Apple Watch Series 8 45mm A2815 Compatible Battery 3.86V 300mAh
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Apple Watch Series 8 45mm A2815 Compatible Battery 3.86V 300mAh - is backordered and will ship as soon as it is back in stock.
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Battery Care Tips
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
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
⚠️ 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.
Apple Watch Series 8 45mm A2815 Compatible Battery 3.86V 300mAh - is backordered and will ship as soon as it is back in stock.
Voltage
3.86V
Amp
300mAh
Apple Watch Series S8 45mm — 3.86V Li-Polymer Replacement Battery (A2815)
This 3.86V, 300mAh Li-Polymer cell is a direct cell replacement for the Apple Watch Series S8 45mm. It fits the 45mm case specifically — not the 41mm variant. Voltage and capacity match the OEM A2815 spec exactly.
- S8 45mm fitment: The 45mm case uses a larger cell footprint (27.40 × 20.00 × 4.20mm) than the 41mm model. The two cells share the same voltage rail but differ in physical size and capacity. Fitting a 41mm cell into the 45mm chassis leaves the battery unseated and the connector under stress.
- Bench tested on actual hardware: We ran this cell through the S8 BMS handshake and confirmed the fuel gauge IC accepted the new cell reference. The BMS held charge termination at the correct upper voltage threshold with no false fault flags.
- Post-swap charging protocol: After fitting the new cell, place the watch on its magnetic charger immediately and charge to 100% before first use. The fuel gauge IC on the S8 platform cannot calibrate its state-of-charge reference without a full charge cycle from near-zero following a cell swap. Skipping this step causes the watch to report 0% or shut down unexpectedly.
Why the S8 45mm shuts down before the battery percentage hits zero
The S8 uses a coulomb-counting fuel gauge IC that tracks charge in and out of the cell over time. When a new cell replaces a degraded one, the IC still holds the old cell's impedance profile in memory. The mismatch causes the IC to miscalculate remaining capacity and trigger a low-voltage cutoff well above the actual depletion point. A full charge-to-100% cycle after swap resets the reference and corrects the shutdown threshold.
Magnetic charger not registering after reassembly
If the watch sits on the charger but shows no charging indicator, the charging contacts on the rear of the case have likely shifted during reassembly. The S8 back glass is held by adhesive, and pressure applied off-centre during resealing can tilt the contact pads by fractions of a millimetre. Remove the back, check the contact pads are flush and centred, then reseal. Confirm charging resumes — the charging indicator should appear within 30 seconds of correct contact.
Compatible Models
Replaces Part Numbers
Technical Specifications
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 S8 45mm shows 0% and won't stay on after I replaced the battery — did I get a dead cell?
The cell is almost certainly fine. The fuel gauge IC lost its charge reference when power was removed during the swap, so it has no valid baseline to read from. Place the watch on the magnetic charger immediately and let it charge to 100% without interruption. Once that full cycle completes, the IC recalibrates and the percentage will read accurately.
Battery on my S8 45mm is draining completely within a day — always-on display was never this bad before.
Always-on display combined with continuous heart rate monitoring pulls significantly more current than the watch estimates during the first few charge cycles on a new cell. The fuel gauge IC is still mapping the new cell's impedance curve, so drain readings can look worse than they are. Turn off always-on display for the first three full charge cycles, then re-enable it. If drain remains the same after that, check that background app refresh is off for any third-party health apps.
Watch won't pair with my iPhone after the battery swap — Bluetooth shows connected but the watch app says unpaired.
Removing the battery cuts power to the BLE stack, which drops the encrypted pairing session stored in the watch's secure enclave. The iPhone still sees a Bluetooth signal but the pairing credentials no longer match. On the watch, go to Settings → General → Reset → Erase All Content and Settings, then re-pair from the Watch app on the iPhone. The watch data restores from the last iCloud backup once pairing completes.
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