Mercedes-Benz CLK Siren Alarm Replacement Battery 4.8V 150mAh

Mercedes-Benz CLK Siren Alarm — 4.8V Ni-MH Replacement Battery (203 820 38 26)

This is a 4.8V, 150mAh Ni-MH replacement battery for the Mercedes-Benz CLK Siren Alarm. It fits the CLK Siren Alarm, CLK 270, and CLK 320 vehicles, matching OEM part number 203 820 38 26. The siren alarm draws from this battery to sound and sustain audible alerts when the vehicle's main power is cut.

• CLK Siren Alarm fit — 270, 320, and base CLK: These models share the same siren housing, connector, and 4.8V battery bay. The BMS in the alarm unit accepts this cell configuration without requiring any recalibration or module reset.
• Bench tested on actual hardware: We cycled this battery through charge and discharge on the bench. The BMS accepted charge from a cold state and held voltage above the siren activation threshold without tripping prematurely.
• Cover latch check after install: After fitting the battery, close the siren cover fully and confirm the tamper switch clicks into position. If the tamper switch is not engaged, some alarm control units interpret the open-circuit signal as a battery fault and log a false error.

Why the CLK siren activates briefly then cuts out

The CLK siren draws a surge of current at activation to drive the sounder. If the battery is below its minimum voltage threshold — common after a long dormant period — the cell cannot sustain that current draw. The alarm unit interprets the voltage drop as a fault and cuts the siren output. Fully charge the new battery for 24 hours before testing to ensure the cell reaches operating voltage.

Tamper fault appearing on the alarm panel after battery replacement

A tamper fault after a battery swap almost always traces back to the siren housing cover, not the battery itself. The tamper switch sits on the cover edge — if it is not fully latched, the alarm control unit sees an open-circuit signal and flags it as a fault. Re-seat the cover, confirm the latch clicks closed, and clear the fault from the alarm panel. If the fault persists, check that the tamper switch arm has not been bent during removal.