How much energy does an alternator use?
 

Hi im new to the site. I just recently bought a scan guage two and saw my mpgs go from about 31 to 39.7 over night. So now the wheels are turning in my head and all i can think about is how to get more out of a tank. I was wondering how much energy an alternator uses and if it is possible to remove it in some way. Now from what I know it just keeps the battery charged. Is that correct? Does the alternator run the whole time your engine is running or only when the battery needs charged? On my car it is to stay between 13 and 15 volts while running. Would it be possible to add a solar panel to keep your battery charged?

There is a mechanical drag (bearing friction) and an electrical drag. The electrical drag will be proportional to the load that your electrical system demands on it.

The electrical drag will just be your voltage x current draw / efficiency, so a 12.8V battery at 50 Amps with a 60% efficient alternator will take 1.07kW. 50 amps was just a guess but you could measure your amp draw if you want an accurate measurement.

According to Wikipedia "Very large automotive alternators used on buses, heavy equipments or emergency vehicles may produce 300 amperes. Very old automobiles with minimal lighting and electronic devices may have only a 30 ampere alternator. Typical passenger car and light truck alternators are rated around 70 amperes, though higher ratings are becoming more common."

So it looks like my 50 A guess was pretty close, somewhere around 1hp (38A) - 2hp (70A).

Hi rsx -

Yes, you can save fuel by running without an alternator. But you'll use up batteries to do so (you'll ruin your starting battery quickly if you regularly run it down). If it's a financial consideration, you have to do the math to see if it's worth doing.

See AndrewJ's build thread - he's doing it. I've also done lots of alternator-less driving in the summer months (though none yet this year).

Solar panels are prohibitively expensive, and you'd have difficulty mounting enough of them to support your battery - without ruining your car's aerodynamics.

i keep hearing about these paper thing printed solar cells.. once these hit the market, it may be possable to solar charge you battery enough to surfice.. especially if you in california or other sunny locations.. but as of now .. its expensive not easy to make it areodynamic and not really functional becuase of those two points..

Howeverm some are experimenting with making there alternator switchable so they can use it when there are wasting power like under breaking etc.. i really like the idea of adding a BIG alternator that i can switch off durring driving and energise it when I use the breaks.. this is using the wasted power of breaking..

for instance get a high amp alternator that takes 4 hp to run.. then the only time its used is when you hit the breaks.. thats 4 hp less your breaks need to work to slow the car... others are doing it so that when there car sees less than 10 volts the alternator then kicks in too.. check the threads on it in the forum and youll see lots of stuff going on in the alternator arena.

My guess is, you would need to have a LOT of stuff turned on, before drawing 50 amps.
But, if your ignition(spark-plugs), OBC, clock, GPS, SGii & radio are drawing much more than 10 amps, I would be very surprised. (10a*13v=130 watts).

IIRC, a car battery wants to charge up to about 13.6V (at least they used to) and the alternator wants to feed it about 14V all the time you are driving.
So, the amount of load on the alt is going to partly depend on the charge state of your battery.

Have you been sitting with the engine off with the radio cranked up to 300 watts? :thumbup:

Look at your SG2 while driving and with the engine off. The battery is getting some charge voltage even if it doesn't need it. But, if it's already up to par, the current drain isn't that much. Maybe less than an amp or two.
Unless, you are driving at night!! Then you are going to need some major amps.
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I read an interesting post today. The idea proposed was to install a relay in the alt output, so the car could run on battery alone.
The relay would pull in, only when the driver hit the brakes.

My idea is to use a smart charger on the battery. Measure it continuously and only charge it, when it started to get low. You could tailor the off-on settings for best fe.

Of course having a 3A 15V solar array might help too. (If it didn't have a lot of drag). :cool:
Or, you could just plug in a smart charger in the garage every night. (& use the alt less the next morning).

Power from the alternator is a complicated thing.

The biggest drain of power is when you start the car - worst in winter, it will drain the battery alot. Any heating system will also put a massive drain on the battery. The actual power reqs of the basics to run the engine are very low, its the starting and "comfort" systems that are the biggest burden aswell as headlamps/wipers. Again in winter you are using them more too :(

If you intend running without an alternator you will need to know exactly the power needs for the regular journey that you do ie home to work. You will need to add additional batteries and charge via solar (again bad in winter) or wind (good in winter) to cover the vehicles needs.

If you get stuck in traffic you are screwed.

TBH I dont think its worth it to rewire new batteries and have a free power source that you have to connect after each drive. Just drive eco, dont put the sterio on full blast, max the heaters, seat heaters, steering wheel heaters and so on.

An alternator mainly only drains power from the engine respective from the draw needed from it.

Welcome to the site Tekkie.

I'm sure you're just settling in and learning as we all are. Alternator deletes/disables are known to increase mileage around 10%. That is quite an increase from a single modification. If you size your battery accordingly you should have no problems. Long trips of course require hooking it back up.

I've done an alternator disable mod on my Paseo and have been running it all summer. You are right that in winter it tax the battery more, but I'm quite sure that I have the battery capacity to spare.

Hi Daox and thanks for the welcome :)

Yes I agree, excluding the alternator is an advantage in direct cost mpg. I guess you live in a warm climate? Im in the UK and I cant see a car battery of say 85AH lasting long if it has to take a drain of starting several times, defrosting the front screen, defrosting the rear screen, heating the cabin, headlights, radio, wipers and so on.

Lets not forget that the cost saved in mpg is offset by the cost of manually charging also.

Also modern batteries are damaged and reduce capacity if discharged past a certain level.

In some climates I guess it can work, but in others I dought its worth even trying.

Deep cycle batteries are a consideration when doing such a thing as deleting the alternator.

The MPG savings are NOT offset entirely by the cost of recharging the battery, especially if you're charging off someone else's connection. Even if you're not charging off someone else's connection, you're only going to pay around 30 cents to recharge a dead battery (or less, depending on where you are), versus the number of miles that you can get up to 3MPG better than you normally would be getting.

Let's assume that you get 1MPG better for a whole tank of gas, where you would normally only see 15 MPG. Now you're getting 16MPG.

At 16 MPG, you run out a full (10 gallon) tank of gas, for a total of 160 miles. With gas at $2.00 a gallon, that's $20.00 to fill up, or 8 cents per mile.

The mileage difference between 150 per tank and 160 per tank is about 80 cents, that you've saved (in this example), and it only cost you about 30 cents to top off the battery a couple times between fills.

Keep in mind, that's only during this example. YMMV, probably will save you even more, since I'm not aware of anyone paying a high enough rate to require 30cent refills of the battery, and only paying $2.00 for gas.