Battery Charging System on a 2007 Saturn Outlook

After the Check Battery Charging System came on the vehicle, it completly shut down and had to be towed.  The mechanic stated that the battery was completely dead.  We attempted to charge the battery but it wouldnt hold the charge.  After replacing the battery, we noticed that the alternator was also bad.  The alternator was replaced and things went well for a couple of days.  While driving at low speeds, everything is fine.  But as soon as I started to accelerate, the voltage meter went down to 8 volts and the battery light came on again.  as I decrease in speed, the voltage went back to 14 volts and operated normal.  So driving below 55 is not and issue, but as soon as i hit 60-65, the light comes on.