WORK­INGS

Soundings - - Contents - BY ROGER HELLYAR-BROOK PAUL MIRTO IL­LUS­TRA­TIONS

You know your alternator is a vi­tal piece of equip­ment, but do you know how to size one for an up­grade?

If the size of a boat’s bat­tery bank is in­creased or the type of bat­tery is changed, the en­gine alternator might have to be up­graded to a higher-amp model. The alternator up­grade must be cal­cu­lated us­ing the bat­tery bank’s ac­cep­tance rate so the charge rate matches. This cal­cu­la­tion is more dif­fi­cult to de­ter­mine on sail­boats be­cause most sailors pre­fer to limit en­gine run­ning time, which typ­i­cally re­sults in deeper dis­charge rates on the house bank.

In gen­eral, the alternator should be ca­pa­ble of de­liv­er­ing rated out­put equal to the max­i­mum ac­cep­tance rate of the house bat­tery bank. This means the alternator’s rat­ing should be ap­prox­i­mately 25 per­cent of the to­tal bat­tery ca­pac­ity for flooded bat­ter­ies, 30 per­cent for gel and 45 per­cent for AGM. When siz­ing the re­place­ment alternator, con­sider the type of bat­tery, cu­mu­la­tive amp hour rat­ing and en­gine time to recharge the bank. Th­ese con­sid­er­a­tions will pre­vent over­load­ing (and over­heat­ing). Con­ven­tional flooded-cell, ab­sorbed glass mat, gel and thin-plate pure lead bat­ter­ies have dif­fer­ent bulk-charge rates when dis­charged, which means they may “ask” for more or less cur­rent — mea­sured in amps — for a sat­is­fac­tory recharge. Th­ese cal­cu­la­tions must be done be­fore pur­chas­ing a re­place­ment alternator.

The stock alternator pro­vided with marine en­gines is sim­i­lar to those used in au­to­mo­biles, with an in­ter­nal reg­u­la­tor and a lin­ear out­put. The faster it turns, the more

cur­rent is avail­able. A cus­tom alternator with a “smart” reg­u­la­tor can max­i­mize charg­ing at less than full en­gine speed. It also can be tai­lored to suit the volt­age re­quire­ments of the bat­tery type for charg­ing.

As the elec­tri­cal load in­creases, the me­chan­i­cal de­mands in­crease pro­por­tion­ally, so you may need a larger or dif­fer­ent type of belt. Cal­cu­late pul­ley sizes so the alternator speed is at the man­u­fac­turer’s rec­om­mended max­i­mum at your en­gine’s full-throt­tle rpm. This will al­low the alternator to pro­duce as much cur­rent as pos­si­ble. I have seen in­stal­la­tions where the pul­ley was too big, and it negated the ex­pense and in­stal­la­tion of a larger alternator.

Al­ter­na­tors are rated cold and im­me­di­ately ex­pe­ri­ence a re­duc­tion in out­put upon get­ting hot, so size them ac­cord­ingly to al­low for this. The cool­ing fans typ­i­cally move air through the unit from the rear and out the front. Some alternator fans are di­rec­tional, so check with the sup­plier to en­sure ad­e­quate cool­ing.

Roger Hellyar-Brook runs a marine con­sult­ing busi­ness, re­pair­ing and up­grad­ing boats of all types. He has spent more than 40 years in the marine in­dus­try and is the former man­ager of the sys­tems pro­gram at The Land­ing School in Arun­del, Maine. Paul Mirto is a dig­i­tal il­lus­tra­tor, long­time boater and former Coast Guards­man. mir­toart.com

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