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Yokogawa S9548FA PLC Backup Compatible Battery 2.4V 2500mAh

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Fits Yokogawa S9548FA PLC; replaces OEM part HHR-21AHF2A1 backup battery.
Supplies 2.4V 2500mAh to maintain SRAM and real-time clock during power loss.
Ni-MH cell slots into standard PLC battery holder with positive terminal forward.
Bench test confirms full voltage delivery to float-charge circuit with no BMS lag.
Swap this cell with the S9548FA powered on and in RUN mode—removing it while powered off causes immediate SRAM memory loss and requires full program reload.
Delivery time

This product ships directly from our Manufacturer’s Warehouse and is usually delivered within 5 – 8 business days to your doorstep.

Discount: As a thank you for your patience, enjoy 5% off on your order
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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

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🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

2.4V

Amp

2500mAh

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.

Compatible Models

S9548FA

Replaces Part Numbers

HHR-21AHF2A1

Technical Specifications

Voltage2.4V
Amp Hours2500mAh
Capacity2500mAh
Rate6Wh
Net Weight75g /2.65 oz
Gross Weight100g /3.53 oz
Approximate Weight100g /3.53 oz
Dimension 51.60 x 34.00 x 17.10mm

Product Highlights

  • Brand: Yokogawa
  • Manufacturer: CS
  • Series: Standard
  • Color: Green
  • Product Type: Ni-MH
  • Battery Type: Ni-MH
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

The PLC lost its program after I swapped the battery — what happened and can I recover it?

The SRAM retention rail collapsed the moment the old cell was removed while the controller was powered off. With no voltage on that rail, program memory is gone — the controller cannot recover it internally. Reload the program from your programming device using the last saved project file. Going forward, always replace this battery with the S9548FA powered on and in RUN mode to keep the retention rail live during the swap.

The battery alarm cleared on install but came back within a day — what's draining this cell so fast?

A warm enclosure is the most common cause. Ni-MH self-discharge roughly doubles for every 10°C above 20°C, so a cabinet running at 40°C can deplete a fresh cell far ahead of its rated service interval. Check the enclosure ambient temperature at the battery compartment. If it exceeds 35°C consistently, increase your battery replacement interval and ensure the cabinet ventilation is not blocked.

The RTC is showing the wrong date and time after replacement — is the cell faulty?

The clock lost sync because the RTC oscillator lost power during the swap, which resets the time registers to their default values. The cell is not faulty. Connect to the CPU through your programming software and set the system clock manually to the correct date and time. If you performed a hot-swap with the PLC powered on and the clock still reset, check that the battery contacts are making clean, low-resistance contact with the cell terminals.

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