Voltage drop as a theory and voltage drop testing are often ignored because the test and its usefulness are often misunderstood. Let's clear up that misunderstanding.
The definition of drop
Voltage is electrical pressure; current (amperage) is the flow of electrons. Each conductor of electricity, whether copper, silver or gold, resists the flow of electrons a certain amount. Naturally, some conductors resist electron flow more than others (copper has more resistance than gold).
As electrons pass through a resistance, which can be a section of wiring, a bulb, switch, or solenoid, some electrons are lost trying to make it through the resistance. When this happens, there's a difference in the potential of the electrons at each end of the resistance, just like at the battery terminals. This difference in potential can be read on a voltmeter connected in parallel as the amount of voltage drop across that part of the circuit.
The amount of voltage drop depends on the length, size, material and temperature of the resistance. As the voltage readings go higher, it indicates a higher resistance and increased opposition to electron flow.
An ohmmeter is a great tool in its own right and we're not suggesting that you toss yours out. It's just that there are several reasons voltage drop works better in a working circuit.
First, ohmmeters only push milliamps (thousandths of an amp) of current through a resistance in order to get a reading. This comes nowhere near the current that may be flowing in some circuits — the starting circuit, for instance.
To help this sink in, consider a battery cable with half of its strands cut. If you took your ohmmeter and checked the resistance of the cable, you would find that it's OK. Then, try to run the starter with your voltmeter connected across the bad section. The starter will crank slowly because the cable impairs needed current flow. Whatever voltage can't make it through the cut will be shown on your voltmeter.
The next reason your ohmmeter isn't as good as a voltage drop test relates to the first reason. Ohmmeters are designed to take readings in unpowered circuits because the meter's internal battery provides the current. But voltage drop is always measured with the circuit working (current flowing). This gives you live readings of what's going on when the circuit is actually working without having to disconnect anything. To use an ohmmeter, the circuit must be dead and you usually have to disconnect something for attaching leads.
The last reason against ohmmeter testing shows how small amounts of resistance have an adverse affect on voltage drop. A review of ohm's law supports this claim.
The equation for ohm's law is E = I x R, where E stands for voltage, I for amperage, and R for ohms — voltage equals the amps value multiplied by the ohms value.
For instance, let's say we're dealing with a starting circuit that may have 200 amps of current flowing during cranking. Using the equation for ohm's law, we'll plug in a value of 0.01 ohms. Why such a small amount? Because it would take an extremely expensive and sophisticated ohmmeter to measure such a small amount of resistance (the ohmmeter in your tool box doesn't read to hundredths of an ohm). Now, plug in the two numbers we know to get the one we don't: 200 amps x 0.01 ohms = 2 volts. You must agree that a 2-volt drop from such a small resistance is quite amazing.
As mentioned, the great thing about voltage drop testing is that you can take readings without breaking any connections. If you're using a digital meter, you don't have to worry much about which lead goes where. If you connect them backwards, the meter recognizes reverse polarity and displays a"-" in front of the reading, but the numbers that you're interested in will be the same.
Testing the charging circuit