About this replacement
BMW X3 / X5 Key Fob — 3.7V Li-Polymer Replacement Battery (MKD35UP)
This 3.7V, 550mAh Li-Polymer cell replaces the OEM battery inside BMW key fob transmitters for the X3, X4, X5, X6, and related models. It powers the remote keyless entry functions — lock, unlock, and trunk release. OEM part numbers covered include MKD35UP, 9442976-01, 6814351-01, and 8717325-01, among others.
- X3, X4, X5, X6 fob platform: These BMW models share the same key fob transmitter housing and Li-Polymer cell format across several production years. The flat 61.60 × 38.20 × 3.20mm footprint fits the PCB cavity directly, and the cell connects to the same two-pad layout used across this generation of BMW remotes.
- Bench tested on actual hardware: We cycled this cell through repeated lock and unlock transmissions on a BMW fob unit. The cell held voltage above 3.5V under RF burst loads, and the protection circuit cut off cleanly at the low-voltage threshold without latching in a fault state.
- Rolling code re-sync after swap: After fitting this battery, stand within one metre of the car and press the lock button five times. This re-establishes the rolling code handshake between the fob and the body control module — skip this step and the fob will appear dead even with a full battery.
Why the BMW fob stops responding entirely after a battery swap
Removing the old cell interrupts power to the fob's microcontroller, which clears its place in the rolling code sequence. The car's BCM tracks a narrow window of valid codes — once the fob falls outside that window, it treats every transmission as invalid. This is not a battery fault. The fob and car need to re-sync before remote functions return.
Push-button start not responding with fob in pocket after battery replacement
If the push-button start ignores the fob even after re-sync, the cell may still be at storage voltage rather than full charge — Li-Polymer cells ship at around 3.7V and the car's passive entry system needs a stronger RF signal than the lock button does. Hold the fob flat against the start button emblem to use the physical backup transponder — this starts the car without RF. Drive the car normally for a short period; the cell will stabilise above 3.8V and passive entry will recover.