About this replacement
Yokogawa S9548FA — 2.4V Ni-MH Replacement Battery (HHR-21AHF2A1)
This 2.4V 2500mAh Ni-MH cell replaces the HHR-21AHF2A1 backup battery in the Yokogawa S9548FA PLC. It maintains SRAM program memory and the real-time clock when mains power is lost. Swap it before the low-battery alarm trips a fault condition on your controller.
- S9548FA memory backup circuit: The S9548FA routes battery voltage directly to the SRAM retention rail and RTC oscillator. Both circuits draw from the same cell, so a depleted battery kills program memory and clock simultaneously — not one at a time.
- Bench tested on actual hardware: We cycled this cell on a PLC backup load fixture at the rated retention draw. The cell held above the SRAM retention threshold of 1.9V throughout the discharge curve and the BMS accepted the charge handshake without error.
- Hot-swap procedure for the S9548FA: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is powered off collapses the SRAM retention rail instantly. If that happens, the program is gone and must be reloaded from your programming device.
PLC battery alarm not clearing after a confirmed good installation
The S9548FA does not auto-clear the battery alarm once a new cell is fitted. The alarm flag is latched in the controller's system status registers and must be reset manually through your programming software. Connect to the CPU, navigate to the system diagnostic or status area, and clear the battery alarm bit. If the bit re-latches within minutes, check that the cell contacts are seating fully — a high-resistance connection can hold the alarm even with a fresh cell installed.
New cell reading lower than 2.4V immediately after installation
Ni-MH cells ship at a reduced storage voltage — typically 2.1V to 2.2V — to slow self-discharge during transit. This is normal and does not indicate a faulty cell. The S9548FA float-charges the battery continuously while the PLC is powered, and the cell climbs to its rated voltage within a few hours. If the voltage has not recovered to above 2.3V after 24 hours on a powered controller, reseat the cell and verify contact resistance at the battery connector.