My first thought is to buy something else :-)

Seriously though, make sure you check the battery voltage under load. Those units will do some funny things if the battery voltage drops below ~18V under load.
There is also a battery reset setting, if it's set too high then the BDI will never reset to 100%. A quick check is to have the battery on the charger, then quickly swap it onto the truck before the battery voltage drops down to normal.
The charger needs to push the battery to 2.44V/cell before it's actually 80% charged, 2.5V/cell @ 100%.

As for the code 80, if the pot tests ok on the display, then I'd start checking the over-mast harness. There are usually some spare wires, if you happen to find a bad wire.

Hope this helps...