Okay, the most common cause of what you described is a loose or corroded battery terminal and is really easy to fix. Take a look through this link:
If that isn't helpful, then do this. Keeping in mind the battery or the alternator can be failing, we need to check one thing at a time. What I want you to do is when the car is running, take it to a parts store and ask them to load test the battery. That way we will know if the battery is just weak. In most cases, they will do it free of charge hoping to sell you a battery.
If you can't take it, here is a link that explains how to load test one at home.
Next, if the battery passes, either have them test the alternator at the same time or you can do it yourself. All you need is a multi meter or volt meter. Here is a link that shows how it's done.
When you check the alternator output, with the engine off, battery voltage should be around 12v. With the engine running, it should be near 14v on the meter.
If everything checks good, we will need to check for a voltage draw, but I'm not going to overwhelm you with that at this point.
Do this and let me know what you find. If possible, let me know the voltages you find if you do it yourself.
I will watch for your reply.
Sunday, May 24th, 2020 AT 7:53 PM