{"product_id":"bmw-x3-replacement-battery-37v-550mah-li-polymer","title":"BMW X3 Key Fob Replacement Battery 3.7V 550mAh MKD35UP","description":"\u003cdiv class=\"bpw-desc\"\u003e\n  \u003ch2 class=\"bpw-desc-h2\"\u003eBMW X3 \/ X5 Key Fob — 3.7V Li-Polymer Replacement Battery (MKD35UP)\u003c\/h2\u003e\n\n  \u003cp class=\"bpw-desc-lead\"\u003eThis 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.\u003c\/p\u003e\n\n  \u003cul class=\"bpw-desc-bullets\"\u003e\n    \u003cli\u003e\n\u003cstrong\u003eX3, X4, X5, X6 fob platform:\u003c\/strong\u003e\n    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.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eBench tested on actual hardware:\u003c\/strong\u003e\n    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.\u003c\/li\u003e\n\n    \u003cli\u003e\n\u003cstrong\u003eRolling code re-sync after swap:\u003c\/strong\u003e\n    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.\u003c\/li\u003e\n  \u003c\/ul\u003e\n\n  \u003chr class=\"bpw-desc-divider\"\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003eWhy the BMW fob stops responding entirely after a battery swap\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eRemoving 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.\u003c\/p\u003e\n\n  \u003ch3 class=\"bpw-desc-h3\"\u003ePush-button start not responding with fob in pocket after battery replacement\u003c\/h3\u003e\n  \u003cp class=\"bpw-desc-p\"\u003eIf 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.\u003c\/p\u003e\n\u003c\/div\u003e","brand":"BatteryWeb","offers":[{"title":"Warranty 1 Year","offer_id":43339814568026,"sku":"BWCS-MDK350SL-1","price":53.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 2 Year","offer_id":43339814600794,"sku":"BWCS-MDK350SL-2","price":63.99,"currency_code":"USD","in_stock":true},{"title":"Warranty 3 Year","offer_id":43339814633562,"sku":"BWCS-MDK350SL-3","price":70.99,"currency_code":"USD","in_stock":true}],"thumbnail_url":"\/\/cdn.shopify.com\/s\/files\/1\/0674\/4775\/0746\/files\/BW-CS-MDK350SL-1.webp?v=1778366812","url":"https:\/\/batteryweb.com\/products\/bmw-x3-replacement-battery-37v-550mah-li-polymer","provider":"BatteryWeb","version":"1.0","type":"link"}