well if you know that the batteries are in fact good then the BDI may be bad. You can try adjusting it in the controller if that is a BDI wired into the controller and see if it changes anything. Or if the BDI is self contained adjust it with the trimpots on the back side of the guage and check for any change.
Otherwise the BDI or controller may just be faulty and needs to be replaced.
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