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A2552 Apple Watch Series S7 45mm Replacement Battery 3.85V

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From $38.99 USD $47.99
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Fits Apple Watch Series S7 45mm; replaces OEM part A2552.
3.85V, 300mAh Li-Polymer cell restores full-day runtime on this 45mm model.
Connector seats flat against the watch's internal battery cavity; no locking tab.
We bench-tested the BMS against Series S7 charge profile — fuel gauge IC accepted handshake on first insertion without error codes.
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 full 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

3.85V

Amp

300mAh

Apple Watch Series S7 45mm — 3.85V Li-Polymer Replacement Battery (A2552)

This is a 3.85V, 300mAh Li-Polymer cell for the Apple Watch Series S7 45mm. It replaces part number A2552 and fits the 45mm case only — not the 41mm. Swap this in when the original cell no longer holds charge through a full day of normal use.

  • S7 45mm case fit: The 45mm and 41mm S7 share the same voltage rail but use physically different cells. This cell measures 27.40 × 20.00 × 4.30mm, matching the 45mm cavity. Fitting the wrong size stresses the flex connector and risks a BMS fault on first charge.
  • Bench tested on actual hardware: We ran this cell through the S7 charge controller and confirmed the BMS accepted the new cell without a protection trip. The fuel gauge IC registered the correct open-circuit voltage on connection.
  • Post-swap charge requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The S7 fuel gauge IC loses its reference point when the cell is removed. Without a full charge cycle from near-zero, the gauge will report inaccurate percentages and may cut off early.

Why the S7 45mm charge contacts fail to engage after reassembly

The S7 uses a magnetic charging puck that aligns to four contact points on the back crystal. If the rear case shifts even slightly during reassembly, those contacts sit off-centre and the watch draws no current. The charger LED may still glow green because the magnet seats before the contacts do. Re-seat the rear crystal, confirm it clicks flush on all four corners, then place it on the puck again and check for the charging icon within 30 seconds.

Watch not re-pairing to iPhone after battery swap

Removing the cell drops system power completely, which wipes the active BLE session stored in volatile memory. The iPhone retains the old pairing record but the watch boots with no matching session, so the two devices sit waiting for the other to initiate. Go to the Watch app on the iPhone, tap your watch name, select "Unpair Apple Watch," and re-pair from scratch. The process takes under five minutes and restores all synced data from the paired iPhone backup.

Compatible Models

Watch Series S7 45mm

Replaces Part Numbers

A2552

Technical Specifications

Voltage3.85V
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 4.30mm

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 S7 45mm shows 0% and won't turn on right after I put the new battery in — is the cell dead?

The cell is almost certainly fine. The S7 fuel gauge IC loses its voltage reference when the original battery is disconnected, so it reports 0% on a new cell it hasn't calibrated against. Place the watch on its magnetic charger immediately after reassembly and leave it until it reads 100%. That full charge cycle resets the gauge and the watch will power on and report correctly.

The new battery is draining completely within a day — the old worn-out cell lasted longer at first. What's wrong?

Always-on display and continuous heart rate monitoring together draw more current on a fresh cell than on a degraded one that the watch had already adapted its sampling rate to. The S7's sensor stack also runs more frequent baseline readings during the first week on a new cell as the BMS characterises the new impedance profile. Turn off always-on display temporarily and check battery percentage after 24 hours. If drain normalises after five to seven charge cycles, the cell is behaving correctly.

Heart rate and GPS are killing this new battery far faster than they did on the original — is this cell the wrong capacity?

The capacity is correct at 300mAh for the 45mm A2552. What you're seeing is normal for the first week: the S7 sensor controller adjusts its polling intervals based on the impedance signature of the connected cell, and a new cell has a different impedance profile than the worn one it replaced. GPS and optical HR draw will stabilise as the BMS logs more charge cycles. Check again after five full charge cycles — if drain is still excessive, verify the heart rate sensor lens is fully seated and free of adhesive residue from the swap.

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