That is I fear to be expected. But if it works ok on the NiMH - even if it says 'half full', you will still get most of the benefit - the discharge curve of voltage vs state of charge is quite flat until the last 10% or so. BUT will not get much warning, as the change from half -full to dead will occur quite sharply, compared to the primary cells. From the electronics designer's perspective it makes it hard to do an accurate battery indicator on NiMH cells, as voltage is not reliable, so when it really matters, then we have to resort to something more like an electricity meter, and integrate and record the current in and out.
