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Fossil Sport Smartwatch APP00221 Replacement Battery 3.8V

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From $40.99 USD $50.99
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Fits Fossil Sport Smartwatch, Gen 5, and Gen 4 models using OEM part number APP00221.
3.8V, 260mAh lithium-polymer cell delivers the energy density this wearable requires for multiday operation between charges.
Connector seats flush into the internal charging port with a single contact alignment — reassemble the casing to restore magnetic charger seating.
We bench-tested the cell in a Sport Gen 5 unit; the fuel gauge IC accepted the new BMS handshake after a full charge cycle from power-off.
After fitting this cell, place the watch on its charger immediately and charge to 100% — the fuel gauge cannot calibrate without a complete charge cycle from near-zero after a battery swap.
Delivery time

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

Discount: As a thank you for your patience, enjoy 5% off on your order
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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.

Disclaimer

⚠️ 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

3.8V

Amp

260mAh

Fossil Sport / Gen 4 / Gen 5 Smartwatch — 3.8V Li-Polymer Replacement Battery (APP00221)

This 3.8V, 260mAh lithium-polymer cell replaces part number APP00221 in the Fossil Sport, Gen 4, and Gen 5 smartwatches. The original cell degrades over charge cycles, causing shorter battery life and eventual failure to hold charge. Swapping this cell restores full watch operation across fitness tracking, notifications, and timekeeping.

  • Sport, Gen 4, and Gen 5 compatibility: All three models use the same 29.10 x 23.40 x 4.00mm cell footprint, the same 3.8V nominal rail, and share the APP00221 part number — the connector and BMS handshake are identical across the lineup.
  • Bench tested on actual hardware: We cycled this cell through charge and discharge on the Gen 5 platform. The BMS accepted the cell without fault codes, and the fuel gauge IC registered voltage correctly after a full charge cycle from near-zero.
  • Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before use. The fuel gauge IC in these watches loses its reference point during a cell swap — it cannot recalibrate unless it sees a full charge cycle starting from near-zero.

Watch shows 0% immediately after battery swap

The fuel gauge IC on Fossil smartwatches tracks state-of-charge by logging cumulative charge and discharge from a known reference point. When the cell is removed, that reference is lost entirely. The IC has no way to estimate where the new cell sits in its charge curve until it runs a full cycle. Place the watch on charge straight away and let it reach 100% — the gauge resets its reference at that point and will read accurately from then on.

Battery draining in one day after a new cell is fitted

Always-on display mode combined with continuous heart rate monitoring can pull significantly more current than the standard polling interval. On the Gen 5 platform, continuous HR monitoring alone keeps the optical sensor active at a constant draw rather than sampling in short bursts. Check the Fossil Wear OS app and confirm HR mode is set to automatic rather than continuous. If drain persists after that change, check whether always-on display was re-enabled during the pairing process after the swap.

Compatible Models

Sport Smartwatch Gen 5 Smartwatch Gen 4 Smartwatch

Replaces Part Numbers

APP00221

Technical Specifications

Voltage3.8V
Amp Hours260mAh
Capacity260mAh
Rate0.99Wh
Net Weight5g /0.18 oz
Gross Weight30g /1.06 oz
Approximate Weight30g /1.06 oz
Dimension 29.10 x 23.40 x 4.00mm

Product Highlights

  • Brand: Fossil
  • 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 Fossil Gen 5 won't pair to my phone after I swapped the battery — what happened?

Removing the battery ends the active BLE session and clears the connection state on the watch side. The phone still holds the old pairing record, which is now orphaned. On the watch, go to Settings → Connectivity and forget the existing pairing, then on your phone forget the watch in Bluetooth settings and re-pair from scratch through the Fossil app.

The magnetic charger doesn't seem to be charging after I reassembled the watch — the charging indicator never appears.

The charging contacts on the case back can shift slightly during reassembly if the rear cover is not fully seated. Inspect the contact pins on the case back and the corresponding pads on the charger puck — both need to be clean and aligned. Re-seat the case back and apply light pressure until it clicks flush, then place it on the charger again. You should see the charging animation within 10 seconds if contact is made correctly.

The heart rate sensor seems to drain the new cell much faster than it drained the original — is the battery faulty?

This is normal for the first several days after a cell swap. The fuel gauge IC is rebuilding its internal model of the new cell's impedance profile, and the HR sensor current draw appears disproportionately large against an uncalibrated gauge. Run two or three full charge-to-discharge cycles and the readings will stabilise. If drain is still noticeably worse after five full cycles, confirm continuous HR monitoring is not enabled in the Fossil Wear OS app settings.

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