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Peter Kennedy has been in  business since 1991 designing, installing and servicing marine electrical systems. The purpose of this blog is to offer support to both professional installers and  do-it-yourself boat owners who wish to undertake this work themselves.

Peter Kennedy has been in  business since 1991 designing, installing and servicing marine electrical systems. The purpose of this blog is to offer support to both professional installers and  do-it-yourself boat owners who wish to undertake this work themselves.

Alternator upgrade case study

Posted by Peter Kennedy on 2/10/2015 to Alternators

Thanks for your question.  When you are choosing a replacement alternator the availability of a serpentine kit greatly increases your options.  You are no longer limited by the belt and can opt for an alternator up to 200 Amps if needed.   

You may not need to go to the trouble and expense of a serpentine kit though. I would suggest you start by looking at your needs.  It’s not much use having a really big alternator if it isn’t going to be working hard, so you need to look at your battery size;  a battery bank can only take so much charge.  The rule of thumb is that an alternator can charge a battery bank with four times as many amp hours as its output in amps.  For example a 100 Amp alternator would be fully utilized charging a 400 Amp hour battery bank.  (4 x 100 alternator amps) You could further modify the rule of thumb to say that if there were other loads on the system at the same time then they could be factored in to the calculations.  So a typical calculation might be like this:

 Battery amp hours: 400

Typical load on system 20 amps

Alternator capacity (400÷4 + 20) = 120 Amps

In this case a 120 Amp alternator is the biggest you could use to its full capacity.  You could use a bigger alternator but it wouldn't be really working to its full capacity,  it wouldn't hurt though.

 Next you have to look at your belts.  A single half inch belt could support an alternator up to 100 Amps.  A single 3/8” belt could support up to 80 Amps. Any more than that and you have to go with double belts or the serpentine kit.  Personally I think if you are going to go to the trouble and expense of getting a serpentine kit you might as well go and get a big alternator, because even if you don’t use it to its full capacity now you might like to use it more later.  So in that case at least go to a 150 Amp

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