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Old 05-14-2021, 01:22 PM   #31 (permalink)
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Originally Posted by Isaac Zachary View Post
If an alternator drags down the fuel economy by 10%, half of which is inefficiency, if you made it 99.9% efficient then you'd only increase overall efficiency by about 2.5%.

I do question whether alternator technology really hasn't improved over several decades. Are modern alternators really only 50% efficient?

As far as the battery goes the lead acid battery is tried and tested. You can make a lithium battery work. But now it needs thermal management, both cooling and heating, plus electrical management because they need to be balanced and if you leave your lights on you don't want a lithium ion battery discharged below it's threshold. Not to mention lithium ion can't be used in a 12V system, so you'd have to use another voltage and you couldn't use common 12V parts.
It would seem to me that if a 50% efficient alternator drops MPG by 10% and you could improve that efficiency to 99.9%, the alternator would then use only half the power from the engine and it would only drop the MPG by 5%. Could be wrong though.

Obviously making an alternator 99.9% efficient wouldn't be possible, but if the efficiency could be improved from, say, 50% to 80-90%, that could still show a measurable fuel savings and be worth doing depending on what the drawbacks are. My theory is since at least to the best of my knowledge the EPA's MPG test cycle doesn't include accessory use, the gains on the test cycle wouldn't be significant enough to be worth the cost or potential downsides.

Of course in the real world the efficiency of an alternator would become more important the more load it's under. It's not too uncommon for an alternator to be under a 700W load on more modern vehicles (50A X 14V). If the alternator is only 50% efficient then 700W of power is being wasted as heat, which I would consider pretty significant as that would require almost 1 extra HP from the engine to overcome the inefficiency.

Mechman alternators use a 6 phase design that they claim is more efficient than a standard 3 phase alternator by somewhere around 10-20% if I remember correctly. I put one of those on my buddy's Bronco because he has electric fans and a big sound system and it does run noticeably cooler than the stock alternator, but I can't say for certain that it runs cooler because it's more efficient as it probably has a better cooling fan also.

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