When a vehicle battery won't maintain a charge, it's possible that you're dealing with a parasitic draw. Testing the battery is the only way to find out. To find a parasitic battery drain, use the process of elimination to narrow down the problem.
Charge your battery if necessary. Dead batteries (and even weak ones) don't give accurate results on a draw test. The dome light is a good indicator of battery strength. If the light is weak, pulsating or refuses to turn on, you'll need to charge the battery.
Shut off everything in the car. Pull the keys out of the ignition. Ensure all service lights are off, lighted compartments closed and doors shut. Open the hood, and disconnect the trouble light underneath if your vehicle has one. Wait for 30 minutes before moving on to Step 3. (This enables normal drain from interior components to finish cycling.)
Set your battery tester to 10 amps DC. Disconnect your positive battery connection (red cable) and position it safely away from metal components. Connect the meter's positive probe to the battery's positive post while simultaneously holding the negative probe in the air; keep it away from anything metal.
Place the negative probe on the end of the removed positive cable to complete the circuit. Yes, you're putting a negative on a positive. Yes, this is ok. If you have a severe drain, you're likely to pop a fuse or two-identifying your problem.
Check the meter reading. A normal reading is usually under .035 amps. If you have a minor drain that causes a higher reading, you'll need to check each component.
Remove the first fuse in the box, and check to see if the load problem is resolved. If not, replace the fuse and check the next one. Repeat the process with all fuses. If the fuses check out fine, you have a wiring problem.
Remove the tester and reconnect the battery. Follow the hot wire to the next connection. Remove the positive cable and connect the positive probe of the tester in its place. Connect the negative probe to a metal object. Check the meter. If the drain isn't resolved, reconnect as normal and follow the hot wire to the next connection. Repeat this step at each connection point until you narrow down the problem.
Never use a tester with less than 10 amps DC. Connection to a car battery will give inaccurate results and can destroy the tester.