I would be concerned about your battery voltage. Defenately sounds like it's time for a set (Be sure to buy two). As far as the alternator, I would put a set of batteries in it and recheck. You have to keep in mind the alternator was attempting to charge what seems to be a bad battery. You're always gonna show low voltage when this is the case. Also wait at least 2 minutes after cranking to check the running voltage, during the first two minutes the glowplugs are still operating and you won't get a "true" measurement.