Considering how fast it's going out, it would have to be a pretty big drain.
Basic testing
1. Fully charge your battery using an external charger. Disconnect the charger.
2. Measure voltage across the terminals. It should show at least 12.5V.
3. Start the car and measure across the battery terminals again. It should read well over 13V. If the car had a hard time starting on a full charge, that points to a bad starter or bad connections. If you don't see over 13V at idle, this points at a bad alternator.
4. Rev the engine to around 2500 RPM. Voltage should be above 14V. If not, alternator is suspect.
If things are pointing to a alternator issue, make sure the battery light in the cluster comes on when you turn the key to run before you start it. Once started then the light should go out. If the light didn't come on, you have a problem. You likely have the older cluster that requires a working lamp in order for the alternator to charge.
If all this stuff checks out, go make sure ALL the high current connections are clean, snug and in good condition.
1. Battery posts
2. Negative cable connection to chassis
3. If you have a trunk mounted battery, then the junction block on the firewall. Depending on year and model, there may actually be two circuits from the battery. The main lead that eventually goes to the starter and a smaller lead for the injection electronics.
4. Main terminal on the starter. This is the one where the main battery lead and alternator lead connect.
5. Alternator connections. Especially the thicker one.
6. Main engine ground strap. This is the one down by the motor mounts. Not the one that attaches at the valve cover.
"Howdy, Folks!"
1986 Delphin 528e - Roof rack equipped lumber hauler.
1989 Zinnoberrot 325iX Sedan - I miss this car. (Deceased)
1998 Avus Blau 328iC - Someone else's project now
2008 Platinum-Beige X3 3.0si - Current project
2012 Alpine White X3 xDrive35i - My new snowmobile.
2020 Estoril Blue 440i xDrive cabrio - This car is a blast to drive.
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