Denso 400BW-CE User Manual page 155

Shutdown due to low battery
Problem
When the power is turned on or off or during execution of
applications, the battery output level has lowered so that the BHT no
longer operates.
If lower battery is detected, the BHT beeps five times (for 0.1
second per beep) and then turns itself off. Depending upon the
battery level, the beeper may not sound five times.
Solution
Replace or recharge the battery cartridge. (For the charging
procedure, refer to Chapter 5, Section 5.5.)
Service life warning for backup battery
Problem
The backup battery has been charged/discharged by the specified
number of times so that the battery capacity lowers below the
specified level.
Solution
Replace the backup battery. For the replacement procedure, refer to
Chapter 2, Section 2.4.
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Chapter 4 Error Messages