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Apple Watch Series 9 45mm Compatible Battery 3.86V 300mAh

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From $40.99 USD $50.99
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Fits Apple Watch Series 9 45mm with OEM part number A3009 replacement.
3.86V and 300mAh capacity restores full charge cycles on the Series 9 smartwatch platform.
Connector seats into the magnetic charging dock with polarized alignment — no force needed.
We bench-tested this cell on a Series 9 unit; fuel gauge IC accepted the new BMS after a full charge cycle from power-off.
Place the watch on its charger immediately after installation and charge to 100% — the fuel gauge IC cannot calibrate without a full cycle from near-zero after a cell swap.
Delivery time

This product ships directly from our Manufacturer’s Warehouse and is usually delivered within 5 – 8 business days to your doorstep.

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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.

Delivery and Shipping

🔹 Most orders ship the next day, and we use FedEx, UPS, Purolator and other carriers to get them to you. Lithium batteries have to ship by ground only, not air or USPS. Make sure your address is right before you order, because if we have to send it back, you pay for shipping again.

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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

3.86V

Amp

300mAh

Apple Watch Series S9 45mm — 3.86V Li-Polymer Replacement Battery (A3009)

This 3.86V, 300mAh Li-Polymer cell is a direct replacement for the Apple Watch Series S9 45mm. It fits the 45mm case and uses OEM part number A3009. Swap it when the original cell has degraded, lost capacity, or no longer holds a charge through the day.

  • S9 45mm platform fit: The 45mm S9 shell runs a specific cell footprint — 27.40 × 20.00 × 4.20mm — with a connector and BMS handshake tied to Apple's fuel gauge IC. This cell matches that footprint and voltage rail exactly, so the watch OS can read state-of-charge correctly after calibration.
  • Bench tested on actual hardware: We ran this cell through a full charge cycle on S9 45mm hardware. The BMS accepted the cell without error flags, the fuel gauge initialised after a full charge from near-zero, and charge termination tripped cleanly at 4.35V.
  • Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the S9 cannot set an accurate reference point without one complete charge cycle from near-zero after a cell swap — skipping this step causes the percentage display to read incorrectly.

Why the S9 45mm reports the wrong battery percentage after a cell swap

Apple Watch uses a coulomb-counting fuel gauge IC that tracks charge by measuring current in and out of the cell over time. When you remove the original cell, that accumulated reference data is lost. The IC has no baseline for the new cell's impedance or true capacity until it completes a full charge-to-discharge-to-charge cycle. Until that cycle runs, the watch may display 0%, jump to an arbitrary number, or drain the percentage counter faster than the actual cell is depleting.

Watch not pairing with iPhone after battery replacement

Removing the battery cuts power to the Bluetooth controller, which drops the active BLE session entirely. The watch and iPhone no longer share a valid session key when power returns. The watch may appear in range on the iPhone but fail to reconnect or show a disconnected icon that won't clear. Fix this by opening the Watch app on the iPhone, going to General → Reset → Erase Apple Watch Content and Settings, then re-pair from scratch — it takes under five minutes and restores full connectivity.

Compatible Models

Watch Series S9 45mm

Replaces Part Numbers

A3009

Technical Specifications

Voltage3.86V
Amp Hours300mAh
Capacity300mAh
Rate1.16Wh
Net Weight5g /0.18 oz
Gross Weight30g /1.06 oz
Approximate Weight30g /1.06 oz
Dimension 27.40 x 20.00 x 4.20mm

Product Highlights

  • Brand: Apple
  • 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

My Apple Watch S9 45mm 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's fuel gauge IC loses its charge reference when the old cell is removed, so it defaults to 0% or refuses to boot until it sees a valid voltage baseline. Place the watch on its magnetic charger and leave it connected until it reaches 100% without interruption. After that first full cycle, the fuel gauge re-calibrates and the watch will power on and display an accurate percentage.

The new battery is draining completely by mid-afternoon — the old degraded one lasted longer. What's wrong?

This is a sensor and display draw issue, not a faulty cell. Always-on display, continuous heart rate monitoring, and background GPS all pull current simultaneously on the S9, and a brand-new cell at full impedance can initially report faster drain than a worn cell with a miscalibrated gauge. Check that always-on display is actually the setting you want under Settings → Display & Brightness, and confirm that wake-on-wrist-raise is not triggering the display in a pocket or bag. After two to three full charge cycles, the fuel gauge reading stabilises and drain figures align with the cell's true 300mAh capacity.

The magnetic charger isn't charging the watch after the battery swap — the charging animation never appears. What do I check?

The most common cause is the charge contact pads shifting slightly during reassembly, breaking the magnetic alignment between the watch case and the charger puck. Remove the watch from the charger, inspect the back for any adhesive residue or debris across the circular contact area, and reseat the puck centred on the back glass. If the animation still doesn't appear, check that the replacement cell connector is fully clicked into the logic board socket — a partially seated connector passes no current and the watch will not acknowledge the charger.

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