User`s manual

Battery life and replacement procedure Maintenance and inspection
MELSEC System Q, Hardware 10 – 3
10.3 Battery life and replacement procedure
The batteries installed in the CPU module and SRAM card are used for data retention of the pro-
gram memory, standard RAM, and latch device during the power failure. Special relays SM51
and SM52 turn on due to the battery voltage drop. Even if the special relays turn on, the program
and retained data are not deleted immediately (refer to section 10.3.1). But if the special relays
are not observed, the contents of the buffered memory may be lost.
SM51 and SM52 turn on when the battery voltage of the CPU module or SRAM card is lowered.
To identify the specific battery of the memory of which voltage is lowered, check the contents of
the special registers SD51 and SD52.
The cases for memory retention by the CPU module battery and the SRAM card battery are
described below. The important points are:
– The CPU module battery cannot retain the SRAM card memory.
– The SRAM card battery cannot retain the CPU module memory.
 = retained
— = not retained
NOTE After relay SM51 turns on, replace the battery as quickly as possible.
When the battery-low special relay SM52 turns on, immediately change the battery.
Special registers SD51 and SD52
Bit 2 Bit 1 Bit 0
Status indication for Battery for SRAM card Battery for CPU module
Tab. 10-3: Bit pattern SD51 and SD52
Power supply
module
Voltage of buffer battery
for CPU
Voltage of buffer battery
for SRAM memory card
Memory retention
CPU SRAM card
On
Sufficient
Sufficient

Too low 
Too low
Sufficient

Too low 
Off
Sufficient
Sufficient

Too low  —
Too low
Sufficient —

Too low — —
Tab. 10-4: Memory retention