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Old 09-12-2006, 01:43 PM   #1
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Calling all EV experts...or just knowledgeables in general

Why don't EV (er, electric vehicles) have an alternator? Couldn't one be used to, uh, recharge the batteries....so the car could actually stand a chance at getting a decent range? Sorry to sound arrogant, but I haven't been able to find the answer through my buddy, Google. Contrary to popular belief, he doesn't hold the answer to everything.

**I have a co-worker who wants to swap gasoline to electric. He's been checking out different websites, and vehicles. He (nor I) can understand why an alternator(read: recharge device) cannot be incorporated into the function of the engine to keep the battery charged up. I am 100% certain there is an explanation for this. I just haven't found out what it is yet. Can one of you help?
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Old 09-12-2006, 01:52 PM   #2
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Some EV's have alternators running off the tail shaft of the drive motor to keep the 12V accessory battery topped up.

But, I think you're asking whether an alternator on the tailshaft could in turn recharge the main battery pack.

Essentially that would make it a perpetual motion machine.

In reality, it takes more energy to run the alternator (generator, whatever) than you get back from it from conversion losses: electric power - to mechanical power - back to electric power, with energy lost in every step of the conversion.
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Old 09-12-2006, 02:06 PM   #3
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hey "theclencher", see that...I'm not so stupid after all. Okay, MetroMPG, I didn't realize the energy loss was so severe. So this must've been experimented with, right? I know, I know. If there are all these EV's out there getting the same lousy short range that they all do there must've been a reason why there's no recharge-on-the-fly method, right? Otherwise we'd all be driving them. Just didn't know what the reason was. But "thank you". You've answered my question.
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Old 09-12-2006, 04:13 PM   #4
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Quote:
Originally Posted by Brian D.
If there are all these EV's out there getting the same lousy short range that they all do there must've been a reason why there's no recharge-on-the-fly method, right? Otherwise we'd all be driving them. Just didn't know what the reason was.
The Alternator in an EV is the motor itself and a thing called REGEN is used to charge the batteries where the motor is turning as if a generator and the controller pumps the output of the generator back into the battery and you get braking action. SO if you want a quick charge you hook up to a truck and let it pull you for a while and ride the REGEN brakes to charge up or simply go down a long steep hill. Typically a car alternator is about 60% efficient - that's one of the reasons that they have lots of cooling in them - EV motors are more efficient than that and have higher power output.
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Old 09-12-2006, 05:17 PM   #5
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Quote:
Originally Posted by theclencher
Serious!?!?
Yep. It's not S.O.P., but some cars have it (conversions, not production EVs).

Quote:
Wouldn't it make more sense to simply run a circuit from the propulsion batteries to the aux battery if that was the concern?
Generally, yes. You'd use a DC-DC converter to take the traction pack voltage (e.g. the ForkenSwift will be 48v) down to the accessory battery voltage (14v - ish).

I suspect DC-DC converters are more efficient than the alternator route.
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Old 09-12-2006, 05:58 PM   #6
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As metro said, a DC-DC is the norm (no moving parts, more efficient, smaller, lighter, etc.) as shown here:


Also, there is a fair amount of complexity (and expense) in rigging regen braking which is missing from the above circuit (typical of controllers for DC series-wound motors).
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Old 09-12-2006, 06:17 PM   #7
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Hey, I recognize that drawing! http://evconvert.com is an awesome site.

Sadly it's been in limbo in recent months since the site owner had a death in the family.
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Old 09-12-2006, 06:22 PM   #8
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Yeah DC Series wound motors don't do regen very well and also are not as efficient as a good brushless motor only the guys that use them don't know what "HIGH" efficiency is just like some people don't know what "HIGH" mpg is.
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Old 09-12-2006, 06:29 PM   #9
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Metro: yes, I hope Jerry is doing OK.

As for the motor dilema, whether AC or DC motor, you'll still need a sophisticated PWM controller that can put your peak power to the motor, and also dump the regen current back to your storage devices when braking. BTW, batteries don't recharge very quickly, so much of the regen current will probably be lost: another reason why many conversion designs don't bother. If you wanted to use ultra-caps for the instantaneous regen and corresponding instantateous traction current, that might work but it'll add a lot of electronic complexity.

A highly developed AC motor/controller setup (e.g. Siemans) can easily run $10K.
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Old 09-12-2006, 09:25 PM   #10
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Why hooking an alternator to a motor to recharge the battery that is running the motor in an electric car will not work is explaned in the second law of thermodynamics, or more so, entropy.
"in all energy exchanges, if no energy enters or leaves the system, the potential energy of the state will always be less than that of the initial state."

so far we have not learned to brake the laws of physics, altho some have tried, but the physics police have caught them.
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