Car battery is flat, how can I tell the cause from multimeter results?

The battery isn’t charging properly - if the charging light was switching off after some time then it could be an intermittent electrical fault or perhaps the drive belt to the alternator is slipping, which is easy to check.

First disconnect the battery from your car and get the battery fully charged.you need to clean the connectors on the battery and in your car to get optimum contact.

When the battery is fully charged re connect it to your car and start your engine,you can now measure the voltage on your battery(the voltage with the engine running should be 13-14V if the alternator charges your battery.

When you turn your engine off the voltage should drop to about 12,7-13,8V and it should not drop below 12V even if you do not use your car for a couple of days.

If your alternator does not charge your battery you need to test if it gives off the correct voltage(with the engine running),measure the voltage on the +out on your alternator to ground connection,next measure the voltage on the battery it should show the same voltage if it does not you have a poor connection.

To find a poor connection start by measuring the voltage drop from +out on the alternator to +on your battery(with the engine running)the voltage drop should be less than 1V if the voltage drop is larger you have a poor connection but we do not yet know if it is on the +ve or -ve side so we need to measure the ground connection and to do this you measure the voltage drop from your engine block to battery -ve,if the voltage drop is more than 1V you have a poor ground connection if not you have found the ground connection to be fine and can focus on locating the poor connection on the +ve side.

All of the above is if your alternator is functioning correctly if it is not you will have to replace your alternator ,or diagnose the controll circuit of your alternator(some cars has a digital charging controller that might fail but you can most likely diagnose this via the obd2 port).

Your battery is probably in poor shape after beeing undercharged for some time.

Most of the measurments above is done by setting the multimeter to its 12-20v range.

you can check from the youtube video how to check if your alternator is dead. CLICK HERE

But I will add more. Turn on your engine and (this should be done if you know what are you doing.) and try with your steel screwdriver close enough to your alternator body (do it from behind the belt, not from the front, your screwdriver will be splashed to your head), if you feel the screwdriver attracted to your alternator is mean your alternator coil working fine. but the diode could be dead inside or the regulator inside is worn.

If you check 11.6V while turning off and turn on the engine only gives you nothing that is not normal.

Check fuse (alternator need firing cable or voltage starter for rotor) if that is blown you will not getting anything from the alternator, check the cable from the alternator.