Tool Briefing: What killed the battery?

How to test for parasitic battery drain


 

When connecting the probe to a DVOM, make sure the probe is connected to the meter’s voltage input, not the current input. If there’s an arrow on the probe, make sure it points in the direction of current flow. Set the probe to its lowest scale and if the meter does not have auto ranging, make sure it’s set to read millivolts (some volt meters have a built-in current probe setting). If there’s more than one wire connected to the positive battery post, make sure the probe is clamped around all of them, or clamp the probe around the negative battery cable. If you don’t get a reading, turn the probe around on the battery cable; remember, current flow is directional. The probe is susceptible to electrical noise, so keep it away from any source of interference.

Voltage Drop

If key-off current is high after 30 minutes, you can pull fuses one at a time until the current draw drops to isolate the high-draw circuit, as outlined in the test procedures. Or you can use a DVOM and a simple voltage drop test.

 

A voltmeter measures the difference in electrical energy between two points in a circuit. Although it’s an imperfect analogy, it helps to think of voltage as pressure. If you probe two points in an active circuit (one that has current flowing through it), the meter should read zero volts because the voltage (pressure) is the same at both points. If there’s any resistance at all between those two points, there will be a measurable voltage across that resistance.

In reality, there’s always resistance between two points in an active circuit, even if they’re only half an inch apart, but it might be so small that it can only be detected with a DVOM that can read millivolts.

But here’s the thing to remember: If current is not flowing, there will be no voltage drop, even in the millivolt range. How do we use this to find parasitic battery drain?

 

Removing fuses to find a high current draw may not be possible because removing/reinstalling the fuse can “wake up” a control unit and ruin the test. Instead, set your DVOM to the millivolt scale and read the voltage drop across each fuse while it’s installed. If the circuit is completely off, no current is flowing and there will be no voltage reading. If current is flowing through the fuse, there will be readings in the millivolt range.

Depending on the fuse type and rating, you can use that reading to determine the current. For example, if the voltage drop across a 5-amp standard fuse is 0.3 mV (3 tenths of a millivolt), the current flowing through that fuse is 20 mA (milliamps). The VW service bulletin mentioned in the Identifix Tech Tips includes a two-page chart showing the current draw for each fuse rating and each measured voltage drop.

In addition to a control unit that’s “still awake,” a faulty alternator can also create a parasitic draw, and so can slime, battery acid residue or salt on the battery cable connections. A voltage drop test across each connection in the starting and charging circuits is a good idea in this situation, and just to complete the picture, use a high-current probe to look at starter current draw.

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