2002 Ford Windstar bad batteries

In July, 2007 we replaced the original battery with an Interstate battery. We have had to replace the battery for the third time. The new batteries have always failed after the van sits over night or this last time for a day and a half. The second time the alternator was tested and was good. We are baffled by this and need some clues as to what to check next. ( My husband is a retired Firestone store manager and had his best tech check it out and he found nothing wrong.)