Split­ting the charge

Canal Boat - - Back Cabin: -

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.

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