About this replacement
BMW F44 228i xDrive Series — 6V Ni-MH Replacement Battery (84 10 6 833 994-02)
This is a 6V, 300mAh Ni-MH rechargeable cell for the integrated alarm siren fitted to BMW F44 and F90 platform vehicles. It fits the F44 228i xDrive, F44 M235i xDrive, F90 M5, F91 M8, and 52 additional variants using part number 84 10 6 833 994-02 or 84 10 6 833 994. The siren draws from this cell to sound theft alerts independently of the main vehicle battery.
- F44 and F90 platform siren compatibility: These model lines share a common siren housing and connector spec. The cell dimensions — 55.00 × 37.10 × 14.70mm — match the OEM bay exactly, and the BMS handshake expects a 6V Ni-MH chemistry. Substituting a different voltage or chemistry will cause the alarm module to report a battery fault.
- Bench tested on actual hardware: We cycled this cell through charge and discharge on the bench to confirm BMS acceptance and proper float behaviour at full charge. The cell held stable voltage and the siren module cleared all fault codes throughout the test.
- Siren cover re-engagement after swap: After fitting this cell, seat the siren cover firmly until you feel the latch click. The tamper switch inside the housing must make full contact. If it does not, the alarm panel flags a tamper fault — which some BMW modules display as a battery error rather than a cover fault.
Why the BMW siren battery discharges faster in outdoor temperature cycling
Ni-MH cells self-discharge faster than Li-ion, and that rate accelerates with thermal swings. A siren mounted in the engine bay or wheel arch sees large temperature deltas between summer and winter. Each cycle stresses the cell chemistry and raises the self-discharge baseline. On vehicles parked for extended periods, the siren cell can drop below the activation threshold before the annual alarm test is even triggered.
Siren fires briefly then cuts out mid-alarm
This happens when the cell voltage collapses under the siren's output current draw — the BMS detects an undervoltage condition and cuts power to protect the cell. A degraded or deeply discharged Ni-MH cell cannot sustain the current the siren demands for a full alarm cycle. The fix is a fresh cell followed by a full 24-hour charge before testing. Confirm the siren completes its full test cycle with voltage sitting above 5.4V under load.