Digital Systems 333C PLC Replacement Battery 3.6V 2700mAh

Digital Systems 333C — 3.6V Li-MnO2 Replacement Battery

This is a 3.6V, 2700mAh lithium manganese dioxide cell for the Digital Systems 333C programmable logic controller. It powers the PLC's real-time clock and SRAM memory backup circuit. When this cell fails, the controller loses stored programs and system settings the moment main power drops.

• 333C memory backup circuit: The 333C uses this cell solely to hold SRAM contents and keep the RTC ticking when mains power is absent. The 3.6V nominal voltage matches the backup rail directly — no regulation stage between the cell and the memory array.
• Bench tested on actual hardware: We ran this cell against the 333C backup circuit and confirmed the BMS held stable voltage across the SRAM retention threshold. The cell accepted the PLC's float charge without event and maintained correct RTC tick rate throughout the test.
• Hot-swap protocol for the 333C: Always replace this battery with the 333C powered on and in RUN mode. Removing the cell while the controller is de-energised drains SRAM instantly. If the PLC was off during the swap, a full program reload from the programming device is required before resuming operation.

PLC losing program memory after battery swap on the 333C

The 333C holds its ladder logic and configuration in volatile SRAM. That SRAM is only kept alive by this backup cell when mains power is absent. If the old cell is pulled while the controller is powered down — even briefly — the SRAM array collapses and all stored data is gone. There is no internal capacitor bridge on this controller to cover the gap. After any cold swap, reconnect the programming device, reload the full project file, and verify outputs before returning the PLC to service.

Battery alarm not clearing on the 333C after a confirmed good installation

A new cell does not automatically reset the battery fault flag in the 333C. The alarm is latched in software and must be cleared manually through the programming terminal once the replacement is confirmed seated and reading voltage. A freshly shipped Li-MnO2 cell may initially read slightly below 3.6V — this is normal storage voltage and rises within a few hours on the PLC's float charge. Clear the alarm only after the cell has stabilised above 3.5V at the backup rail.