You need to test the battery and the alternator. The battery voltage should be about 12.4 volts when the engine's off. The alternator should be suppying about 13.5 volts at the battery terminals when the engine's running. If those conditions are good, look for poor connections at the battery terminals. If that's good, the alternator is working, the battery could be losing charge because of a glove box or trunk light staying on and draining the battery, or the battery could be failing. Best way is to charge the battery with a charger, get a mechanic to load test the battery and alternator to see what's wrong for sure. But look for a light staying on first.