The Apple Watch Series 4 (40mm) Replacement Battery (224.8mAh / Model A2058) is an OEM-grade, high-density lithium-ion polymer cell engineered for professional smartwatch service technicians, wearable repair labs, and micro-soldering workshops. Designed to restore factory-rated battery runtime, stable standby current, and consistent processing power to 4th Generation Apple Watches, this replacement battery directly rectifies common aging symptoms including rapid battery drain, sudden power dropouts under active workout/cellular tracking, persistent Apple logo boot loops, and pouch swelling that threatens to push the OLED display off the housing.

Equipped with an integrated hardware Battery Management System (BMS) circuit and precision-soldered terminal flex ribbon, this 40mm Series 4 battery incorporates rigorous safeguards against overcharge, over-discharge, short-circuit current surges, and wireless inductive charging thermal spikes. Precision-contoured to match the internal tray geometry of the Apple S4 SiP platform, it provides reliable daily backup time and smooth magnetic power transfer.

Key Engineering Features & Bench Advantages:

• OEM Capacity & Voltage Alignment: Formulated to exact factory specifications (224.8mAh / 3.81V nominal / 0.858Wh), ensuring sustained voltage stability across heart-rate telemetry, haptic engine feedback, and background sensor polling.
• Integrated Hardware Protection BMS: Onboard safety micro-circuitry guards against over-voltage, deep discharge conditions, short-circuit faults, and thermal escalation during magnetic wireless charging cycles.
• Low Equivalent Series Resistance (ESR): Manufactured using high-purity cobalt pouch materials to keep internal resistance minimal, curbing localized heat buildup during inductive charging.
• Zero Cycle Count: 100% brand-new cell with zero charge-discharge history, delivering maximum battery life expectancy and factory-standard capacity retention.
• Pre-Shaped Factory FPC Connector: Pre-aligned terminal flex ribbon sits flush against the motherboard battery socket, preventing strain or micro-fractures on delicate board-level connector pins during reassembly.

Technical Specification Matrix

Parameter Profile	Product Specification Details
Compatible Device Model	Apple Watch Series 4 (4th Generation, 40mm)
Battery Model / Part Number	A2058
Chassis Compatibility	40mm Aluminum, Stainless Steel, and Hermes Editions
Associated Watch Model Codes	A1977 (GPS), A1975 (Cellular North America), A2007 (Cellular Global)
Battery Chemistry	High-Density Lithium-Ion Polymer (LiPo)
Nominal Capacity	224.8 mAh (0.858 Wh)
Nominal Voltage	3.81 V
Cycle Life Expectancy	500+ Full Cycles (Retains 80%+ Nominal Capacity)
Primary Bench Utility	Battery Degradation Swap, Swollen Cell Replacement, Boot-Loop Restoration

Recommended Bench Operation Guide:

1. Safety Pre-Discharge: Drain the depleted battery below 25% prior to opening the watch to reduce thermal runaway risks during mechanical extraction.
2. Display Separation: Warm the perimeter of the watch face on a heating plate set to 65°C–70°C for 2–3 minutes to soften the factory waterproof gasket. Carefully slice through the adhesive perimeter with an ultra-thin curved pry blade, maintaining tool depth under 1.5mm to avoid severing the Force Touch sensor gasket or display ribbon traces.
3. Internal Disconnection: Prop the display open to a 90-degree angle, remove the tiny tri-point bracket screw, and disconnect the battery connector first before unclipping the OLED display and digitizer ribbons.
4. Adhesive Release & Extraction: Apply 1–2 drops of high-purity isopropyl alcohol around the battery cavity to soften the bottom adhesive strip. Use a wide, flat nylon spudger to gently lift the battery out (avoid using metal picks or tweezers that could puncture the pouch).
5. Pre-Close Testing & Final Seal: Connect the new 224.8mAh battery, verify smooth boot-up, check wireless charging on a magnetic dock, and confirm stable standby. Seat the battery with fresh adhesive strips, clean the housing lip thoroughly, apply a new custom-fit waterproof display gasket, and clamp the display assembly under uniform pressure until cured.