Chevrolet Repair: Battery Mysteriously Running Down, starter problem, berreta

Question
Hello Van,
A few days ago I purchased a 1988 Chevy Berreta.
All was fine until a couple of days later...the battery keeps going down.This is a NEW battery,I'm sure because I have papers showing date of purchase.
Well the first time this happened I charged it up good,drove the car 3 miles and it ran down again that quick!I charged it up again and it lasted 6 miles before it happened again.
This time someone tested the alternator by taking
one of the cables off while it was still running.It still ran.So they said the alternator was o.k.They thought it is either a bad battery cable or bad starter selenoid.However it starts easily.
Surprisingly,this time the battery has stayed charged up all night and all day today!I even drove the car a couple of miles without a problem.
However I want to be sure it is going to be o.k.before I go very far.
Could you tell me what may be causing this annoying problem?Obviously it is something electrical.I am frustrated because I bought this car just a few days ago and am having trouble already.But it does run good when the battery works!
I kindly thank you for your help.

Answer
Hello Betty,
That is not a great way to check the alternator. It will tell if it is charging enough to operate the ignition, but if you go to one of the larger parts stores, they will often take a battery/alternator tester out to the car and really test it, including the diodes.
The solenoid has little or nothing to do, other than during starting. Very remote chance that it could be your problem, other than the possibility that the battery really isn't dead.
Next time it won't start, turn the headlights on, and see if they are dim also, which would indicate a dead or low battery.
If the headlights are bright, even when you try to start the engine, you could defanitely have a solenoid or starter problem, or a loose cable.
The cables at the battery, and at the other ends must be clean and tight. Not only clean on the outside, but in where the metal contacts the metal of the battery.

If the battery is going dead, and the alternator is good, it may be that the ignition switch is actually going to accessories when you shut it off. Make sure that isn't happening.

Check this link.

electric.articles.mbz.org/drain/

Van