[Copprhead]

If you get only 12.8V with fully charged batteries, there is something wrong. Reasons might be:

• 1.) Batteries not really fully charged (maybe the charge regulator stops charging too early?)
  2.) Batteries old/broken
  3.) Cables too long or not thick enough
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  3.) applies also to your extension cord, it might be too thin. We have the same problem with our fridge. With extension cord the fridge only runs with really full batteries, then the voltage gets too low and the fridge stops voluntarily to protect the battery from deep discharge.