I think, there can be 02 issues: Either the battery has grown weak (which is highly likely), or, there are battery drain issues.
In case of weak battery, to check and test it, first check the voltage via multimeter across terminals, when car is in OFF position, and nothing is working (lights, engine, etc. everything switched OFF). For a good, healthy battery, the reading is usually, above 12 (however slight, but its above 12, and not below it)
Then start the car, and again check voltage. It must be 12 (if its different, then alternator / generator may not be working and charging the battery properly. Above 12 or below 12, both may not be good for the battery and ultimately, associated systems).
If the reading on battery, when everything is OFF, is below 12; it means the battery has grown weak and requires replacement.
