The battery has a dead cell or short and needs replacing.
My advice is to charge the battery with a battery charger before starting the engine. You will be putting a heavy load on the alternator if you allow it to charge a dead battery. This could damage the alternator.
Inside the alternator
If you have already checked the connections on the battery I would suggest testing the alternator and the battery. If the battery will not take a charge or the alternator will not give a charge you could have similar problems.
If the alternator is good and the battery does not charge, then the battery may be bad. There could be problem with the electronics. You will need to take it to a place where they sell batteries. They have equipment to check out the battery and the wiring.
could be a bad battery
the battery could have a dead cell, this could cause it to not charge. Have the battery tested to be sure.
It varies depending on how much other electronics you are using on the vehicle at the time. Could take hours. I do not recommend you charge any dead battery with the alternator if you can avoid it. it puts too much of a strain on the alternator and may damage it. Charge a dead battery with a battery charger.
could be the alternator. could be the battery, the alternator charges the battery but does not stop the battery holding a charge.could also be a bad starter motor. take alternator off and have it tested. most places will test for free
Could be the alternator is not charing correctly.
Well, It could be your battery or alternator or both needs to be replaced but it could just be loose or corroded battery cables
voltage regulator.
You can start a car without a battery if you hook up a jump pack or use jumper camples but it could fry your alternator. It is pointless to charge a car without a battery. When you charge the car you are charging the battery.