Micheal M
I more or less sure that a code 15 means that the battery voltage is lower than the setting of the truck controller and 51 means your capacitor voltage is to low, I sugest you get your battery tested, if it checks out ok then you may need laptop service tool to alter power controller parametters. I relise this advice is abit late in coming to you, and you may have allready solved your problem.

Regards Titus

Ive been told these are battery related error codes, but the battery is new and the charger has been tested and works fine so we're a little confused to say the least. We are sending out a new battery today, hopefully that acts differently....