Originally Posted by Captm
Not exactly. The loads have no direct effect on how much amperage the battery will allow to flow through it.
The voltage regulator turns on the alternator to supply power to hold the "12v bus" at a certain voltage. Once the bus gets to that voltage, the regulator switches off the alternator. This is usually a range; i.e., switch it on at say 13.5v, and off at 14.5v. The switching happens quickly, so to the naked eye, it appears to be holding the bus at a more or less steady voltage.
If the battery voltage is below whatever voltage the bus is at, then the voltage (pressure) of the bus will cause some power to flow (push) through the battery. How much? Depends on the resistance of the battery, the resistance of the wire, and how much voltage differential there is between the bus, and the battery.
Let's say the battery will allow 10a of current to flow through it at whatever voltage the bus is at. Well, then the alternator is producing 10a because the battery is drawing 10a from the bus, and that pulls down the bus voltage and so the voltage regulator keeps the alternator switched on to hold the bus voltage up - but since there is only 10a being drawn from the bus, only 10a will be flowing from the alternator to the bus.
If you then turn on the lights, the auxiliary lights, your 12v air compressor, etc., etc., you might add another 50a of load to the bus. The loads will pull the bus voltage down (suck the pressure out), so the voltage regulator will switch the alternator on to supply power to keep the bus voltage up.
In this example, the load would be 10a + 50a = 60a. So if you had a 30a alternator, it would not be able to produce enough power to hold the bus voltage up, in that case, the loads would draw the bus voltage down below the battery voltage, and so power would flow from the battery to the bus.
The load on the bus would be 60a, and if the alternator can supply 30a of that, then the other 30a will flow out from the battery to the bus - until the battery is dead. Turn off some things and get the load down to say, 20a...then there would be an extra 10a available from the alternator to feed the battery, and since the battery's voltage is below the bus voltage, the battery will now be a load on the bus, and power will flow through the battery again, recharging it.
But if the bus voltage is at a (more or less) steady 14v - then the resistance of the battery is only going to allow X amps to flow through it at that pressure.
And the resistance of the battery changes. The resistance is highest when the battery is low, and also when the battery is almost full. If you take a regular ol' benchtop battery charger, with one of the analog meters, and hook it up to a dead battery, what you see is that only maybe 1a flows at first. After a while, it slowly goes up until it reaches some point - say 10a - and it stays there for some hours, and then slowly drops back down to 1a again.
That is exactly what happens with a normal alternator/voltage regulator setup charging a battery. The amount of amps that will flow through the battery, is controlled BY the battery.
If you want more amps to flow through the battery, you have to kick up the voltage to create a greater pressure differential, but you can't because the voltage regulator is hard wired to a certain voltage range. There are special voltage regulators though...
Even if the alternator can potentially* produce 160a, it won't unless there is something pulling 160a out of it. It will only produce however many amps are being drawn from it.
Same thing with the main breaker box (service entrance) to your house that can potentially supply 100a, 200a or 400a or whatever - if the house is empty and nothing is plugged in, then no amps are flowing.
Same thing with a portable generator; it might be able to supply 1600w, but if there is only a 100w of load plugged in, then it's only making 100w of power.
* Electricity is called "potential" for a reason. It doesn't actually exist until something causes it to flow. It's not like plumbing where there is water in the pipe all the time. With electricity, there is no electricity in the wire - until some load causes power to flow out from the supply. Until there is a load, then the supply is only a "potential".
Last edited by dwh; 06-14-2012 at 03:50 PM.
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