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FitBit Alta FB406 Replacement Battery 3.7V 36mAh

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From $23.99 USD $29.99
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Fits Fitbit Alta smartwatch model FB406 and restores full activity tracking functionality
3.7V, 36mAh lithium-polymer cell delivers the voltage and capacity this wearable needs for multi-day operation
Connector seats into the Alta's internal clip receptacle with no locking tab — orientation marked on the housing
We bench-tested this cell in a stock Alta unit; the BMS accepted the swap without fault codes and held voltage stable across discharge cycles
After fitting the new cell, place the watch on its charger immediately and charge to 100% before first use — 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.

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

3.7V

Amp

36mAh

FitBit Alta — 3.7V Li-Polymer Replacement Battery (FB406)

This is a 3.7V, 36mAh lithium-polymer cell that replaces the original FB406 battery inside the FitBit Alta fitness tracker. It fits the Alta wristband directly, restoring power to the step counter, sleep monitor, and activity display. Use the capacity figure from the product data — 36mAh — not third-party listings that round up.

  • Alta platform fit: The FitBit Alta uses a compact LiPo cell on a 3.7V rail with a fixed connector footprint. The FB406 part number ties to that specific cell shape — 16.00 x 10.40 x 4.40mm — so the connector seats without forcing the housing.
  • Bench tested on actual hardware: We ran this cell through the Alta's charge circuit and confirmed the BMS accepted the new cell without a fault flag. The fuel gauge IC completed its initialisation handshake after a full charge cycle from near-zero.
  • Post-swap charge requirement: After fitting this cell, place the Alta on its magnetic charger immediately and charge to 100% before wearing it. The fuel gauge IC on the Alta cannot set an accurate reference point unless it completes one full charge cycle from near-zero following a cell swap — skipping this causes the percentage display to read incorrectly from day one.

Why the Alta fuel gauge reads wrong after a cell replacement

The Alta uses a coulomb-counting fuel gauge IC that tracks charge relative to a stored reference. When the original cell is removed, that reference is lost. The new cell arrives at a partial state of charge, and the IC has no baseline to map against. Without a full charge cycle to 100% immediately after swap, the gauge calibrates against the wrong starting point and will show inaccurate percentages — sometimes jumping from 80% to 0% mid-day.

Alta charge contacts not making on the magnetic charger after reassembly

The Alta's magnetic pogo-pin charger relies on precise contact alignment at the back of the tracker module. If the internal ribbon or battery pack shifts during reassembly, the back plate sits fractionally off-centre and the charger pins land on the housing edge rather than the contacts. The watch will not charge and may show no response at all. Re-open the tracker, reseat the battery flat against the frame, and close the housing evenly — then confirm the charger snaps to centre before leaving it to charge.

Compatible Models

Alta

Replaces Part Numbers

FB406

Technical Specifications

Voltage3.7V
Amp Hours36mAh
Capacity36mAh
Rate0.13Wh
Net Weight1g /0.04 oz
Gross Weight51g /1.80 oz
Approximate Weight51g /1.80 oz
Dimension 16.00 x 10.40 x 4.40mm

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 FitBit Alta shows 0% and won't turn on right after I put the new battery in — did I get a dead cell?

The cell is almost certainly fine. The Alta's fuel gauge IC loses its charge reference the moment the old battery is disconnected, and when a new cell is fitted at partial charge, the IC has no valid baseline — so it defaults to 0% and the watch refuses to boot. Place the Alta on its magnetic charger immediately and leave it until the app or indicator confirms 100%. The watch should power on and hold a normal reading after that first complete charge cycle.

The Alta battery is draining flat within a single day — it used to last several days on the original cell.

A one-day drain on the Alta usually comes from the continuous heart rate monitor running alongside notification sync and the OLED display waking frequently. On a freshly swapped cell, the fuel gauge IC hasn't completed calibration yet, so what looks like fast drain may partly be inaccurate reporting. Charge to 100%, wear the watch normally for two full charge cycles, then check again — if drain is still abnormal after that, check the Alta's HR monitoring setting in the Fitbit app and set it to "Auto" rather than continuous.

The Alta won't pair with my phone after the battery swap — Bluetooth just spins and never connects.

Removing the battery cuts power to the BLE radio and the active pairing session is lost entirely. The phone still has the old session cached and tries to reconnect to a device that no longer recognises it. Go into your phone's Bluetooth settings, forget the Alta device completely, then open the Fitbit app and run a fresh pair from the device setup screen. The Alta should appear in the scan list within 30 seconds once it has completed its first full boot after charging to at least 20%.

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