via email
ATONY REPLIES: If the relay is wired as I think it should be, then it should pass less than one amp so it is more than adequate. But I do not think there is a problem with the relay; even if there was, any automotive relay should do the job reliably – but do not mount it on the engine.
What the boater did and your symptoms made no initial sense, but I think I know what is going on: one side of the split-charge diode has failed ‘open circuit’, so the battery bank that the relay is connected to is no longer receiving a charge. The alternator will then be trying to charge that battery via three little field diodes and ‘back-feeding’ through the relay. Luckily the warning lamp will limit the current and protect the diodes.
Sterling have made two sorts of split-charge diodes, a simple one and a ‘zero volt-drop’ version – a box of electronic tricks. If it’s the former, test it using a multi-meter on the continuity (‘beep’) setting. Connect the meter leads to the alternator connection and to one of the battery connections on the diode, then reverse the leads. One way round the meter should beep and the other stay silent. Do the same for the other battery terminal. If on either half of the diode the meter beeps both ways or stays silent both ways, the diode is faulty.
If it’s a ‘zero volt-drop’ one, I doubt you can test it yourself. I would advise changing it, and changing the wiring to use a decent quality voltage sensitive relay because they will be far easier to test in the future
Incidentally, the three-bank solution you have is not optimum for battery performance, it complicates things and is arguably detrimental to battery life. One big bank for the fridge and domestics is usually considered much better.