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Apple Watch 38mm A1578 Replacement Battery 3.8V 200mAh

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From $25.99 USD $31.99
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Fits Apple Watch 38mm models A1553 and A1554, replacing OEM battery part A1578.
3.8V, 200mAh Li-Polymer cell powers display, sensors, and wireless connectivity on this wearable.
Connector solders directly to watch mainboard; no locking tab — physical fit is fixed during assembly.
We bench-tested the BMS against a drained OEM pack; fuel gauge IC stabilized after a full charge cycle from power-off.
After fitting the new cell, place the watch on its magnetic charger immediately and run to 100% — the fuel gauge 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

3.8V

Amp

200mAh

Apple Watch 38mm — 3.8V Li-Polymer Replacement Battery (A1578)

This is a 3.8V, 200mAh Li-Polymer cell that replaces the original A1578 battery inside the Apple Watch 38mm. It also fits the Watch 42mm and models A1553 and A1554. Swap it in when the original cell no longer holds a charge across a full day of use.

  • 38mm and 42mm fitment: Both watch sizes use the same A1578 cell spec — same voltage rail, same connector pinout, and the same BMS handshake with the S1 or S1P processor module. One part covers both.
  • Bench tested on actual hardware: We ran this cell on a Watch 38mm unit and confirmed the BMS accepted the new cell without fault codes. The fuel gauge IC initialised correctly after a full charge cycle from near-zero.
  • Post-swap charging requirement: After fitting this cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the Watch platform loses its voltage reference when the old cell is removed — without a full charge cycle from near-zero, the percentage readout will be inaccurate and the watch may shut down unexpectedly at mid-charge levels.

Why the Apple Watch BLE session drops after a battery swap

Removing the battery cuts power to the W1 or W2 wireless chip, which discards its active Bluetooth session with the paired iPhone. The watch reboots as an unknown peripheral. The iPhone holds the old pairing record but the watch no longer matches it. Re-pair through the Watch app on the iPhone — Settings → General → Reset → Erase Apple Watch Content and Settings — then set up as new.

Watch draining to zero within one day after cell replacement

If battery life collapses to under 24 hours after the swap, always-on display combined with continuous heart rate polling is usually the cause. The new cell has a slightly different internal impedance than the aged original, and the sensor stack draws harder against it during the first several charge cycles until the BMS recalibrates. Check that Theatre Mode and Wrist Detection are both enabled, then run three full charge-to-discharge cycles. If drain persists past cycle three, confirm the display assembly was reseated fully — a lifted gasket keeps the screen backlight at full draw.

Compatible Models

Watch 38mm Watch 42mm A1553 A1554 MJ302LL/A MJ2Y2LL/A MJ312LL/A MJ322LL/A MJYK2LL/A MJ332LL/A MJ362LL/A MJ3A2LL/A MJ3E2LL/A MJ3F2LL/A MLLD2LL/A MLCK2LL/A MLFA2LL/A MLFC2LL/A MJ2T2LL/A MJ2V2LL/A MJ2U2LL/A MJ2W2LL/A MJ2X2LL/A MLCF2LL/A MLCG2LL/A MLC62LL/A

Replaces Part Numbers

A1578

Technical Specifications

Voltage3.8V
Amp Hours200mAh
Capacity200mAh
Rate0.76Wh
Net Weight4g /0.14 oz
Gross Weight54g /1.90 oz
Approximate Weight54g /1.90 oz
Dimension 24.74 x 16.72 x 4.72mm

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

Almost certainly not. The fuel gauge IC inside the watch lost its voltage reference when the original cell was disconnected, so it has no baseline to read from. Place the watch on its magnetic charger immediately and let it charge to 100% without interruption. After one full charge cycle from near-zero, the percentage readout recalibrates and the watch powers on normally.

The heart rate sensor is going non-stop and draining the new battery far faster than the old one ever did — is the replacement cell faulty?

The new cell has a lower internal impedance than the degraded original, which causes the heart rate sensor's current draw to register differently against the BMS during the first few cycles. This is normal and settles within three to five full charge-discharge cycles as the BMS maps the new cell's impedance curve. In the meantime, open the Watch app on the iPhone, go to Privacy → Health → Heart Rate, and confirm Background Heart Rate is set to your intended frequency rather than continuous. If drain remains abnormal after five cycles, check that the sensor window on the case back is clean and fully seated.

My Apple Watch won't connect to my iPhone after replacing the battery — Bluetooth is on but the watch just shows the disconnected icon.

Power removal wiped the active BLE session from the W1/W2 chip, and the iPhone's saved pairing record no longer matches the watch's current state. On the iPhone, open the Watch app, tap your watch, scroll down and tap Unpair Apple Watch, then re-pair from scratch. The watch will restore from its last iCloud backup once pairing completes.

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