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FitBit Inspire 3 Replacement Battery FB424 3.87V 75mAh

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From $33.99 USD $41.99
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Fits Fitbit Inspire 3 smartwatch and replaces OEM battery part number FB424.
3.87V, 75mAh Li-Polymer cell restores multi-day runtime to this wearable fitness tracker.
Connector seats flat against the watch PCB with no locking tab required.
We bench tested the cell on a Fitbit test rig — BMS accepted charge current immediately with no fault delay or cutoff cycling.
After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% — the fuel gauge IC cannot calibrate without a complete charge from near-zero after a battery 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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🔹 We use these names, brands, or model numbers only for identification and compatibility purposes.

Voltage

3.87V

Amp

75mAh

FitBit Inspire 3 — 3.87V Li-Polymer Replacement Battery (FB424)

This 3.87V, 75mAh Li-Polymer cell replaces the original FB424 battery inside the FitBit Inspire 3 smartwatch. It fits the full Inspire 3 lineup — the watch that tracks daily activity, heart rate, sleep stages, and workout metrics. Swap this cell when the original no longer holds a charge or the watch dies before the end of the day.

  • Inspire 3 compatibility: The Inspire 3 uses a specific 19.30 x 11.80 x 3.40mm cell footprint with a fixed connector orientation. This cell matches that footprint exactly — voltage rail, physical dimensions, and connector pitch all align with the FB424 spec.
  • Bench tested on actual hardware: We ran this cell on an Inspire 3 unit. The BMS accepted the new cell without fault flags, heart rate and SpO2 sensors initialised normally, and the fuel gauge IC began calibrating correctly after a full charge cycle from near-zero.
  • Fuel gauge reset after swap: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before use. The fuel gauge IC on the Inspire 3 cannot calibrate its reference point without one full charge cycle from near-zero following a cell swap.

Why the Inspire 3 shows 0% immediately after a battery swap

The Inspire 3 uses a coulomb-counting fuel gauge IC to report battery percentage. When the original cell is removed, that IC loses its stored reference data entirely. The new cell has no charge history, so the gauge defaults to 0% or an undefined state. Plug the watch into its charger straight after reassembly and allow it to charge to 100% — one full cycle resets the IC and restores accurate percentage readings.

Charge contacts not making on the magnetic charger after reassembly

If the watch sits on the charger but shows no charging animation, the rear charge contacts have likely shifted during back-panel reassembly. Even a 0.5mm misalignment breaks the magnetic contact. Remove the watch from the charger, inspect the rear contacts for debris or adhesive residue, re-seat the back panel flush, then confirm the charger snaps into place magnetically with even contact across all four pads.

Compatible Models

inspire 3

Replaces Part Numbers

FB424

Technical Specifications

Voltage3.87V
Amp Hours75mAh
Capacity75mAh
Rate0.29Wh
Net Weight2g /0.07 oz
Gross Weight27g /0.95 oz
Approximate Weight27g /0.95 oz
Dimension 19.30 x 11.80 x 3.40mm

Product Highlights

  • Brand: FitBit
  • 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 Inspire 3 is draining the new battery in one day — it used to last several days. What's wrong?

Always-on display mode combined with continuous heart rate monitoring is the most common cause. Together they draw roughly three to four times more current than the default auto-brightness and on-demand HR setting. Open the Fitbit app, go to Advanced Settings for the Inspire 3, and switch Always-On Display off — battery life should return to the multi-day expected drain rate within one charge cycle.

The heart rate sensor seems to be running constantly and the new cell is warm after a short workout. Is that normal?

A new Li-Polymer cell has a slightly different internal impedance than the aged cell it replaced. During the first week, the Inspire 3's sensor power management recalibrates to the new impedance profile, which can cause the HR sensor to poll more frequently than steady state. Mild warmth is within spec for a 3.87V Li-Polymer cell under sustained sensor load. If warmth persists beyond the first week, check that the wrist detection sensor setting is not stuck in continuous mode inside the Fitbit app device settings.

The Inspire 3 won't pair with my phone after the battery swap — it shows up in Bluetooth but the sync fails. How do I fix it?

Removing the battery drops power to the BLE radio mid-session, which orphans the active Bluetooth pairing record on both the watch and the phone. The fix is a clean re-pair: on your phone, forget the Inspire 3 in Bluetooth settings, then open the Fitbit app and remove the device from your account. Restart the watch by holding the button for eight seconds, then add it again through the Fitbit app as a new device.

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