Tool Briefing: What killed the battery?

How to test for parasitic battery drain


Tool Briefing offers real-world tips and advice for using specific tools to accomplish specific tasks. The Customer Concern is shown below, along with a brief list of factory bulletins addressing the same problem. Remember, the tool information might apply to many other vehicles too.

STEPS

TESTS/PROCEDURES

When testing for parasitic drain, use an ammeter in series with the battery cable and battery. Place a shunt in series with the negative battery cable and connect the meter in parallel to the shunt. (Note: Don’t disconnect battery power from the vehicle. Use a jumper battery to keep the electrical system energized to eliminate problems with antitheft recoding and PCM relearn).

1. Start the engine, then turn on and then off every electrical device on the vehicle.

2. Turn the key off, close and lock the doors and close all windows and sunroof. Open the shunt to place the meter in series with the negative battery terminal. (Note: Disable any hood alarm switches and close the driver’s door latch but keep the door open so the fuse box is accessible.)

3.Leave the vehicle undisturbed for 30 minutes, allowing onboard computers and processors to enter ‘sleep mode.’ After 30 minutes, parasitic load should be 50 milliamps or less.

4. Should parasitic load exceed 50 milliamps, pull fuses from the fuse box one at a time until current draw drops below 50 milliamps. Once the high-draw circuit is identified, use wiring diagrams to locate the faulty component.

Vehicle Application:

• All VW from 1995 through 2006

Avg. Reported Mileage:

• 94,765 miles

Direct hits:

• 372575

 

Customer Concern:

The battery goes dead after a couple of days. The technician is testing for parasitic battery drain and needs to know what values are acceptable.

Tech Tips:

Parasitic loads in the 300 milliamp range usually indicate a computer or processor that has not entered sleep mode. Refer to VW TSB 27-07-11 for a step-by-step alternative method to locate a draw using voltage drop.

 

Tools Used:

• DVOM

• Low amp probe

• Battery tester

• Memory saver

Assuming the battery is healthy and the charging system is working properly, this tech decided to look for a key-off current draw to find out what's killing the bettery. A low-current parasitic draw will indeed drain a battery in just a few days, but there's more than one way to search for it. As a side note, after repeated discharges, don't forget to recharge and test the battery.

Current Draw Through a Shunt

The test procedures tell you how to avoid creating new problems. By using a shunt to connect an ammeter (amp meter) to the positive battery circuit, you can preserve the radio security code and the idle speed memory in the Powertrain Control Module (PCM). Even more important, the shunt will also keep the vehicle security system happy, avoiding the need to possibly reprogram keys on some vehicles.

 

The shunt is a jumper wire connected in parallel with the positive battery cable. Getting the shunt connected to both the battery post and battery cable before disconnecting the cable from the battery can be a bit complicated, but it can be done. However, it’s far easier to use a memory saver connector and a booster battery pack that has a 12V power port. The memory saver plugs into the vehicle’s OBD II connector, supplying both power and ground to every control unit on the CAN (Controller Area Network) bus.

If you’re making the shunt yourself, make sure it has an inline 10- or 20-amp fuse, just in case. If you’re using a high-quality Digital Volt/Ohm Meter (DVOM) as the shunt, the fuse inside the meter will typically handle 10 amps continuously or 20 amps for brief periods. If the meter suddenly stops working, check the fuse inside. When the shunt is connected, unplug the memory saver so all the current flows through the meter or shunt.

Low Current Probe

A low current probe is easier to use than a shunt because it clamps around the battery cable. Direct current (DC) flowing through a wire generates a magnetic field around the wire. A hall-effect sensor in the current probe detects the strength and direction of that field and generates a voltage signal proportional to the current.

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