# not really about motors, but about replacing the alternator with something more...



## miernik (Nov 28, 2009)

This is not really about a motor for moving the vehicle, but similar:

I want to put something besides the alternator to be able to charge a large battery in the vehicle more efficiently while idling. Let's leave aside the reasons I want to do this, I just need it.

Currently there is a 12V 55A Bosch alternator. I want to achieve as much charging power as possible. 14.4V * 55A is about 800 W. I am looking into increasing that power 10 times or more.

I could put a larger alternator, I saw they go up to 300A and maybe 24V that would be: 300 A * 28.8V = 8640 W - that's something.

But I read that alternators are very low efficiency, something around 50%.
I would like this installation to be more efficient, and I read that some motors used as generators would have over 80% efficiency. That's something!

Can you point me in the right direction what kind of motor would be good for this, and how can I attach it to the belt instead/besides the alternator?
I would actually prefer to receive something around 48V+20% DC out of this. 200A 58V DC would be perfect, 60A 58V DC also nice. Of course the generator can produce AC and rectify it if that'll be more efficient.

The ICE engine will stay, but the car (van actually) is to be converted to an electric hybrid, but that's another story. This is about high-power charging when idle. If this is not the right forum to ask for tips, maybe someone can point me to the right one.

I am looking for advice about motor/generator type for this, and mounting tips. Has someone actually done something like this? I read something about ambulances having lots of generator power.

Thanks.


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## DJBecker (Nov 3, 2010)

Perhaps you should check the information you are getting.

Automotive alternators are considerably more than 50% efficient. And they are much more efficient in general than automotive DC generators.

Yes, generators can be over 80% efficient, but only when sized and driven so that the output power is in a specific range. In a car the drive speed will vary 10x, while the output voltage is expected to be regulated within a few percent over a 100x load difference. Alternators do a much better job of adjusting for the wide operating points while retaining efficiency.

With both the efficiency is improved by oversizing. Using more copper for lower resistance and more iron for a better magnetic circuit reduces losses. The trade-off of weight vs. conversion efficiency means that comparing an automotive optimized part to a fixed plant part isn't apples-to-apples.


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## major (Apr 4, 2008)

miernik said:


> I am looking for advice about motor/generator type for this, and mounting tips. Has someone actually done something like this? I read something about ambulances having lots of generator power.


Might be this you're thinking about. http://www.aurasystems.com/pages/prod_home.html


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## miernik (Nov 28, 2009)

DJBecker said:


> Yes, generators can be over 80% efficient, but only when sized and driven so that the output power is in a specific range. In a car the drive speed will vary 10x, while the output voltage is expected to be regulated within a few percent over a 100x load difference. Alternators do a much better job of adjusting for the wide operating points while retaining efficiency.


The speed will vary 10x only when the car is driving (and with my driving max 50 km/h not even so), but please note that my application is charging the battery while idling, so the RPM can be adjusted to the most optimal specifically for this purpose, and charging always done at this RPM.

Also the load while I charge will be constant, because the batteries will be built to charge at a specific rate, and only charged at that rate.

What's more, the batteries will not be hooked-up to the alternator directly, but through a charge controller, so something like maximum power point tracking (MPPT) which is used for solar panels, can be done - so constant voltage is not important, the MPPT controller will adapt to the voltage.

And it doesn't have to be a DC generator, maybe an AC induction motor or permanent magnet AC motor would be better.

So how much efficiency do you think you can get from a good high-power automotive alternator, if you say it can get more then 50%? Maybe 60% - which doesn't make much of a difference? Or is it more? I've never seen anything more then 50-62% range quoted for alternator efficiency.

What I am looking for is maximum efficiency for use at constant speed and constant load - while idling, about 3-5 minutes at a time, and squeezing maximum electrical energy out of the motor in that time.


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