Did they actually test the battery, or just check for voltage? 8.4 is definetly too low, should have around 12.4+ for a fully charged battery. Could simply be a bad battery. Howver, there are other possabilities, such as a drain in the system (something staying on or a short to ground) or the alternator not charging properly. What kind of voltage do you vave with it running? While that's a good indicator that it's charging ok, having it load tested is the best bet, if it's not keeping up the proper amount of amperage or has bad diode, it will not cherge the battery. A bad diode can drastically shorten a batteries life, as it sends an AC voltage spike through the system.
