B+ for a new al­ter­na­tor, but D+ for wiring

Canal Boat - - Ask The Experts -

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

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