Why is your battery dead in the morning?
If you left a dome light on overnight or left a door ajar, that constant current draw could drain your battery to the point where you’d have a dead battery in the morning. If that’s a one-time event, your best bet is to slap on a battery charger and get the battery back up to full charge and you should be good to go. But what if you have a battery dead in the morning condition that occurs several days in a row or intermittently? That’s the sign of a computer module that hasn’t gone into “sleep” mode.
Why do car modules stay awake?
Several modules have a set time to stay awake after you’ve shut down the vehicle. The PCM/ECM may stay awake for 15-mins to monitor engine coolant temperature. When you shut off an engine, the hottest coolant will rise to the top of the engine. If the temperature rises above a factory set point, the PCM will activate the radiator fans, even if the keys are out of the ignition. Since the engine isn’t running, the coolant won’t circulate, so the coolant must flow through by itself.
As the radiator fans reduces the temperature of the radiator, the liquid coolant contracts and draws in coolant from the top of the engine through the still open thermostat. As that hot coolant is drawn into the radiator, the suction draws replacement coolant into the bottom of the engine and that’s what creates circulation. That’s why the radiator fans run even when the engine is off-to prevent cylinder head and cylinder head gasket overheating.
The Body Control Module stays awake for a while
In many vehicles, the body control module (BCM) and the
remote keyless entry module (RKE) stay awake for a period of time to detect a driver’s lock or unlock command from the remote fob. The BCM and RKE in late model cars with the push button to start feature may stay awake in low current draw mode at all times and then “wake up” when they detect the presence of the fob.
But all modules should go into sleep or limited power mode after a while. If they don’t you’ll wind up with that dreaded battery dead in the morning condition.
Every vehicle is different with some entering sleep mode in 15-mins and others taking as long as 45-mins. Although it differs between car makers, the usually sleep mode current draw is less than 50Ma at all times. That’s a small current draw, but it’s enough to completely draw a good car battery in about 30-days with no other activity. That’s why car makers recommend driving the vehicle or placing it on a battery maintainer if you’re not going to drive it for extended periods.
If a module doesn’t go to sleep it can draw several amps, enough to create a battery dead in the morning condition.
Three ways to find what’s causing your battery dead in the morning condition
These tests are referred to as parasitic battery drain tests
• Bad alternator diode test. This is rare, but the one-way diodes in the alternator can sometimes short-to-ground, creating a large dead-short that can start to drain your battery as soon as you shut off the engine. It’s best to use a clamp on amp probe for this test because a high current draw can blow the internal fuse on your multimeter.
But if you don’t have access to a clamp on amp probe, set your meter to the amp
feature connect it in series between the alternator and battery positive post. If you see any current draw, that’s an indication you have a shorted diode in the alternator.
• Current draw test using a low amp clamp-style current probe. Clamp the probe onto the negative battery cable. Wait the required time for modules
to go to sleep. If current draw remains above 50Ma, remove fuses one a time until you see current draw drop to around 50Ma. Do NOT put the fuses back in until you’re done with the test. If you put the fuse back in, the modules on that circuit will wake up and you’ll have to wait for them to go back into sleep mode.
• Find parasitic battery drain using the voltage drop method. Set your multimeter to a low DC volts setting and probe across both sides of each fuse. The fuses with a zero reading indicate circuits where no current is flowing, and thus aren’t candidates for further testing.
The fuses with a mV reading are your most likely suspects. At that point you can either use a low amp clamp on current probe or connect your meter in series.
Then remove each of the fuses that gave you a mV reading to determine which circuit has the highest current draw. Then grab a wiring diagram for that circuit and disconnect one module at a time until the current drops to an acceptable level. The module that causes the current drop is the one that isn’t going into sleep mode.
Video showing how to check for parasitic battery drain using current draw and voltage drop
©, 2017 Rick Muscoplat
Posted on by Rick Muscoplat
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