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Apple iWatch Series 4 40mm A2058 3.85V Replacement Battery

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Fits Apple iWatch Series 4 40mm, part number A2058 battery replacement.
3.85V, 220mAh Li-Polymer cell restores full smartwatch runtime and sensor operation.
Connector seats flush against the watch's internal charge contacts with no locking tab required.
We bench-tested the BMS on a Series 4 platform — fuel gauge recognized the new cell after a full charge cycle from power-off.
After fitting this 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 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

3.85V

Amp

220mAh

Apple iWatch Series 4 40mm — 3.85V Li-Polymer Replacement Battery (A2058)

This is a 3.85V, 220mAh Li-Polymer cell replacing part number A2058 in the Apple iWatch Series 4 40mm. It fits models A1975, A1977, and A2007 — all 40mm Series 4 variants sharing the same battery bay and connector. Swapping this cell restores the watch's ability to run fitness tracking, heart rate monitoring, and notifications at full capacity.

  • Series 4 40mm compatibility: The A1975, A1977, and A2007 model numbers cover GPS, GPS + Cellular, and regional variants of the 40mm Series 4. All share the same 23.80 × 18.92 × 4.44mm battery footprint, the same flex connector, and the same BMS communication protocol — one cell covers the entire 40mm Series 4 lineup.
  • Bench tested on actual hardware: We cycled this cell through charge and discharge on a Series 4 unit, confirmed BMS handshake on reconnect, and verified the fuel gauge IC accepted the new cell without throwing a service warning on the watch face.
  • Fuel gauge calibration after swap: Place the watch on its magnetic charger immediately after fitting this cell and charge to 100% before powering the device on. The Series 4 fuel gauge IC loses its reference point when the original cell is removed — skipping this step causes the percentage readout to jump or read incorrectly from the first boot.

Why the Series 4 shows a service alert after a battery swap

watchOS includes a battery health check that flags replacement cells until the fuel gauge IC completes a full calibration cycle. The alert appears in Settings → Battery → Battery Health and typically reads as "Unable to determine battery health." This is a calibration state, not a fault. Running one full charge from near-zero to 100% and then using the watch until it prompts a charge clears the flag in most cases.

Watch not pairing with iPhone after battery replacement

Removing the battery terminates the active BLE session between the watch and the paired iPhone. When the watch reboots with a new cell, the phone-side Bluetooth stack does not automatically re-establish the link. Open the Watch app on the iPhone, go to General → Reset, and select "Reset Network Settings" on the watch — this clears the stale session. After that, keep the watch and phone within one metre until the pairing indicator confirms connection.

Compatible Models

iWatch Series 4 40mm A1975 A1977 A2007

Replaces Part Numbers

A2058

Technical Specifications

Voltage3.85V
Amp Hours220mAh
Capacity220mAh
Rate0.85Wh
Net Weight3.8g /0.13 oz
Gross Weight28.8g /1.02 oz
Approximate Weight28.8g /1.02 oz
Dimension 23.80 x 18.92 x 4.44mm

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 Series 4 shows 0% and won't turn on after I fitted the new battery — did I get a dead cell?

The cell is almost certainly fine. The fuel gauge IC on the Series 4 loses its charge reference the moment the original battery is disconnected, so it reads zero even on a full cell. Place the watch on its magnetic charger before pressing any buttons, let it charge uninterrupted to 100%, then power it on. The percentage should read accurately after that first complete charge.

The new battery is draining in a single day — my old one used to last two days even when it was tired. What's wrong?

Always-on heart rate monitoring and background app refresh draw significantly more current on a new cell than on a degraded one, because a fresh cell can actually sustain the sensor load without voltage sag cutting sessions short. Check Settings → Battery on the watch and look for any app with unusually high background activity. Disabling always-on heart rate monitoring in Privacy → Health → Heart Rate drops the average current draw noticeably and will extend charge intervals back toward the multi-day range.

The magnetic charger doesn't seem to be making contact properly since I replaced the battery — the charging ring won't appear.

This usually means the back crystal shifted slightly during reassembly, misaligning the internal charge contacts with the magnetic puck. Remove the watch from the charger, press gently but firmly on the back glass around its entire edge, then re-seat the charger — the back plate only needs to move fractions of a millimetre to break contact. If the ring still won't appear, check that the charger cable reads at least 5V output with a USB meter before assuming the contact point inside the watch is the issue.

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