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In their most basic form, electrical circuits consist of a power source (battery, generator, etc.), a load item (motor, lamp, solenoid, etc.) that is driven by the power source, and the conductors (wires, connectors, chassis, etc.) that connect the source and load together.
In a perfect circuit, the load item would be the only source of resistance and we would have no resistance from the conductors. We are relying on the conductors to deliver the full source voltage to the load; however, since there are no perfect conductors (yet), we almost always experience some small degree of voltage loss from the source to the load. We generally use wires as our conductors, but if the wires are too small, or too long they won’t be able to supply adequate voltage to the load item. Resistance is also added to the wiring by any connectors, switches, fuses in the circuit. On top of that, any extra resistance from corrosion or damage to the wiring or connectors we may have too much voltage loss by the time it reaches the load item.
The difference in voltage from the source to the load is what we normally refer to as “voltage drop.”
According to the NEC (National Electrical Code), the rule in an electrical circuit (not just automotive) is that you should not have more than 5% total power loss from the power source to the load. For vehicles with a typical battery voltage of 12.6V, that means a total circuit voltage drop of no more than 0.6V. (12.6V x 5% = 0.6V) That is the maximum allowed total voltage loss from both the positive and the negative side wiring.
One way of determining the voltage drop in a circuit is to compare the observed voltage reading taken directly from the source to the voltage reading taken directly at the load. Subtract the load voltage from the source voltage and the difference is the total voltage drop. For example, if you take a voltmeter reading across the positive and negative battery terminals and it reads “12.60” volts, and then you take a reading across both terminals of the load item (while under power) and it reads “12.35” volts. Subtract 12.35V from 12.60 and you get 0.25V. That is the total circuit voltage drop and it is less than the 0.63V drop allowed, so the voltage drop in that circuit would be acceptable.
However, the above method does not distinguish if the voltage drop is on the positive or negative side of the circuit and there are important differences between the two. Most autos, trucks, boats, etc. use the negative side as the common ground throughout the vehicle. Because of this, having too much resistance (and/or voltage drop) on the ground side the circuit can cause back-feed voltages (known as ground loops) through other components or circuits that can possibly cause other electrical issues. It is for this reason that negative side voltage drops are crucial and should be kept to a minimum, usually less than 0.2V.
The technique that is commonly used to directly measure the voltage drop on either a positive or negative leg of a circuit is to connect your meter probes from the source to the load. (Or anywhere in between) For example, connecting a voltmeter’s positive lead to the positive battery post and then connecting the negative lead to the load positive connection (again while under power) will directly display the voltage differential between those two points. If the voltages are the same at both points, your meter will read “0.0V”. If the battery was “12.60V” and the load positive point is only “12.10V” then your meter display the difference or “0.50V” directly. No need to do any subtracting and you can tell that the voltage drop is generated on the positive side of the circuit. Probe from the negative battery post to the negative terminal of the load and you will directly read the negative side voltage drop.
This is a very efficient technique, as you are directly reading the voltage difference between any two points in the circuit. This allows you to truly pinpoint the different points or components in the circuit that could be causing too much voltage drop. For example probing on both sides of a connector will display the voltage drop across that connector alone. This will enable you to precisely zero in on a problem in the circuit.
The Power Probe III, Power Probe IV, and the Power Probe Hook all have built-in LED and tone indicators that can instantly alert you if the circuit has more than 0.5V of voltage drop from the source battery, but they cannot do point-to-point type voltage drop testing. For this type of testing, you would need a digital multimeter like the PPDMM.
Resistance in a circuit is what causes voltage drops, but the amount of acceptable resistance can vary considerably depending on the current requirements of the load being operated. Refer to the attached “Resistance vs. Voltage Drop” chart for examples of how different resistances affect current draws and voltage drops. Starting on the left of the chart it shows a perfect circuit with zero circuit resistance. As we move from left to right we are adding more resistance to the circuit and you can see how the added resistance has drastic effects on high current circuits but may have very little effect on low current circuits. However, you should notice that the circuit voltage drop is always consistent regardless of circuit draw or resistance. A 5% power loss is always right at 0.6 volts of voltage drop.
Circuits with very high currents may have slightly higher voltage drop allowances; however, here are some general guidelines for acceptable voltage drop limits, and remember the total circuit voltage drop should be no more than 0.6 Volts:
• starter circuit (including starter solenoid) = 0.60 volt
• battery post to battery terminal end = zero volts
• battery main cable (measured end to end) 0.20 volt
• starter solenoid = 0.20 volt
• negative main cable to engine block = 0.20 volt
• negative battery post to starter metal frame = 0.30 volt
• battery positive post to alternator b+ stud= 0.5 volts with maximum charging load applied (all accessories turned on)
• battery negative post to alternator metal frame = 0.20 volt
· Any connector = 0.10 volt