Likely a bad alternator or connections. Sounds like the alternator is undercharging the battery and you will have problems as the battery runs-down. Check the battery cables and make sure the connectors are clean and tight. There have been problems with the connector to the alternator, e.g., cracked wires near the connector.
Get the electrical check done, or as a start measure the voltage at he battery while the vehicle is running, and also when it is shut-off.