Join Date: Dec 2001
Location: Belleville, Illinois
Thanked 22 Times in 17 Posts
Re: Dual Alternators
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Most likely one's doing all the charging in that case too.
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When a battery is really low either alternator by itself might not have enough output to bring the voltage up to the regulator set point. In that case they will both put out as much as they can. That situation is likely to last a very short time however.
All voltage regulators are set about the same, but they aren't exactly the same, even if they're supposed to be. Whichever alternator has the higher set point will do all the work. The other will see that the system voltage is higher, even if only a hundredth of a volt, than its set point and so will shut down.
That won't cause a problem by itself, but the complexity of wiring, belts and mounting won't buy you much either. You will have more output, but you can get that with a bigger alternator. You will have redundancy in case one fails, but the complexity is a poor tradoff IMO. Plus, there's the issue of maximum charging rate.
I have an ammeter in my Jeep with a 56 amp Delco. I almost never see the alternator putting out full capacity. The only time is if the battery is really run down from leaving lights on or something like that, and then it only puts out for a minute or two before it starts backing down.