Why your charging cable's wattage display goes up and down (and why that's a good thing)
Our charging cables have a small illuminated display that shows the power flowing to your device, live, in watts (W). It's the feature people love most — and the one that generates the most questions. The most common: "Why is the number lower than the cable's rating?" and "Why does it keep dropping?"
The short answer: the number is supposed to change. The display isn't showing the cable's maximum — it's showing what your device is actually receiving at this exact moment. That figure is the result of a constant negotiation between your charger, the cable and your device, and it moves for entirely healthy reasons. Here they all are.
Your charger sets the ceiling
A cable carries power; it can't create it. Plug a 100W-rated cable into a 20W wall plug and the display will show up to about 20W — that's the plug's limit, not the cable's. Older plugs, computer USB ports, car chargers and airline seats often supply far less than a modern USB-C wall charger.
This is actually the display's superpower: it instantly reveals which of your chargers, sockets and power banks are genuinely fast and which are quietly slow. Try your usual charging spots and compare — most people find at least one surprise.
Your device has its own limit
Every phone, tablet and laptop has a maximum charging speed set by its manufacturer. Many phones top out somewhere between 20W and 45W no matter how powerful the charger and cable are. If your phone shows 27W on a 100W cable, nothing is wrong — that's your phone charging as fast as it's designed to.
A fuller battery charges more slowly — on purpose
Lithium batteries charge fastest when they're low, then taper off as they fill, slowing dramatically past roughly 80%. Every modern device does this deliberately: it protects the battery's long-term health. So a number that falls steadily during a charge is the system working exactly as designed. Near 100% you might see just a few watts — or the display may switch off entirely — as your device tops up in small sips.
Everything else that moves the number
Temperature. Devices slow charging when they're warm — in sunlight, under a pillow, or during heavy use — to protect the battery. Expect lower figures on hot days or while gaming.
Battery-care software. Features like Apple's Optimised Battery Charging or Samsung's Battery Protect intentionally pause or slow charging (often overnight) to extend battery lifespan. If your phone sits at a low wattage at 2am, that's your phone being sensible.
Using the device while it charges. Watching video or navigating while plugged in changes the balance between charging the battery and powering the screen, so the number shifts continuously.
Multi-port chargers. Most share their total output across ports. Plug a second device in and the first one's wattage often drops — unplug it and watch the number climb back.
Ordinary fluctuation. Even in steady conditions the figure drifts by a few watts as charger and device continually renegotiate. Rock-steady numbers would actually be the odd behaviour.
When should you investigate?
Compare like with like. If the display reads much lower than it usually does with the same charger, same device, and a similar battery level, try a different socket, then a different charger — tired sockets and ageing chargers cause most slow-charging mysteries. If you've ruled those out and still think the cable is at fault, contact us and we'll put it right — every MediaDevil cable is covered by our warranty.
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