I had the same problem when I bought my car a few years ago. I would leave the garage with a fully charged battery, but return after 8 or 10 miles with a nearly dead one. The initial diagnostics indicated a bad voltage regulator so I replaced it, but still had the problem.
I also asked "what else can go wrong."
I was referred to this link which I found very helpful
http://www.coolcatcorp.com/faqs/Lucasalternators3.html
and this one
http://www.coolcatcorp.com/faqs/Lucasalternators2.html
My problem was solved when I replaced the 3AW Relay with a Solid State Replacement for 3AW Relay
http://www.coolcatcorp.com/Merchant5/merchant.mvc?Screen=PROD&Store_Code...