charger-cable-gate

Use when pairing a USB-C charger, cable, and device and wondering why charging is slow or not working — PD revision rules (2.0/3.0 PPS/3.1 EPR 140-240W), cable e-marker 3A vs 5A current caps, charge-only cable traps, proprietary protocols (VOOC/QC) vs standard PD, and per-archetype wattage needs; outputs OK / CAPPED / SLOW / NO-CHARGE verdicts with the exact bottleneck.

Charger & Cable Gate — USB-C PD Compatibility

Fast charging fails silently. You plug a 65W laptop brick into an ultrabook with a bargain cable and get... 27W, or nothing. The chain is charger protocol × cable current rating × device appetite, and the weakest link caps everything — with no error message, just slower charging. This skill checks the whole triple and names the bottleneck.

When to Use

• "Why is my laptop charging slowly / battery draining while plugged in?" — the #1 USB-C mystery
• Buying a replacement charger or cable — what actually delivers the watts you're paying for
• Decoding wattage labels (what does 30W vs 65W vs 140W actually mean for a given device class)
• Sorting a cable drawer: which cables are 3A, which are e-marked 5A, which are charge-only junk
• Checking whether a phone with proprietary fast-charge (VOOC/SCP families) will fast-charge on standard PD gear (spoiler: no)

Don't use for: Thunderbolt/USB4 data speeds (40Gb/s is a data-chain question, different math), MagSafe/Qi wireless (different physics), or USB-A legacy charging (5V-only world, no negotiation).

The Rules (memorize these five)

1. PD fixed steps: 5/9/15/20V (SPR), extended to 28/36/48V only under PD 3.1 EPR. Power = V × A at the negotiated step.
2. The 3A cable wall: without an e-marker chip (or explicit 5A rating), a cable caps at 3A → 60W max (20V × 3A). This is the single most common 65W+ laptop undercharge cause.
3. EPR needs an EPR cable: 140W/240W only works over a 50V-rated cable; a 100W e-marked cable won't carry EPR.
4. Charge-only cables can't negotiate. PD handshakes live on the data lines; a charge-only cable = dumb 5V mode forever. Classic hidden trap in cheap multipacks and some KVM/dock paths.
5. Proprietary ≠ PD. VOOC/SuperVOOC/SCP devices charge at ~10-27W on standard PD chargers regardless of the brick's rating; the full 65W+ rate only exists on the vendor's own charger+cable pair.

Workflow

1. Identify the three links (device archetype, charger wattage + protocols, cable type). Labels and original packaging beat memory; assume the worst for unmarked cables (3A, no e-marker).
2. Get the verdict:

   bash

   python3 scripts/pd_compatibility_checker.py check \
       --device laptop-ultrabook --charger 30w-pd --cable 5a-emarked
   

   Output names the negotiated watts, the device's full/min needs, and the verdict: OK (full rate) / CAPPED (works, below max — fine overnight) / SLOW (device throttles or refuses) / NO-CHARGE (5V fallback at best).
3. Decode wattage marketing:

   bash

   python3 scripts/pd_compatibility_checker.py watt --watts 65
   

   prints the V×A operating points and the device-class meaning.
4. Browse the archetype tables:

   bash

   python3 scripts/pd_compatibility_checker.py table
   

5. Buy the fix in this order: e-marked 5A cable (cheapest big win for 65-100W), then adequate brick, then EPR cable only for 140W+ laptops.

Verdict Semantics

Verdict	Meaning	Action
OK	≥ device full-speed watts	none
CAPPED	between min and full	fine for overnight; upgrade if you need daytime top-ups
SLOW	above zero but below device min	device throttles / battery may still drain under load
NO-CHARGE	no common PD contract	replace the broken link (usually the cable)

Common Pitfalls

1. Blaming the charger first. The cable is the culprit more often than the brick — a 3A cable silently caps a 100W setup to 60W.
2. "USB-C to USB-C, so it's fine." Connector shape says nothing about e-markers, EPR rating, or data lines. Test, don't assume.
3. Trusting wattage stickers on multi-port chargers. "65W" often means 65W total, single port — split across ports it can be 45+18 or worse. Check per-port specs.
4. Assuming proprietary phones fast-charge on PD. They don't; expect standard-PD rates (~18-27W) on any third-party brick.
5. Buying 240W EPR cables for everything "to be safe." Fine, but EPR cables cost more and older (pre-3.1) devices can't use the headroom; a certified 100W e-marked cable covers 99% of gear.
6. Charging a laptop through a dock/hub port. Every hop adds negotiation and current caps; dock "PD passthrough" is often 60-90W even when the dock's brick is bigger.

Verification Checklist

• Device archetype confirmed (full-speed and min watts known)
• Charger per-port (not total) wattage identified
• Cable current rating checked (e-marker/5A print on jacket)
• Charge-only suspicion ruled out (does the cable transfer data?)
• Re-run the check after any swap — verify the negotiated watts actually moved (some devices report charge rate in settings)

One-Shot Recipe: diagnose "slow laptop charging"

bash

python3 scripts/pd_compatibility_checker.py check \
    --device laptop-performance --charger 100w-pd --cable 3a-standard
# -> CAPPED at 60W of 100W: the cable. Buy e-marked 5A, re-check.

Check the OS battery/charge readout after swapping — the negotiated number should jump within seconds of replug.