About this replacement
BWM X3 / X5 / Z4 Key Fob — 3V Lithium Vanadium Pentoxide Replacement Battery
This is a 3V, 20mAh lithium vanadium pentoxide cell for the BWM X3, X5, and Z4 keyless entry remotes. It replaces the original coin cell that powers the fob's RF transmitter for lock, unlock, and trunk release. Dimensions are 20.60 × 20.00 × 7.80mm — measure your existing cell before ordering if unsure.
- X3, X5, and Z4 fob compatibility: All three models use the same fob housing footprint and RF transmitter circuit, drawing from the same voltage rail. One cell covers all three platforms without modification.
- Bench tested on actual hardware: We ran this cell through the fob's transmitter circuit and confirmed stable 3V output across repeated button presses. The cell held voltage without sag under the short RF burst load.
- Rolling code re-sync after swap: After fitting the new cell, stand within 1 metre of the car and press the lock button 5 times. This re-syncs the rolling code between the fob and the car's receiver module — skipping this step is the most common reason the fob appears dead after a battery change.
Push-button start not responding with fob in pocket after battery swap
The push-button start system reads the fob passively via low-frequency induction, not the RF transmitter button. If the new cell is still at storage voltage — typically 2.85–2.95V rather than the full 3V — the passive transponder may not broadcast strongly enough for the car to detect it in your pocket. Walk through the rolling code re-sync first. If the car still won't start, hold the fob flat against the start button itself; the car has a backup inductive reader built into the button surround that works at close range.
Fob range noticeably shorter right after fitting a new cell
A freshly installed lithium vanadium pentoxide cell often sits at storage voltage rather than peak operating voltage. The RF transmitter output scales with supply voltage, so reduced range immediately after a swap is normal. After 4–6 button presses the cell reaches its rated 3V and range returns to normal. If short range persists beyond that, check the battery contacts in the fob housing for oxidation and clean them with a dry cotton swab.