# Why don't/can't you charge batteries from an AC motor running an alternator?



## ElectricVirgin (Mar 27, 2011)

I've always wondered...

Why don't people just use a small AC 110v electric motor to power a standard "off the shelf" 250 amp 14.4 volt automotive alternator to charge battery packs?

It's less than $300 bones for a high quality alternator and another $500 for an electric motor to run it. 

I think I read somewhere that alternators just start free spinning when they sense the voltage has reached 14.4.

What am I missing?


Please be gentle. I'm a virgin.


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## frodus (Apr 12, 2008)

Because of low efficiency.


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## ElectricVirgin (Mar 27, 2011)

frodus said:


> Because of low efficiency.


I'm not sure I understand what you mean. The least expensive Manzanita charger is $2,000.

Are you saying that an electric motor/alternator charging system cannot provide as much energy to the power pack as a $2,000 Manzanita? 

Or are you saying that it cost more money in electricity per hour to run an electric motor/alternator to charge a battery pack than it costs to run a Manzanita charger... 

Thanks.


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## rwaudio (May 22, 2008)

ElectricVirgin said:


> I'm not sure I understand what you mean. The least expensive Manzanita charger is $2,000.
> 
> Are you saying that an electric motor/alternator charging system cannot provide as much energy to the power pack as a $2,000 Manzanita?
> 
> ...


The Manzanita is fully adjustable as far as voltage is concerned, exactly how do you plan on charging a 96v to 310v (common range for EV's) battery bank off of a single 14.4v alternator? If you had a common 144v pack (nominal voltage) for example you would then need 12 alternators hooked up to that motor to charge the battery bank, at 288v you need 24 alternators etc. This quickly becomes insanely expensive not to mention huge.

If you are looking for an alternate charging idea have a look at the dc/dc converter based charger that I'm building (on my blog, or in my build thread). A few others on here have done the same thing with good results.


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## frodus (Apr 12, 2008)

ElectricVirgin said:


> I'm not sure I understand what you mean. The least expensive Manzanita charger is $2,000.
> 
> Are you saying that an electric motor/alternator charging system cannot provide as much energy to the power pack as a $2,000 Manzanita?
> 
> ...


I mean just that, efficiency
http://en.wikipedia.org/wiki/Energy_conversion_efficiency

power out = power in * efficiency

so, in the conversion of power from your wall to an AC motor, there are electrical to mechanical losses. Then mechanical loss from one motor to the other. Then more mechanical to electrical loss. And that only charges ONE 12V battery. You can't just hook up 14.4V to all batteries, you would create a short.

So, in the same amount of time, and the same INPUT power, you'll get less out of a conversion from electricity to mechanical then mechanical to electrical than you would out of an electrical to electrical conversion.


And 250A and 14.4V is 3600W. Take into account 65% or so for efficiency and you're at ~5500W input. Then lets say mechanical loss is negligable, but then you have the AC motor running at peak efficiency RPM, and its running at 80%. Now you have ~7000W going INTO the AC motor, but only 3600W out. Thats almost 50% efficiency.

Compare that with electrical conversion of power and you're looking at over 80% efficiencies.

So you'd be spending 30% more on energy and take longer to charge, or require more energy in a short period to get it to charge in the same time as a Manzanita.


I mean, give it a shot, but you're gonna need a motor that can produce about 10HP continuous. And I can tell you, its not gonna run on normal 110V service, you need MUCH more current (60+amps).


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## peggus (Feb 18, 2008)

Efficiency as in Energy out/Energy in. 

You can use a motor + generator to charge batteries, it was quite common 100 years ago.

It is big bulky, susceptible to mechanical wear (though can be made very reliable), and inefficient.

If you use a automotive generator and single phase AC motor you'll be lucky to exceed 60% efficiency. A manzanita charger is in the 90%+ range, IIRC.


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## major (Apr 4, 2008)

peggus said:


> You can use a motor + generator to charge batteries, it was quite common 100 years ago.
> 
> It is big bulky, susceptible to mechanical wear (though can be made very reliable), and inefficient.


We've got one in the lab. WWII surplus I think  3Phase 208V motor with a commutator DC generator. IIRC, goes up to like 30 or 40 volts, maybe 75A output. Just used it last summer on a WarP9 to break it in. Running about 15V, 70A, but the old MG regulation wasn't what it used to be. Kept on creeping up on me. The unit has to weigh 200 pounds, maybe 250.


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## wb9hco (Nov 11, 2009)

Regardless of efficiency, you can raise the output voltage of an automotive alternator quite easily.

see:
http://www.scribd.com/doc/23250178/Alternator-Secrets

The windings don't 'know' or care what voltage that they are exposed to short of the break down voltage of the insulation.

The rectifiers can and most likely should be replaced by external full wave three phase rectifiers. This allows for better cooling and integrated SCR control. This same principle could be extended to a small 'pony' power pack to extend your range or provide for emergency recharging on a trip.

I am all for efficiency, but I have to temper all that with the final cost.
I can only build what I can afford.

Paul


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