To begin with the voltage regulator is not underneath the alternator, it is on the REAR of the alternator. If you are only getting 13 volts across the battery, I would click on the Fastlane above and just get a voltage regulator. The last time I bought one from them it was $14 and change and has lasted a long time.

This regulator has an electronic voltage regulator and the brush pack. When the brushes wear out, it usually stops charging altogether, but low voltage could very easily be caused by a bad regulator. Before replacing it, make sure all connections including ground straps are tight.

As far as the difference between Voltage and Amperage, that is a question about electricity, not MB's. Voltage is the amount of Electromotive Force that is available to push current through the circuit. Amperage is the measure of current flow. The amount of current flow (Amperage) will increase as the Voltage increases IF the load (resistance) remains the same. In the case of an automotive electrical system, the load is always changing. That's why a voltage regulator is needed to adjust the voltage applied from the alternator to the battery to match the load.

The alternator load is changed by the different accessories that are operational and the state of battery charge.

Since Voltage is the measurement of electromotive force, you measure across the battery with the voltmeter, while Amperage, being current FLOW is measured with the ammeter in SERIES with the circuit.

An oversimplified way to think about Voltage and Current is to think about a water hose. The pressure at the faucet is like Voltage. The gallons per minute through the hose is like Current and the size of the hose is like Resistance. If the hose stays the same diameter, the more you turn on the faucet, the more pressure you apply to the hose thus you get more water flow. If you put on a smaller hose with the same faucet setting you will get less flow.

Even though Ohms law is quite simple to understand, troubleshooting the electrical systems of a car is not that easy. But it certainly is not easy if you don't understand Ohms law.

Hope this helps,