I will try to help you as for what l know. When the truck is programmed at the factory it is set the bdi for a new battery as for what that means is the controller looks for good Pacific gravity and correct voltage per cell on a new battery.
After charge and the battery has sat over night each cell of a new battery should read at least 2.1volts per cell if the battery can not get to that reading then the gauge will not reset to 100percent.what l would do is run the battery down until the truck goes into lift interrupt than recharge the truck a full charge cycle let it set overnight than check the cell to see if they are at 2.1 volts per cell it is possible that the battery was sitting at the factory before you got it.if the cells are at 2.1 then l would think there is a problem with the gauge or controller. If they only get to 2.07 or 2.o8 then the gauge would only reset to80 percent than get in touch with the battery supplier

If this is a curtis bdi you will want to charge it too see a high voltage then let it drain below 25%. That should reset it. Basically the bdi wants to see a large range of voltages. Its something that there really isn't good documentation for.