# What's better: alternator or charger?



## vpoppv (Jul 27, 2009)

No, this isn't another perpetual motion thread! I was just wondering if it is more efficient to recharge a battery in a car using an alternator than on a 12v charger? I am guessing that an alternator doesn't need an AC to DC converter and would be more efficient and lose less power to heat. There is also the factor of cost of gas versus cost of home electricity. Then there is also the environmental issue: are you creating more carbon from the ICE recharging as opposed to more efficient coal burning. It certainly is more convenient to put a charger on a battery than pull out the one in my car and put in the one to be charged, then take it out again when the trip is over, but I would do it if the consensus is that it is more efficient....


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## madderscience (Jun 28, 2008)

If by charger you really mean a DC/DC converter, it is going to be way more efficient that an alternator. 

Consider that a DC/DC converter is basically a switching power supply that steps your traction battery voltage down to 12V (actually probably 13.6V or so to allow proper charging). No moving parts. 

If you want to use an alternator to power the 12V system in an electric car, you have to take traction battery energy, throttle it with the main controller (at a small loss), convert it to mechanical energy (spinning the drive motor) with a loss, then transfer some of that mechanical energy via a belt with some friction loss, to mechanically spin the alternator which converts the mechanical motion back to electrical power with yet another loss. If the controller, motor, belt, and alternator have a combined energy transfer efficiency of 50%, then you are lucky. good DC/DC's are likely closer to 90% efficient through much of their operating range. They also work when the car is stopped.

An actual AC to DC car battery charger probably won't be as efficient as a DC/DC since they are usually built simple and as cheap as possible but you are still better off than using an alternator. ANYTHING is better than using an alternator. (have I made my opinions clear yet? )

Note for the cheapos amongst us: some laptop power supplies and other computer related power supplies have been used successfully as DC/DC converters, but you will need to do your homework and may have some trial and error to find one with enough oomph and the right output voltage.


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## vpoppv (Jul 27, 2009)

That is some excellent information! Thank you! However, I was not clear and I apologize for that. The comparison I want to make is ICE alternator to 110v plug-in battery charger. I don't have a DC to DC converter. I make a 30 mile (one way, so 60 miles total) trip once a week in my ICE Metro and I was wondering if I could use that trip to charge one of my discharged pack batteries. The way I do it now I just put all my batteries (6 in series for 72v) on individual 12 amp car battery chargers. It is way more convenient to just use the chargers, but I was just wondering if it would be beneficial to pop one of the batteries in my ICE for that trip once a week....


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## Guest (Oct 31, 2009)

In my Humble opinion, using your ICE powered auto's alternator is NOT as efficient or convenient as the bank of AC powered "Chargers" at your home. I feel it is unreasonable to remove a battery from the EV to charge in your ICE car with an Alternator and voltage regulator programed for a Starting, Lights, and Ignition rated battery, not for an EV battery. And all to save 10 cents on your electric bill? How much the extra fuel needed to run the alternator to charge an EV battery, I suspect more than 10 cents...


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## icec0o1 (Sep 3, 2009)

Your gas consumption will definately go up and cost you more than charging with even a 70% efficient AC charger. But moreover, time is money. Your time is valuable, far more than the fuel costs so do whatever is easier or simpler.


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