Battery Draw and Parasitic Drain Diagnosis
A 2020 heavy-duty truck with dual batteries (Group 31, 950 CCA each) is brought in with a complaint of dead batteries after sitting overnight. The batteries are 6 months old and test good with a conductance tester. The alternator output is 14.2V at idle with all accessories on. A parasitic draw test is performed using a clamp-on ammeter around the negative battery cable. After all modules go to sleep (about 20 minutes), the draw is 350 mA. The manufacturer's specification for maximum parasitic draw is 50 mA. The truck has an aftermarket alarm system and a GPS tracker installed. Fuses are pulled one at a time to isolate the circuit, but the draw remains until the fuse for the aftermarket alarm is removed. Then the draw drops to 45 mA.
1. What is the likely cause of the excessive parasitic draw, and what should be done?
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The parasitic draw of 350 mA is well above the 50 mA specification. Removing the fuse for the aftermarket alarm reduces the draw to 45 mA, which is within spec. This indicates the aftermarket alarm system is the source of the excessive draw. The alarm may be miswired, drawing power when it should be asleep, or it may be faulty. The recommended action is to inspect the alarm installation for proper wiring and connections, and if necessary, repair or replace the alarm system. If the alarm is not essential, it could be disconnected, but the customer should be consulted.
2. What other steps should be taken to ensure the draw test is accurate?
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Ensure all doors, hood, and trunk are closed and latched, and all accessories are off. Verify that all modules have gone to sleep by waiting the specified time (often 20-60 minutes) and monitoring the draw until it stabilizes. Use a clamp-on ammeter with a low range (e.g., 0-1A) for accuracy. Also, check that the aftermarket devices are not keeping modules awake via data bus activity. If the draw remains high after removing the alarm fuse, further isolate by pulling other fuses or disconnecting modules.