93 Lincoln Conti. Died Out

Have a 93 conti. and the other day mom was driving it and it died at the intersection and she tried cranking it and cranked the battery dead. i got it to a friends house a block away and put charger on it and and 20 minutes later it fired right up and ran good has been running for couple of days and ran today and she pulled up to a red light and it died and same thing she cranked the battery dead i towed her over to a parking lot near by and tried jump starting it and would work. I left and came back later and tried jumping it later and it cranked for a few seconds then dead i do know the battery is going bad but even with the high boost charger on it it still wont start what could be the problem.