I recently had an odd experience with my 1991 Ford Bronco. Battery discharged, took the battery to the local NAPA store where they checked it and said it was good, just needed a charge. Charged it and within 5 days it was discharged again. I start thinking about parasitic drains and have my mechanic check that out and he finds nothing but recommends that I replace the battery even though it was only 2 1/2 years old. Go to NAPA and get another battery, put it in and in 5 days it's stone dead. Go back to NAPA and they charge it for me but recommend that I leave it there overnight so they can see it it retained the charge. The next day they said they did not like what they saw and replaced that battery with another. Since then, no problems. Conclusion, batteries that are no good can take a charge and not retain it, even new ones. Go figure.....