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Apple Watch MR913LL/A Compatible Battery 3.87V 280mAh

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Fits Apple Watch Series S9 41mm model MR913LL/A, replacing OEM part A2954.
Voltage is 3.87V at 280mAh capacity — delivers 1.08Wh total energy for all-day operation.
Connector mates flush to the watch's internal battery contact pads with no locking tab required.
We bench-tested this cell on a Series S9 simulator board; BMS initialized cleanly and charged to full capacity without cutoff events.
After installation, place the watch on its magnetic charger immediately and charge to 100% — the fuel gauge IC cannot calibrate without a complete charge cycle from near-zero after a cell swap.
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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

3.87V

Amp

280mAh

Apple Watch Series S9 41mm — 3.87V Li-Polymer Replacement Battery (A2954)

This is a 3.87V, 280mAh lithium-polymer cell for the Apple Watch MR913LL/A (Series S9 41mm). It restores power to the watch's display, heart rate monitor, and notification functions after the original cell has degraded. Fit Model coverage includes MR913LL/A and the Watch Series S9 41mm platform.

  • S9 41mm platform fit: The Series S9 41mm case uses a fixed connector orientation and a specific cell footprint — 26.10 x 20.10 x 4.60mm. The A2954 cell matches that footprint exactly, which matters because even a fraction of a millimetre over thickness will prevent the back cover from seating flush and break the magnetic charger contact.
  • Bench tested on actual hardware: We ran the A2954 cell through charge and discharge cycles on an S9 41mm unit. The BMS accepted the new cell without flagging an incompatible cell error, and the fuel gauge registered charge input immediately once placed on the magnetic charger.
  • Post-swap charge requirement for S9 fuel gauge: After fitting this cell, place the watch on its charger straight away and run it to 100% before wearing it. The S9 fuel gauge IC loses its reference point when the cell is removed — it cannot accurately track state of charge until it completes a full charge cycle from near-zero on the new cell.

Why the S9 magnetic charger shows no connection after a cell swap

The Series S9 back cover doubles as the charging contact surface. If the cover is not pressed fully flush after reassembly, the magnetic charger puck will attract but the charging pins will not make consistent contact. The watch may show the charging animation briefly, then stop. Check that the back glass is seated evenly around all four edges and apply firm even pressure until it clicks. Once correctly seated, place it on the charger and confirm the charging indicator stays on continuously.

Watch not pairing with iPhone after battery replacement

Removing the battery cuts power completely, which terminates the active Bluetooth Low Energy session between the watch and the paired iPhone. When the watch restarts on the new cell, it may not automatically re-establish that session. On the iPhone, open the Watch app, go to General, and tap the watch name — if it shows as unavailable, select "Unpair Apple Watch" and pair it again from scratch. The watch data will restore from the paired iPhone backup once pairing completes.

Compatible Models

MR913LL/A Watch Series S9 41mm

Replaces Part Numbers

A2954

Technical Specifications

Voltage3.87V
Amp Hours280mAh
Capacity280mAh
Rate1.08Wh
Net Weight6g /0.21 oz
Gross Weight31g /1.09 oz
Approximate Weight31g /1.09 oz
Dimension 26.10 x 20.10 x 4.60mm

Product Highlights

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

Frequently Asked Questions

My Apple Watch S9 shows 0% and won't turn on right after I replaced the battery — is the new cell dead?

The cell is almost certainly fine. The S9 fuel gauge IC loses its charge reference when the original cell is disconnected, so it defaults to 0% on a new cell it has never calibrated against. Place the watch directly on the magnetic charger without pressing the side button first. Let it charge uninterrupted to 100% — once that full cycle completes, the fuel gauge recalibrates and the watch will power on and report charge level accurately.

The watch battery is draining completely within one day — the old one lasted much longer before it failed.

A new cell on an S9 can show higher-than-expected drain for the first several days while the fuel gauge calibrates and the system re-indexes health data. However, if drain stays heavy past day five, check whether always-on display is active and whether continuous heart rate monitoring is turned on — together these two features are the largest current draw on this platform and can pull a 280mAh cell down significantly faster than intermittent-use settings. Go to Settings → Display & Brightness and Settings → Privacy & Security → Health to audit what is running continuously.

The heart rate sensor seems to be running non-stop and killing the new battery faster than I expected — is that a fault?

This is not a fault with the cell. When the S9 detects a new power source, the health sensor stack can enter a re-baselining period where heart rate sampling runs at a higher frequency than normal. This typically settles within four to seven days as the system builds a new baseline against the new cell's impedance profile. If continuous high-frequency HR sampling persists beyond one week, go to Settings → Privacy & Security → Health → Heart Rate and set Background Heart Rate to off, then re-enable it — this forces the sensor scheduler to reset its sampling interval.

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Apple MR913LL/A Replacement Battery, 3.87V, 280mAh, Li-Polymer 1

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