Battery choices

[JackG]

Originally Posted by Jaxonalden

Jack,

I totally disagree with everything you wrote and stand behind my thread. The car does not run off of the alternator, it pulls power from the battery. The battery is a constant, large capacity source of ~12 volts of electricity. The alternator does nothing more than recharge the battery at 13.5-14.5v. Anything less than 13.5v and the battery will not recharge ("Alt" idiot light). The alternator produces A/C power thats converted to D/C through diodes and the voltage regulator does just that, it regulates the flow of current to the battery. Flow starts when battery volts fall below 13.5v and shuts it off at 14.5v.

I'm a Electrical Engineer, and I understand that you believe that's how things work, but it's simply not the case. The battery is in parallel with the alternator, and the car loads see them as a unified source of power. There is no mechanism that prevents the flow of current from the alternator to the car, limiting it to supplying only the battery. Current does not flow into the battery, and then out to the car.

As you point out yourself, the battery is ~12V, while the alternator produces ~13.6V. An accurate analogy of voltage is water pressure in a pipe. If you have two pipes connected together in a "Y" that then go to a sprinkler, and one pipe has a higher pressure than the other, which pipe will be feeding the sprinkler? The one with the highest pressure, of course! The lower pressure pipe will in fact reverse it's flow until the pressure equalizes, which is how the battery charges!

Since the battery's built-in voltage (pressure) is 12v, even when it fully charges it will not take over the sourcing of current. If the alternator were turned off, the battery would return very quickly to it's 12v potential, which would signal the alternator to immediately turn back on.

The "Alt" idiot light actually lights up when the battery is sourcing the current instead of the alternator. Remember the real current gauges in cars? The current gauge had a "0" in the middle, and it swung both positive and negative. It measured current flow in/out of the battery. Positive showed when the battery was charging, and negative showed when the battery was sourcing power. At idle, the gauge would go to 0 or just slightly negative, as the alternator was not spinning fast enough to provide 13.6V. When you revved the engine or were travelling at a steady speed, it swung positive. A big negative swing of the meter was your signal that your alternator had quit. The idiot light shows the same thing, that something is wrong, and your battery is providing the car's power.

One last thing, the voltage regulator does exactly what it says... it regulates voltage. The alternator will produce voltages far in excess of ~13.6v, so it has to be regulated. The battery is self regulated, in that as it comes to a full charge, it will accept less and less current. The voltage regulator keeps the voltage at safe levels that will not overcharge the battery or damage any electronic equipment in the car. There is no current regulation going to the battery, except for the max voltage potential applied to it by the alternator as it runs the car.