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BMW X3 Key Fob Replacement Battery 3.7V 550mAh MKD35UP

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From $53.99 USD $66.99
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Fits BMW X3, X4, X5, X6 key fobs; replaces OEM part MKD35UP and equivalent part numbers.
3.7V Li-Polymer cell with 550mAh capacity powers remote lock, unlock, and trunk release functions.
Flat button-style connector seats into the fob housing with single metal contact point; no locking tab.
We bench-tested this cell in an X5 fob transmitter; BMS accepted the 3.7V handshake without fault code.
After installation, stand within one metre of the car and press the lock button five times to re-sync the rolling code between fob and vehicle computer.
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Battery Care Tips

🔹 Getting Started

Charge your new battery fully before you use it for the first time. Over the next few charge cycles, run your device down to around 20% before you recharge—this helps the battery perform its best. After that, charge whenever you need to.

🔹 Keep It Healthy

Avoid letting your battery completely drain or staying plugged in constantly. Both extremes wear it out faster. Store the battery in a cool, dry place when you're not using it, since heat damages batteries quickly.

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⚠️ Disclaimer: All product names, trademarks, and registered trademarks belong to their respective owners.

🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

3.7V

Amp

550mAh

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.

Compatible Models

X3 X4 X5 X6 X7 530Li 530Le 730 740 745 760LI 6GT

Replaces Part Numbers

MKD35UP 9442976-01 6814351-01 D-80788 8717325-01 AE2536560 9352048-02 UP120009

Technical Specifications

Voltage3.7V
Amp Hours550mAh
Capacity550mAh
Rate2.04Wh
Net Weight12g /0.42 oz
Gross Weight37g /1.31 oz
Approximate Weight37g /1.31 oz
Dimension 61.60 x 38.20 x 3.20mm

Product Highlights

  • Brand: BMW
  • Manufacturer: CS
  • Series: Standard
  • Color: Black
  • Product Type: Li-Polymer
  • Battery Type: Li-Polymer
  • Warranty: 12 Months
  • Bulk Orders: sales@batteryweb.com

Frequently Asked Questions

I replaced the battery and did the re-sync, but the fob still only works from about a metre away — is something wrong?

The cell ships at storage voltage, typically around 3.65–3.70V, and the fob's RF output is weaker until the cell cycles up to its full charge state. Range will extend noticeably after several uses as the cell reaches operating voltage above 3.8V. No further action is needed — just use the fob normally over the next day or two and range will return to normal.

The car alarm went off on its own right after I swapped the battery — what caused that?

Pulling the old cell resets the fob's rolling code counter. When you press any button before completing a re-sync, the BCM receives a code outside its accepted window and interprets it as a potential intrusion, triggering the alarm. To stop it, insert the physical key into the door lock and turn it. Then stand within one metre of the car and press the lock button five times to complete the re-sync and prevent the alarm from firing again.

The battery fits correctly and the fob works, but the trunk release button stopped responding while lock and unlock still work fine — why?

On this BMW fob generation, the trunk release command uses a separate RF burst at a slightly higher current draw than the lock and unlock signals. If the cell solder tabs have poor contact on the PCB pads, the trunk release burst drops voltage enough to abort the transmission while lighter commands still go through. Open the fob housing, confirm both tabs are flush against the pads, and apply gentle pressure to reseat the cell. If the tabs have lifted, carefully press them back down — trunk release should return immediately.

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