Disconnect the battery and measure the voltage. set the multimeter to volts D.C. should see a reading of at least 12V. leave the multimeter connected for 3 minutes and monitor the voltage. What do you get?
Reconnect the battery to the car, leave the ignition off, and once again check the voltage over 3 minutes. should once again see a reading of at least 12V. There may be a very small drop in voltage over time, i.e start at 12.7V then 12.65. What do you get?
Start the engine and once again measure and monitor the voltage across the battery. If you got say 12.7V engine off you should see a reading of at least 13.7V. i.e at least 1 Volt more. What do you get?
I'm off to the gym now, post up your results and I'll help where I can.
Richie