Au­tomat­ing split charg­ing

Canal Boat - - Back Cabin: Experts -

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.

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