When your car battery runs out overnight, either the battery is at the end of its life, or you have left something on, such as a light. However, sometimes it can happen that something absorbs power without you being aware of it. In that case it is parasitic absorption, and can cause the same effect as leaving the headlights on: a battery that does not start the next morning.
Steps
Step 1. Remove the cable from the negative side of the battery
Step 2. Connect the black lead to the com input of a multimeter and the red lead to the 10A or 20A input
The meter must be able to indicate at least 2 or 3 amps for the test to work. Plugging the red wire into the mA input will not work and may spoil the indicator.
Step 3. Insert the multimeter (set the switch to reading the Amps) between the negative cable and the negative pole of the battery
Wait a few seconds (or a few minutes) to allow the machine to return to sleep - when you plugged in the multimeter, the machine's electrical system "woke up".
Step 4. If the ammeter reads more than 25-50 milliamps, something is using too much battery power
Step 5. Open the fuse panel and remove them, one at a time
Remove the main ones last (higher amperage). Perform the same steps for the relays that are inside the panel. In some cases the relay contacts may not break creating an eddy current. Be sure to check the ammeter after removing any fuses or relays.
Step 6. Check that the ammeter indicates normal values
The fuse that caused a significant reduction in value after being removed was responsible for the current. Consult your car manual to identify the circuits protected by that fuse.
Step 7. Check each fuse-protected device
Disconnect any light bulbs, heaters, etc. to find the parasite element.
Step 8. Repeat steps 1 & 2 to verify your repairs
The ammeter will tell you the exact number.
Step 9. You can also try disconnecting the large wire leading to the alternator
The alternator may in some cases have a short-circuited diode that allows current to flow into the alternator power cable and through the machine metals back to the battery. This will cause a very rapid discharge of the battery. Be sure to check the ammeter before disconnecting your alternator.
Advice
Parasitic absorption occurs when a device is using battery power with the machine off, and the key removed. Therefore, when performing the tests, make sure that the interior light, the light of the bonnet and the trunk, etc. are off
Warnings
- Don't forget to check your cigarette lighter and sockets. Sometimes coins in these places can cause short circuits.
- Some second-hand anti-theft systems may make this test too long or noisy. If that's your case, ask a professional mechanic for help.
- In more and more models built after 2003, disconnecting the battery will cause the PCM to reset requiring the modules to be reinitialized. In some cases this can only be done with a tool from the manufacturer. It is best to have cars of this type repaired by a dealer or an auto electrician.
- Be careful when working around a car battery. Protect your eyes and skin.
