B+ for a new alternator, but D+ for wiring
Q I have fitted a new alternator (pictured) to my boat. I was told that all I need to use is a positive battery wire from the terminal marked B+ to the battery positive, but it doesn’t charge. Do I need to add a wire to the battery warning light terminal? If so, does there actually need to be a warning lamp?
DANIEL RIDDLES, via email
A TONY REPLIES: The basic wiring requires the B+ to connect by a cable thick enough to carry the full charging current to a battery positive. To comply with the Boat Safety Scheme it should run to the switched side of the master switch), or to a diode charge splitter (if you have a split-charge system).
The D+ requires a cable run from the warning lamp. This can be a thin cable because it should never carry any more than the warning lamp current. Feed the warning lamp from the ‘ign on’/Aux terminal on the ignition switch. If you do this without fitting a warning lamp it will charge, but in the event of a high resistance on the B+ circuit (if a terminal worked loose, say, or you inadvertently turned the ignition off or disconnected the battery with the engine running), you may well blow the field diodes (and need a new alternator). So fit a lamp: not an LED or ‘grain of wheat’ bulb, but a minimum of about 1.5W (or alternatively a suitable resister in parallel with the bulb) to ensure the D+ terminal receives enough current to energise the alternator.
As there is no B- marked, we can assume the case of the alternator provides the negative.
You also have a terminal that looks as if it is marked NIH or HIN. This is probably the feed for the sensing terminal on an electric rev counter, and may be left disconnected. (There is a slim chance that it is a battery sense connection, but battery-sensed alternators are rare – if this is one, connect it to the battery positive).
D+ terminal goes to warning lamp