[cs123]

For basic in car tests I use a multimeter.

Ideally you will see 13.8 to 14.2 volts from the alternator. Start the car. Put the voltmeter across the battery terminals, measure the voltage.

The turn on accessories likes headlights, ac, demister etc to create electrical load and measure again.

You will generally see the volts lift slightly as the engine RPMs increase off idle.

For basic battery test you can do the same without the engine running. Check voltage with accessories and lights on.

They are just basic tests but show up if you have a problem 99% of the time.