Car Battery Keeps Dying Overnight? Here Is What Drains It
You charge your battery and it dies again by morning. Here is how to find the parasitic drain that is killing your battery.
Three Reasons Batteries Die Overnight
A battery that dies overnight has one of three problems: a parasitic drain where something is drawing power while the car is off, a failing battery that can no longer hold a charge, or a bad alternator that is not charging the battery while driving. You can test each one yourself with a basic multimeter that costs about 15 dollars.
Test 1: Check Battery Health
With the car off, set your multimeter to DC voltage and touch the red probe to the positive terminal and the black probe to the negative terminal. A healthy battery reads 12.4 to 12.7 volts. Below 12.2 means the battery is discharged. Below 12.0 means it is dead or dying. If the battery is more than 3 to 5 years old and reads low, replacing it is the cheapest fix. Car batteries cost 100 to 200 dollars.
Test 2: Check the Alternator
Start the car and measure voltage at the battery terminals again. With the engine running, a healthy alternator produces 13.5 to 14.5 volts. Below 13.5 means the alternator is not charging properly and the battery runs down during driving. Above 15 volts means the alternator is overcharging which can damage the battery. A replacement alternator costs 200 to 500 dollars with labor.
Test 3: Find a Parasitic Drain
With the car off and all doors closed, disconnect the negative battery cable. Set your multimeter to DC amps (milliamps setting). Connect the red probe to the cable and the black probe to the negative terminal. A normal drain is 25 to 50 milliamps for the clock and alarm system. Anything above 75 milliamps indicates a parasitic drain. To find the source, start pulling fuses one at a time while watching the meter. When the reading drops to normal, you have found the circuit causing the drain. Common culprits are trunk lights that stay on, aftermarket stereos, and malfunctioning door switches.
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