There are many factors that can affect your battery and the charge it holds. To get a grasp of the problem, first you want to narrow it down to just a few areas: battery, wiring and charging. Assuming you can start the car, it's easy to quickly tell if your charging system is at least functioning on some level, which can help you rule that out.
In order to do this quick test, you will need an accurate method of measuring voltage. You can buy a cheap multimeter for $10 to $20. They are very handy to have, and versatile. But for this task, I personally use a little gadget that plugs into your car's cigarette lighter and tells you the voltage. They are more specialized, but run only about $4, making it a great investment in your vehicle's electrical health.
In a nutshell, if the voltage goes up when the engine starts, your car is charging. That's it. Obviously there are
other factors at play, but most of the time, if the voltage goes up, look elsewhere for your problem, like the battery itself, the wiring or maybe a parasitic drain. The readings below are for a Jeep which had a slow drain on the battery from an improperly wired stereo system.
Step 1 - Take Voltage Reading With The Engine Off
Your starting voltage ideally should be anywhere from 12.0 volts to 12.6 volts. Any accessories (lights, radio, etc.) will affect this reading and make it read a little lower. If you're sitting in the car, and there's no lights on and nothing obviously powered on, you should see a reading in the above range.
|This reading is a little low, but still within range.|
Step 2 - Take Voltage Reading With The Engine On
You measured the voltage, and you started the car. Did the voltage go up? Then congratulations, you are charging. In general, you should be reading 14.0 to about 14.6 volts with the engine running. There are several factors that affect this number, but it's a good general range you should be looking for. If you see the reading in this range, then most likely you don't have a charging problem. I like to leave the engine running a few minutes to a) let the battery charge and b) make sure there are no fluctuations in the voltage.
|This is a little low, but again, within range|