The iPhone 15 generation moved to USB-C, and with it came a new complaint category: the phone charges, but never at the speed the customer expects. Fast-charge faults split cleanly between cable, adapter, port, and software - and shops that test in the right order close these tickets in minutes.
What Fast Charging Should Look Like
With a 20W or higher PD adapter, a 15 should pull roughly 20W in the first half hour, tapering as the battery fills. A USB power meter between adapter and cable gives you the ground truth in seconds - it is the single best $20 tool for this fault class.
The Diagnosis Ladder
| Reading on Meter | Meaning | Action |
|---|---|---|
| 5V / 1-2A flat | PD handshake never happens | Try a known PD cable, then PD adapter |
| 9V negotiated, current low | Handshake OK, charge path limited | Clean port; inspect pins under magnification |
| Normal watts, still slow% | Charging fine, battery health low | Check battery health; quote battery job |
| Intermittent cutouts | Port or flex fault | Replace port flex |
The USB-C Specific Traps
- Cable confusion: many customers use C-to-C charge-only or low-wattage cables; the 15 accepts both fast lanes, but the cable must support 60W PD for full speed.
- Port debris packs differently in USB-C than Lightning; lint compacts at the front lip and blocks the plug from seating fully - visible as a shallow click.
- Optimized charging: overnight slow charging is a feature; test midday at 30-60% battery before declaring a fault.
Test cables, port flexes, and meters: charging ports, for iPhone 15, and flex cables.
FAQ
Does a non-Apple adapter damage the phone? Certified PD adapters are safe; uncertified ones risk unstable handshake and slow charging.
Port flex replaced, still slow - now what? Verify with the meter that the new flex negotiates 9V; if yes, the battery is the limiter.
Can cleaning alone restore fast charging? Often - lint-related slowdowns are among the most common 15-series tickets.