Here is an entry from the operating instructions:
REPLACING THE BATTERIES
If the display shows poor information or no information at all, this means that
that the batteries need to be replaced.
Remove the battery cover and replace the old batteries with two new ones.
Batteries. When replacing the batteries, pay attention to the polarity of "+" and "-".
If the display does not work properly after a restart, remove the batteries and replace them.
Remove the batteries for a few minutes and then reinsert them.
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