Denso BHT-300B User Manual page 144

Abnormal shutdown
Problem
The BHT was shut down abnormally* and has been left without a
battery cartridge loaded or with a discharged battery cartridge loaded,
so unsaved data is lost.
(*"Normally shut down" refers to "the BHT is turned off with the PW
key or by the auto power-off feature.")
Solution
Refer to Chapter 2, Section 2.3.6 "BHT Turning-off Notes."
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