Didn't say what the car is, but big Fords are famous for corroding the INSIDE of the positive ( red wire ) battery cable clamp. Pull the cable clamp off the post & use a small pocket knife blade to scrape the inside of the clamp & the sides of the post. If it works better, most all car parts stores carry a small wire brush designed just for the job.
You need to read the voltage at the battery when running. If it's below 12 then take a reading at the alternator. If it's below 12 then you need a new alternator.
I had the same problem with my 88 T-Bird. The shop told me I must have a dead short somewhere.
I ask. how much to fix that.
they said, shop fees x how many hours it takes to find it plus parts.
I was looking at like $15 per hour times whatever.it took to find it.
I stilled owed on it SOoooo, I let it get repoed.
In the end it was cheaper.
Backwards jumper cables can fry some pcm circuitry.
If your vehicle is new enough (you don t mention year & model), it is possible your computer is controlling the charging rate and could have a problem.
-Ken
Check: interior,trunk, underhood lights for bad switch . Suggest that if your not up to this have a reputable auto tech evaluate vehicle. This will cost less in long term.
Sounds like there is a short somewhere that is draining your battery
So my car wouldn't start so I took the battery to advanced auto and they said I needed a new one so I paid $140 for it and two days later it wouldn't start again without being jumped off and wouldn't stay running long. so I figured it was the alternator. So I took the alt up there and they said it was still good so I was wanting to know what the most likely problem is. This all started the day after I hooked the jumper cables up backwards..
