Check your battery voltage with the engine off by selecting DC Voltage on your multi-meter and putting the red probe on the positive terminal and the black problem on the negative terminal. A fully charged battery will show around 12.6V (Volts). If the Voltage is 10.6V or lower your battery is fully discharged (flat).

Start the car and repeat the test, the voltage should show between 13.5V and 14.5V. If this is not the case, the issue could be (but less likely to be) your battery as it should be actively charging when the engine is running.

If the voltage is still at 12.6V or less with the engine running the battery is not being charged! It's time to troubleshoot the charging circuit!