I have an 85 Z28,with 305/TPI combo.The car sits most of the time.In the last ten years,I have put about 5000 miles on it.Anyway,I went to run it yesterday and the battery was dead.no surprise there.I put the booster charger on it and it started right up.However,the voltmeter would not come up off zero and it died on a quick trip around the block.The windows came up super slow and it would not start again.After another boost,the voltmeter came to life,car ran great and all acc.worked like normal.The car started up repeatedly afdter this last jump.This is a first for this car.The alternator is lowwwww mileage,but ten years old.I'm still thinking shot battery,or the alternator???.comments????................BILL

 

try swapping it with a known good battery from another car or a new battery first.