USB-C is a shape, not a promise
The plug fits. The device charges at a crawl, or not at all, or dies the moment the encoder gets busy. Nothing on the outside tells you why.
USB-C describes a connector. Nothing else. Not voltage, not current, not what either end will agree to.
Two cables can look the same. One carries twenty times what the other does.
What actually happens when you plug in
Power Delivery is a negotiation. The source lists what it can offer. The device picks one. They settle on a voltage and a current limit.
Common steps are 5 V, 9 V, 15 V and 20 V. A source that only offers 5 V charges a phone. It will not run a laptop. The port shape tells you nothing.
If the deal fails, both ends drop to a trickle. That is the device that charges and never gains.
Above 3 A a cable needs a chip that declares its rating. Without one the deal caps at 60 W. It does not matter what the charger and device could have agreed.
Sizing a rig instead of guessing
Add up everything at once, at peak rather than idle. Encoder box under load. Modems transmitting. Camera running. Phone charging. That total is nothing like any one of them alone.
| Symptom | Usual cause |
|---|---|
| Works alone, fails when everything runs | Total draw exceeds the negotiated limit |
| Charges slowly on one charger, quickly on another | The slow one advertises fewer voltage steps |
| Fine on a short cable, unreliable on a long one | Voltage drop, or a cable with no e-marker |
| Drops out when the modem transmits | Peak current spikes past the limit for a moment |
Buying so it works
- Check the watts the battery can deliver, not the capacity it stores. A 30,000 mAh pack with an 18 W output will not run an encoder.
- Count the ports and the shared budget. Many packs quote a total that drops per port once you use two.
- Use cables rated for the current you need, and keep them short.
- Buy one inline meter that reads voltage and current. It turns every question here into a measurement.
The habit that saves a shoot
Test the whole rig on the battery you plan to use. Everything running. For as long as you plan to run it.
Power problems show up under sustained load and heat. A bench test never gets there.
Sources
- USB Implementers Forum, USB Power Delivery documents
- USB Implementers Forum, specifications
- FAA PackSafe, lithium batteries
The Rig Wire reports on streaming technology from public documentation, source repositories and vendor releases. We did not bench-test what is described here. When we have tested something, we say so.