Below is a structured troubleshooting workflow appropriate for board-level diagnosis of an Apple AirPods Pro (2nd Generation) Charging Case after saltwater exposure. Saltwater damage is significantly more destructive than fresh water because chloride ions create conductive residues and rapidly accelerate galvanic corrosion, especially around fine-pitch ICs and flex connectors. 1. IMMEDIATE CHEMICAL MITIGATION Safety First Before beginning: Disconnect all power immediately. Do not connect Lightning power or attempt charging until cleaning is complete. Disconnect the battery as soon as access is possible. Wear ESD protection: Grounded ESD wrist strap ESD-safe mat Plastic or ceramic tools whenever possible Avoid metal probing near the battery terminals. Cleaning Materials Recommended: 99% Isopropyl Alcohol (IPA) (preferred) 99% minimizes water introduction. 95–99% electronic-grade IPA is acceptable. Soft ESD brush Microscope (10–20× minimum) Lint-free swabs Compressed clean dry air (oil-free) Avoid: 70% IPA Tap water WD-40 Contact lubricants Vinegar (only appropriate for alkaline battery leakage, not saltwater contamination) Cleaning Procedure Step 1 Disconnect battery first. Never clean the board while the battery remains electrically connected. Step 2 Flood affected areas with 99% IPA. Target: Lightning connector Battery connector Charging IC region Power management IC Flex connectors Corroded vias Test pads Coil area Bottom side of PCB The objective is dissolving chloride residue rather than merely drying moisture. Step 3 Use an ESD brush to gently scrub: White deposits Green copper oxide Darkened solder joints Corroded component leads Step 4 Repeat IPA flushing several times until no visible contamination remains. Step 5 Dry using: Clean compressed air Warm airflow below approximately 50°C Allow complete evaporation before electrical testing. 2. DISCONNECTED POWER TESTING Battery Isolation Disconnect battery completely from the motherboard. Never measure battery voltage while connected to the board if corrosion damage is suspected. Multimeter Setup Mode: DC Voltage Range: 20 V DC (or auto-ranging) Probe Placement Red probe: Battery positive terminal Black probe: Battery negative terminal Do not short adjacent terminals. Expected Battery Voltage The AirPods Pro 2 charging case uses a single-cell lithium-ion/polymer battery. Typical values are: Battery Condition Voltage Fully charged ~4.20 V Normal operating 3.7–4.1 V Low battery 3.3–3.6 V Deep discharge <3.0 V Protection circuit may trip ~2.5–2.8 V (varies by protection design) Practical Interpretation 4.0–4.2 V: Excellent 3.6–4.0 V: Normal 3.2–3.6 V: Low but generally recoverable 3.0–3.2 V: Borderline; proceed with caution <3.0 V: Deeply discharged and potentially damaged 0 V: Open circuit, protection latched, or battery failure A battery measuring below approximately 3.0 V after saltwater exposure is at increased risk of permanent degradation, though voltage alone does not prove irreversible failure. 3. MOTHERBOARD POWER RAIL TESTING Battery Must Remain Disconnected Leave battery disconnected. Apply External Power Connect: Lightning cable Powered from: Quality USB power supply USB power meter if available Initial Lightning Measurements Measure voltage arriving at: Lightning connector VBUS Expected: Approximately 5.0 V (USB supply) If absent: Lightning connector corrosion Open fuse/filter Damaged connector Broken trace Input Protection Stage Locate: ESD protection Input filter Ferrite bead Input fuse (if present) Expected: Approximately 5 V before and after protection devices. A significant voltage drop across a protection component indicates damage or a short downstream. Main Power Management Circuit Identify the PMIC input. Expected: Approximately 5 V at the PMIC input (if the design routes VBUS directly there). Downstream rails are typically generated by buck/LDO converters once the PMIC is active. On a functioning board, common regulated rails are in ranges such as: ~3.3 V ~1.8 V ~1.2 V (processor/core) The exact rail names, sequencing, and values are Apple-specific and not publicly documented. Battery Connector Voltage With battery disconnected: Measure voltage present at battery connector. Common possibilities: No voltage until PMIC enables charging. A charging voltage (typically above the nominal battery voltage) only after proper PMIC initialization. Exact behavior depends on the charging IC state machine and should not be assumed from voltage alone. Additional Checks Resistance to ground: Battery positive rail Should not be near 0 Ω. A very low resistance (for example, only a few ohms) may indicate a shorted capacitor, PMIC, or another failed component. Interpret resistance readings in the context of the circuit; many power rails normally measure tens to hundreds of ohms. 4. INTERPRETATION OF RESULTS Diagnostic Logic Tree Saltwater Exposure │ ▼ Clean PCB Completely │ ▼ Disconnect Battery │ ▼ Measure Battery Voltage │ ├──────── ≥3.2V │ │ ▼ │ Test Motherboard │ └──────── <3.0V │ ▼ Battery Deeply Discharged │ ▼ Continue Board Testing Board Test Apply Lightning Power │ ▼ 5V Present at Lightning? │ ┌───────┴────────┐ │ │ No Yes │ │ Repair Continue Input Stage │ ▼ PMIC Input Receives 5V? │ ┌─────────┴─────────┐ │ │ No Yes │ │ Repair Trace/ Check Regulated Protection Output Rails Requested Scenario If: Motherboard receives proper Lightning voltage PMIC input is normal Main regulated rails initialize as expected Battery measures less than 3.0 V Conclusion: The motherboard has demonstrated that it can accept external power and its primary power-distribution path is functioning. The battery is deeply discharged and may have suffered damage from saltwater exposure. Because lithium-ion cells that have been contaminated or deeply discharged can be unsafe and unreliable, the recommended repair action is to replace the battery rather than attempt to revive it. After installing a known-good battery, retest charging, power-up behavior, and case functionality. If problems persist with a known-good battery, continue diagnosis of the charging circuit, PMIC, and any corrosion-related damage on the PCB. Additional Recommendations Inspect under magnification for lifted pads, cracked solder joints, and corroded vias, especially around the Lightning connector, battery connector, PMIC, and charging circuitry. Check connector pins for blackening or missing plating, as corrosion can create intermittent faults even after cleaning. If available, use a thermal camera or controlled current-limited bench supply to help locate abnormal current draw from shorted components. Avoid injecting voltage onto battery rails without understanding the circuit topology, as this can damage sensitive ICs. After successful repair, monitor charging current and temperature through a complete charge cycle before returning the device to service.