Hi9 Pro - Battery sudden discharge?

I have a problem with my Hi9 Pro, since purchase last October. From fully charged to around 25%, everything is fine - it then suddenly drops below 20%, then rapidly goes below 10%. I have replaced rhe battery and there are no open apps causing heavy useage. Could this be a hidden app causing heavy battery useage? I have looked at Settings, Battery, ‘Battery useage data’ is not avilable.
Any ideas?
Peter
UK.