Why Your Cars Battery Keeps Draining And How to Fix It ( FAST), by Piston Post on YouTube. 13 min 46 s.
- Video length
- 13 min 46 s
- Time to do it
- 1 hour
- Difficulty
- Medium
- Steps
- 9
What you need
Steps
Charge the battery fully with a trickle charger, then check its age. A battery over four or five years old, or one that fails a load test, is the most common reason a car keeps going flat.
Check the easy things first: interior lights, boot light, glovebox light, dashcam, phone charger and alarm light. A single light left on will drain a battery overnight.
Test the charging system. With the engine idling, put the multimeter on the battery posts. You want 13.5 to 14.5 volts. Under 13 volts means the alternator is not charging properly.
Set up the drain test. Switch everything off, take the key out, close all doors and wait 30 to 60 minutes so the car's computers go to sleep.
Set the multimeter to 10A DC and move the red lead into the 10A socket. Disconnect the negative terminal and connect the meter in series between the negative post and the cable clamp.
Read the number. Under 50 mA (0.05A) is normal. Between 50 and 100 mA is borderline. Anything higher means something is staying powered with the ignition off.
Pull one fuse at a time and watch the reading. When the number drops to normal, that circuit holds the fault. Check your fuse chart to see what it feeds.
Trace that circuit: radio, amplifier, alarm, dashcam, boot light or a leaking alternator diode. Repair the wiring or replace the faulty part, then repeat the test to confirm.
Reconnect the negative terminal last and tighten it firmly. Clean any white or green corrosion off both posts with a wire brush before you close the bonnet.
Tips and common mistakes
Never crank the engine with the multimeter wired in series, because the starter current will blow the meter fuse.
A clamp meter around the negative cable does the same job without disconnecting anything, which suits modern cars with sensitive electronics.
Some cars take 40 minutes or more for every module to sleep, so a quick reading can show a false drain.
Short trips under 10 minutes never fully recharge a battery, so drive 20 minutes or more once a week.
Questions
How many milliamps is a normal battery drain?
Most cars settle at 20 to 50 mA once the computers sleep. Anything over 100 mA is worth chasing, and over 500 mA will flatten a battery in a day or two.
My new battery still goes flat. Why?
A new battery only hides the fault for a while. The drain is still there, so run the fuse pull test to find the circuit that is drawing current with the car switched off.
Can a bad alternator drain the battery overnight?
Yes. A failed diode inside the alternator can leak current back through the charging circuit even with the engine off. Disconnect the alternator and see if the drain stops.
This guide was drafted from the video's details and general task knowledge. It has not been checked against the full video. Check the video and relevant product instructions before following a procedure. Report a correction.
