The Offshore Voyaging Reference Site

How To Blow Up Your Alternator

Anytime I write about batteries and charging someone is bound to bring up one of the clever gadgets that fool a stock alternator into charging at a higher current for longer without resorting to external regulation.

Some of these gadgets, particularly the VRC-200 from Nordkyn Electronics, are undoubtedly very clever—here’s another one I wrote about.

And I can certainly see using one of these as a quick and relatively inexpensive way to make the stock alternator charging a small battery bank on a boat used for weekends, and perhaps the occasional week cruise, charge more quickly and efficiently.

But for an offshore voyaging live-aboard boat, these things are not a good idea.

Most alternators that come with our engines will not last long if pushed hard day in day out, particularly if trying to charge a large bank (lithium or lead-acid)—stock alternators are simply not designed for that kind of duty cycle.

That said, the Nordkyn will extend alternator life by monitoring its temperature, but that’s going to mean that most of the time the alternator will not be putting out much because stock OEM alternators heat very quickly as soon as they come under load.

Of course you could use the Nordkyn with a heavy duty alternator, which would be a good combo.

But my thinking always has been, and remains, if we are going to the trouble of installing a high-capacity bank, we might as well do the charging right with a rugged alternator designed for the job, installed right, and with an external regulator that won’t be subjected to the heat inside the alternator.

And then if we are going to do the alternator right, we might as well go the whole hog and do the regulator right too.

Fun Demo

By the way, Victron have a fun demo showing how fast they can burn out an alternator when charging lithium batteries. There’s a lot of good stuff to learn here, particularly the counterintuitive fact that low engine RPM will do more damage.

That said, we offshore boat owners should understand that even a big lead-acid bank can fry alternators too—our 800 Ah at 12 volts (9 kWh) AGM lead-acid battery bank on the McCurdy and Rhodes would happily lap up 250 amps for an hour, at least, if we had had an alternator that big, and regularly sucked 150 amps for two hours out of the alternator we did have.

Anyway, have a watch, it’s interesting:

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Jim Schulz

A picture (video) is worth 1000 words, thanks for sharing. It was especially notable how the low fan speeds at low RPM on stock alternator created so much heat. I have a question that wouldn’t change the takeaway but would help my understanding. Isn’t a 1500 rpm alternator speed about what you’d get at idle with a 2:1 pulley ratio? The narrator says 1500 rpm is higher than idle speed for most boats, but it looks like they were using a 1:1 pulley ratio so effectively they’re measuring alternator rpm, aren’t they?

Anyway, I left with a solid takeaway that even if a stock alternator is capable of producing 60 or 70 amps at low engine speeds it’s still going to be at risk of burning up under regular demanding use. Thanks!

David Branyon

The video left me wondering that exact question: what IS the typical drive speed ratio of an alternator relative to engine speed? And never realized that alternators are that inefficient. In my work world, I deal a little with very large gensets and those alternators are in the high 90’s in generating efficiency. I realize that’s a very different world, but I always just guessed that typical automotive alternators were in the upper 70’s or low 80’s in efficiency. 50%… sheesh. After that, then all the heat problems make much more sense.

Robert Cart

The sad fact is that most people soon realize their original engine alternator is sized for little more than topping up the start battery. They next turn typically turn to Balmar as the gold standard. Truth is the boat market is small so they have done well to capture it. Loads were relatively small and short lived back then. Now things are different. Not only do we have the demand of lithium banks to charge, we are more likely serving bigger loads with more electronics, computers, inverters, air conditioning, induction stoves and even under water disco lights is surely on the way.

Balmar alternators are not actually made by Balmar. They come from a variety of manufacturers. Look at the specs closely and you can find them and match them up. They are really most often not sized for the sustained heavy loads and low RPM of cruising boats.

Moving the regulator off board doesn’t actually help this much. It may increases the life of the regulator circuitry, but that hasn’t been a thing. A good regulator also doesn’t contribute a significant amount f heat. You can move the rectification diodes off and they helps spread the heat load, but the fact is it is better to choose an alternator design for the kind of use you see on a cruising boat. Luckily there are many of these to choose from.

Leech Neville makes some nice ones. I chose Eco Air Eco-Tech alternators. They supply alternators to military applications that need high output for long periods at low RPM and very high efficiency. They aren’t the only game in town. Just look at the specs and compare. Look at low RPM output, look at max RPM, look at efficiency. The high the efficiency, the lower the waste heat. Look at how they are cooled, one fan or two? Large semi tractor trailer rigs, military tanks and other equipment meet more difficult requirements. The video says most alternators can’t handle the low RPM, well, why use an alternator that is not fit for the job? Do we blame the manufacturers or the customers who buy the wrong product? Labeling something “marine” doesn’t necessarily make them better for the task, but surely makes them more costly.