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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