Then something is on pulling power from the battery. Any light on the vehicle or any relay that is stuck closed.
the alternator is bad
Your alternator is not recharging the battery and is defective.
Well it maybe that the battery is to small for the truck or the alternator is bad
If it is running on the battery only then that means the alternator is not charging the battery.
the diodes in the alternator have gone
To test the alternator on a 1985 Dodge truck, first ensure the battery is fully charged. Start the engine and use a multimeter to measure the voltage at the battery terminals; it should read between 13.5 to 14.5 volts if the alternator is functioning properly. Additionally, you can check the alternator's output by disconnecting the positive battery cable while the engine is running; if the engine stalls, the alternator is likely faulty. Lastly, inspect the alternator belt for any signs of wear or looseness, as this can affect performance.
bad alternator, bad voltage regulator, bad battery, bad connections.
Try jumping it off. If it cranks remove the + side on the battery. If it shuts off the alternator is bad .If it does not shut off clean the battery terminals . Have the battery checked at the Auto Zone.
As long as the alternator works and your charging system is correct, the new battery will start your truck and keep it running.
The answer is no . The battery starts you vehicle , the alternator charges the battery so if your alternator is bad your battery is not receiving a charge which will cause a "no start" condition.
bad alternator ? its not charging the battery
Start the truck then disconnect the battery if it's still running the alternator is good