About this replacement
Omron C20 Series — 3.6V Li-SOCl2 Replacement Battery (3G2A9-BAT08)
This is a 3.6V lithium thionyl chloride cell rated at 1700mAh (6.12Wh), cross-referenced to OEM part numbers 3G2A9-BAT08 and C500-BAT08. It fits the Omron C20, C20P, C20PF, C28P, and over 20 additional C-series PLC models. The cell maintains SRAM program memory and real-time clock data during mains power interruptions in industrial automation environments.
- C-series PLC compatibility: These models share the same backup power rail voltage, connector footprint, and SRAM retention threshold. Any cell that drops below 3.0V under load will fail the BMS handshake and trigger a battery alarm on the controller panel.
- Bench tested on actual hardware: We ran this cell on a C-series controller under continuous SRAM retention load and confirmed the BMS accepted the cell without fault flags. Open-circuit voltage on arrival measured within expected storage range for Li-SOCl2 chemistry.
- Hot-swap procedure for C-series controllers: Always replace this battery with the PLC powered on and in RUN mode. Removing the cell while the controller is powered off will drain SRAM instantly — program data and clock values are lost and a full reload from the programming device is required.
Why the C20 battery alarm won't clear after a confirmed good installation
On Omron C-series controllers, the battery alarm flag is latched in software — it does not reset automatically when the new cell is detected. After fitting the replacement cell, you must connect a programming device and manually clear the alarm bit through the PLC's system settings or ladder program monitor. Skipping this step leaves the alarm active even with a fully functional cell installed. Confirm the alarm is cleared before disconnecting the programming device.
New cell reading lower than 3.6V on the controller's battery monitor
Li-SOCl2 cells ship in a passivated state — the internal lithium anode forms a thin oxide layer during storage that suppresses the open-circuit voltage. On a C20 controller, the battery monitor may report a voltage below the nominal 3.6V for several hours after installation. This is not a fault with the cell. The passivation layer dissipates under the PLC's float charge load, and the voltage climbs back to full rated level — typically stabilising above 3.5V within a few hours of continuous connection.