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Apple Watch 1 42mm A1579 Replacement Battery 3.8V 240mAh

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Fits Apple Watch 1 42mm, OEM part number A1579 for first-generation smartwatch battery replacement.
3.8V, 240mAh lithium-polymer cell delivers the energy density required for display, processor, and wireless modules on this 42mm model.
Connector solders directly to the watch mainboard; no mechanical locking tab — alignment critical during reassembly to avoid cold joints.
We bench-tested this cell on a donor Watch 1 mainboard; BMS accepted the new pack without fault codes and held voltage steady under load.
After fitting, place the watch on its magnetic charger immediately and charge to 100% — the fuel gauge IC cannot calibrate its reference without a full charge cycle from near-zero after a cell 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.

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🔹 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.

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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

3.8V

Amp

240mAh

Apple Watch 1st Gen 42mm — 3.8V Li-Polymer Replacement Battery (A1579)

This 3.8V, 240mAh Li-Polymer cell is a direct cell replacement for the first-generation Apple Watch 42mm. It fits the larger case variant of Apple Watch Series 1, sitting behind the display stack in the 28.30 × 20.20 × 5.00mm footprint. Swap it when the original A1579 cell no longer holds charge through a full day of normal use.

  • 42mm case compatibility: The 42mm and 38mm Watch 1 cases use different cell footprints and separate OEM part numbers. This A1579 cell fits only the 42mm variant. The connector, ribbon length, and BMS handshake are specific to that chassis — fitting it in the 38mm shell is not possible.
  • Bench tested on actual hardware: We ran this cell through charge and discharge cycles on Watch 1 hardware. The BMS initialised correctly, the fuel gauge registered state-of-charge without error codes, and the cell held voltage within spec across the test window.
  • Post-swap charge procedure: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on Watch 1 loses its charge reference during cell removal and cannot recalibrate without a full charge cycle from near-zero after the swap.

Why the Watch 1 fuel gauge reads wildly wrong after a cell swap

The Watch 1 uses a dedicated fuel gauge IC that tracks charge state by counting coulombs in and out of the cell. When the cell is removed, that running count is lost entirely. The IC has no stored reference for the new cell's actual capacity, so it estimates from the first partial charge rather than a calibrated baseline. This produces incorrect percentages — sometimes jumping or dropping 20–30% in one charge step — until you complete at least one full charge cycle to 100%.

Watch not pairing to iPhone after battery replacement

Removing power from the Watch 1 during a battery swap terminates the active BLE session between the watch and the paired iPhone. The watch comes back up as an unpaired device from the phone's perspective, even though the watch itself still shows the pairing stored. To fix this, open the Watch app on the iPhone, go to My Watch → All Watches, tap the info icon next to the watch, and select Unpair Apple Watch — then re-pair from scratch. The watch data restores from iCloud backup once pairing completes.

Compatible Models

iWach 1 42mm Watch 1st Gen 42mm

Replaces Part Numbers

A1579

Technical Specifications

Voltage3.8V
Amp Hours240mAh
Capacity240mAh
Rate0.91Wh
Net Weight5.2g /0.18 oz
Gross Weight30.2g /1.07 oz
Approximate Weight30.2g /1.07 oz
Dimension 28.30 x 20.20 x 5.00mm

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 Apple Watch 1 42mm shows 0% and won't turn on right after I replaced the battery — did I get a dead cell?

The cell is almost certainly fine. The Watch 1 fuel gauge IC loses its charge reference the moment the old cell is disconnected, so it has no valid starting point for the new one. Place the watch on its magnetic charger immediately after reassembly and leave it until it reads 100% — do not power it on before that first full charge completes. Once it hits 100%, the IC re-establishes its reference and the percentage will read correctly.

The new A1579 battery is draining in a single day — my old worn-out battery actually lasted longer. What's happening?

This is a known pattern in the first few days after a cell swap on Watch 1. The heart rate sensor runs a higher sampling current against a new cell's lower internal impedance, and always-on display plus continuous HR monitoring together draw more than the fuel gauge initially accounts for. Give the watch three to five full charge-and-discharge cycles — the BMS will settle its current profiling and drain rate will normalise. If it is still collapsing inside a day after five cycles, check that the display cable is fully seated, as a partially connected display backlight draws continuous current.

The magnetic charger stopped making contact after I put the Apple Watch 1 42mm back together — it was charging fine before I opened it.

The charge contacts sit on the rear crystal, which can shift fractionally during reassembly if the adhesive strip is not seated flat. Press the rear crystal firmly around its full perimeter while the watch is on the charger and check whether the charging icon appears. If it does, the contact alignment was off. Re-open the case, replace the adhesive gasket, and reseat the back — apply even pressure from the centre outward to avoid the crystal tilting. The watch should register charge within two seconds of correct contact.

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