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Old 12-16-2002, 04:43 PM
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Question alternator amps

How do you determine the size of the alternator and battery that will be needed to properly support the electrical system of a vehicle ?

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Old 12-25-2002, 11:22 AM
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As an electrician, I was always taught that you should go with no less than 100% of the largest current using device and 25% of all other devices you probably should use the overcurrent protection (fuse size)for computing the final amprage. Once this number has been calcalated, go to the next size bigger Alternator, and you should have yourself covered, without going overboard.
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Old 12-25-2002, 03:29 PM
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The voltage regulator makes sure the alternator only puts out what the car needs. I have upgraded a few older cars that only came with 60Amp alternators to 100Amp alternators. Going to a larger rated alternator doesn't mean she's going to put out more current, but the potential will be there if the regulator calls for it. Most older cars without power windows and accessories usually had around 60Amp alternators from the factory. If I'm upgrading to power windows and stereo amps I'll go to a 100Amp or better. Many limos have two alternators to handle the amount of accessories but when the car is idling with everything turned off the regulator prevents any overcharging. My philosophy is better to have more potential than I need than to not have enough. Same thing with batteries, I have upgraded many 550CCA batteries to 750 or higher CCA's. The battery will only supply whatever current you need to start your car, can't hurt to have a little unneeded reserve. I have never built a whole car from scratch, just upgraded systems that were already there. If your building a car from scratch and have nothing to go on I would look at another car with similar accessories and current demands and go with at least what that car has or better.
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Old 12-25-2002, 05:16 PM
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Simple, see your fuse box? Count up all the AMPs listed there. Don't forget to add in the distributor/igniton since there's usually a fusible link for that.

Add 10% for peak loading and your good to go, if you prefer add 50% for low rpm load handling for things like taxis or limos or cars with big stereos. Don't forget alternators have ideal rpm ratings for output also.
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Old 12-25-2002, 06:23 PM
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Man if you tried that on a newer car you'd be looking at a 500Amp alternator. The fuses are there to protect the circuits from drawing more than the maximum current that any circuit can handle without causing a safety risk. The actual current through each circuit is much less, and the chances of maxing out every circuit at the same time are slim. If you check most new cars they have a fuse box in the passenger compartment and sometimes two more in the engine compartment. That's a whole lot of fuses, yet most alternators still only output about 155Amps max even with all the junk on a newer vehicle.
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Old 12-25-2002, 08:16 PM
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You are correct DM but what is of prime importance? Never overloading the alternator, many think that an alternator just puts out it's rated power and that's it. Fact is an alternator will put out how many amps you draw from it, if it is above the rated output then you risk damaging the diode trio in the regulator or the windings themselves from overheating.

The answer is add up all the accessories and assorted fusible link leads and then decide which ones could possibly all be on at the same time. The starter is one you can leave out since it only draws during starting. Some of the loads you do want to account for are the ignition, heater fan, rear window defrost, headlights, dash accessories, radio, injectors, heated seats, interior fan(s), fuel pump etc. etc. etc. These items could conceivably all be on at the same time.

Once you determine what would be the worst case scenario for load on your vehicle then you can decide what is the right size alternator to use. Remember the loads listed at the fuse panel are overload values and no actual circuit runs at overload. So then it becomes a question of what factor do I use to figure this stuff out?

Common sense is the key, if a car came equipped with a 40-60 amp alt as standard equipment then it would seem most needs from the charging system are met sufficiently with this size alt. Of course A/C adds to this along with the typical accessories that are on most A/C vehicles so these cars usually get 100 amp alts? Adding to this dilemma is the fact that even a garden variety 40 amp alt can put out 100 amps for short periods of time?

So the short answer to my long explanation is alternator size should be sufficient to accomodate all persistent loads. What is that number? somewhere between 40-120 AMPS depending on vehicle and accessories. The question has no hard and fast formula to follow that will give an accurate answer unless you play electrical engineer and determine your own needs. With alternators bigger is always better since the device will be stressed less if underutilized. Drive ratio to engine rpm must be considered too since output is directly related to operational rpm.

Some rules of thumb are;

-Alternators will support a load up to and beyond it's rated output, overload will shorten it's life significantly. Constant overload heats the windings and causes inefficiency at above rated load, this is a recipe for disaster.

-Wire resistance is a load itself so don't be tricked into thinking your wiring is any less important.

-Regulators are rated by load also, make sure you get the right one.

-If you run the battery down cranking your car the alt will not recharge it up to spec, the field excitation is drawn from the battery so it can only maintain the charge it "senses" once excited. Maintain your battery!

-Battery amp rating is pretty useless unless you want to compare batteries, use the biggest battery you can fit. When it's -40 Celcius and a cold start you will be thankful you splurged for the biggun! High compression and long crank times stress a battery to it's max, don't cheese out on a small battery, extra capacity is always needed at some time or another.

Of course wallet size and availability are critical determining factors too.

I hope our discussion helped.
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Old 12-26-2002, 06:16 PM
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...and don't forget to account for the guage of the wire from the alt. to charge the battery if increasing alt. output. On a motorhome I had the builder added a 120 amp. alt. but left the #10 wire for charging. It is the big pump little pipe issue and burnt the alt. I replaced the charge line with #4 battery cable.

Increase the charging line remembering the smaller the wire guage # the more current capacity. Confusing? If the stock wire is #10 and you are increasing the alt. amperage consider using #8, #6, or better, #4. Just provide the electrons lots of space the run... r
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