Can you overload an alternator

[reactive]

Please can you help me understand something.

In the 4x4 world we often install a second battery in a vehicle to power fridges etc. This is normally connected to the charging circuit via an solenoid when the engine runs. If the second battery is very flat a very high charging current could flow.

If an alternator is overloaded the voltage will start to collapse to maintain a constant max. current. The question is can the alternator supply this max. current indefinitely or is an alternator rated below it's max. current. Put a different way, is it possible to overload an alternator.

Thanks

 

[MikeHalloran]

In days of yore, when alternators had mechanical regulators, the regulators included overcurrent protection, so you couldn't kill the alternator by connecting a dead or near-dead battery.

In more recent decades, I have experienced alternator failures, especially in "high efficiency" alternators, that were blamed on dead batteries. Not even auxiliary batteries, just the regular one. Note that modern "high efficiency" alternators are much lighter and much smaller than similarly rated alternators of yore. ... and they obviously also contain less copper and can pass less cooling air even if the fan is working as well as it can.

So I'd have to say that it depends on the particular alternator and its regulator. If you paid extra for "heavy duty", it shouldn't be a problem, except that "heavy duty" is really undefined, so you could still be stuck in the boonies.

If your alternator looks like a scale model (or the one in the Miata), then you might consider replacing it with something sturdier, or finding ways to treat it gently, like limiting the charging current by some means.




Mike Halloran
Pembroke Pines, FL, USA

 

[thruthefence]

Yes, it is possible.

If what you mean, is a catastrophic explosion, or some-such, that's not likely, but the diodes in the rectifier pack have a finite load carrying spec, and it that's exceeded for long, they will fail. One will fail, and while it (the alternator) may put out, it won't be to buss voltage, and if the load continues, the others will fail shortly thereafter.

 

[TheBlacksmith]

The alternators typically have a duty cycle limitation. This may be 10-20% at rated load, if you have a good battery and crank the engine hard in really cold weather, the battery will demand a lot of energy but it will taper down relatively quickly, so it works. If however, your battery is old, or you've added a battery, you may stay "above the duty cycle" long enough to overheat and damage the diodes or probably the field winding.

 

[patprimmer]

I have seen windings turn blue and diodes fail when the continuous electrical draw exceeds the alternator rating, generally from addition of after market high powered lights.

Regards
Pat
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[MikeHalloran]

Yeah, but you had to love those aircraft landing lights....

Mike Halloran
Pembroke Pines, FL, USA