1995 Mercury Marquis Battery loses its power

I bought this car recently and after a few days car doesn't start. Battery was down. So I thought may be problem is with battery. So I recharge my battery and and car would start fine. But after a couple of days again I got the same problem. Auto Electrician says that dynamo is good and when he checked the battery while engine is on, it shows that battery is charging. But again after I drive for 1-2 days, battery will go down. So I changed it. But still same problem. Please advice.