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Once a car has been started and is running, on average, how much electricity is needed for basic functions in the car to continue operating?
I've been trying to find a reference on how much certain things like power steering / power brakes and other items consume, in amps, assuming 12V, but I can't find a good rule-of-thumb here.
To be specific, is there a minimum amount of current an alternator would need to provide to keep a car running? How would one know what that amount was?
It really varies upon the car and it's systems. In 1989 I went on vacation with my family, and the alternator on the 1984 Rabbit Diesel died.
We were in Maine, and the only VW dealer around for several hours said the replacement alternator would be some outrageous cost, and take the better part of a week to get there.
Sears had a battery charger for $16, and I plugged the car in at our cabin. We did all the things we needed to do each day, and got home before dark.
Traveling 700 miles home was a different story. No rain on our route, and the weather was pleasant. However on I-90 in Massachusetts, an RV caught on fire, and we were trapped in traffic for 2.5 hours. People around me didn't like the diesel car idling, so they got me through to the top of the hill, and several helped push me for a bump start after the jam freed up. We got home with energy to spare. Later, I learned that by unscrewing the fuel solenoid, we could have saved even more power.
Low power life meant just using turn signals and brake lights, and no air conditioning, etc. Today that same problem in newer VW Diesel would not have had the same outcome. Today's car takes about 2.5 amps for the injection pump, and just under 2 amps for the instrument panel. A gasoline car would probably have similar draws. If the cooling fan was needed, and electric cooling fan can draw from 10 to 25 amps, usually at the low end of that range. Also many cars have DRL. If they are LED, the draw is less but with Halogen lamps, the draw is about 8 amps (DRLs run the bulb at a lower power level, which extends bulb life).
Since every car is different, your best bet is to measure your own car. To do this you will need to pull the fuse which provides field power to the alternator, or possibly unplug the alternator, depending upon your connections. Then start the car up, and measure the power draw. The easiest type of meter to use for this is a clamp-on DC ampmeter. That will give you an idea, but there are many variable loads. Like fuel pumps, ECUs, brake and turn lights, and so on.
Unfortunately, even the repair manuals don't have the kind of detail on current draw that they used to have 20 years ago. A possible rule of thumb I use is that the draw of a device is 1/2 the fuse rating.
So let's talk just for a moment about alternator failures. Some cars, like by VW diesel, have overrunning clutches in their alternators. This clutch may fail, with intermittent spinning of the alternator. (It's kind of like a quiet ratchet.) When it fails, it usually does so gradually, and the alternator failed light may not come on, or it might flicker on every now and then. This lets the battery not get as fully charged, and by the time the alternator totally fails, you have a poorly charged battery. Starting is usually an indicator of a marginally charged battery. If your car is slow turning over, but starts, that is often the best warning that you might have a charging system issue. In the US most car parts stores have a alternator tester, and will test your alternator / battery, in the car, in their parking lot. They do not charge for this service. If you don't have tools with you, and you are out and about, and detect slow starting, head to the auto parts store and have them check it out.
Also, shed all the loads you can. Lights, blowers, heated seats, defroster and all of that. That will save power so that you can run the ignition, fuel pump and ECU (engine computer) for a little longer.
50 years ago, when cars were simpler, your current draw may have been 1 amp for the ignition coil. Today, there are many more things that need to run, and the accessories on the car are more, and draw more power than in years past. Alternators 50 years ago were 20 or 30 amp alternators. Today the smallest alternator on any vehicle I own is 120 amps.
In summary, I am sorry but I cannot definitively answer your question, but hopefully this gives you a better idea of what some of the factors are.
It depends on what is being powered the car.. What type of car and with what type of systems. If say you're just running the engine in a basic car, then its likely only a couple of amps.. This would be consumed buy the ignition system and fuel pump unless that was mechanical.
Anything after that like lights, wipers, sound system, heater fan motor, heated screens etc would increase the current draw substantially.
