Time isn't related to current flow. If an item such as a light or motor is operating, the amount of current it requires must come from the battery or the generator as long as that thing is turned on.
Every car is different but depending on conditions one might use as little as ten amps while running and another might be using 30 amps. If the two head lights are on, for example, they each draw about five amps. Tail lights, running lights, and dash lights draw additional current. When you press the brake pedal, each brake bulb draws about an amp. An electric fuel pump typically draws between six and ten amps.
Car batteries are designed to give up a huge shot of current very quickly for starting the engine. On modern cars that can be between 100 and 250 amps but it's only for a few seconds. They can't pack that charge back in nearly as fast so it can take ten to thirty minutes to fully recharge it after starting. Whether it's charging from the car's generator, (commonly called an "alternator", but that term is copyrighted by Chrysler), or a home battery charger, 20 amps is about the fastest you want to charge it at to prevent overheating the plates. You can consider it fully charged when the current comes down to about five amps. It will never drop to 0 amps unless the battery voltage is higher than whatever is charging it.
The best way to start testing a charging system for the basic functions is with a digital voltmeter. With the engine not running, a fully-charged battery will measure 12.6 volts. A good but fully-discharged battery will read closer to 12.2 volts.
With the engine running the voltage must be between 13.75 and 14.75 volts. The generator has to put out a higher voltage than what's in the battery to convince the current to flow into and through the battery's plates.
The generator never stops working when the engine is running. It runs the car's electrical system first, and when it's able to produce more current than what's needed it also recharges the battery. Generators need three things to produce current; a coil of wire, a magnet, (electromagnet, in this case), and most importantly, movement between the two. That's why we spin the electromagnet with a belt and pulley. At low engine speeds such as at idle, generators are very inefficient and will not produce all the current they're capable of. That's when any shortfall has to be made up by the battery for a few minutes. Once engine speed picks up and the generator regains the ability to produce excess current, that goes to recharge the battery again.
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Friday, July 5th, 2013 AT 6:08 PM
