I recently bought an Anker power bank and noticed that its display shows the incoming charging wattage in real time. It arrived with just under 17% charge, so I plugged it into my 96W MacBook Pro power adapter using Apple USB-C cables. I tried two different cables, but the power bank consistently appears to draw roughly 57W.
The adapter lists these USB Power Delivery outputs: 20.5V at 4.7A, 15V at 3A, 9V at 3A, or 5.2V at 3A.
Is around 57W a normal charging rate for this power bank? Why does a charger rated for 96W provide less than its maximum? Does this tell me anything about whether my MacBook Air charges at its maximum speed, or does the laptop choose a different rate depending on its battery level and workload? Is there a way to check the MacBook Air's actual charging rate?
4 Answers
Yes, this is normal. USB Power Delivery involves a negotiation between the charger and the device being charged. The 96W figure is the adapter's maximum capability, not a fixed amount it pushes into every device. The power bank requests a supported voltage and current, then draws only what its charging circuitry is designed to accept. If it is showing about 57W, that is probably its current charging limit under those conditions.
The charger and power bank establish a USB-PD contract based on the capabilities of both sides, including the cable. The available 20.5V profile could theoretically reach close to 96W, but the power bank may not support that profile or may cap its input around 60W. In that case, roughly 57W at the display is simply the highest rate it is requesting and accepting. A 96W adapter does not force 96W through the cable.
Your MacBook Air can behave differently. Its charging system continuously adjusts power draw based on battery percentage, temperature, whether the battery is nearly full, and how hard the computer is working. It may draw substantially less than the adapter's maximum while still charging normally. The power bank's 57W reading does not indicate a problem with the MacBook Air.
The adapter's input rating is not useful for determining the laptop's charging rate; it only describes what the adapter consumes from the wall. To estimate the MacBook's actual draw, use a USB-C power meter between the charger and the laptop, or check a system-monitoring utility that reports battery power and charging status. Keep in mind that software readings may show net battery charging rather than the adapter's total input, since some power is used to run the computer.
The Apple cables are unlikely to be the issue if they are genuine, undamaged USB-C cables rated for the required current. A lower-rated cable can limit the negotiated power, but the device still has to support and request the higher rate. The simplest explanation here is that the Anker's input specification tops out at about 60W, with the small difference between that and 57W coming from measurement variation and charging overhead.
So the adapter's listed output is the maximum it can provide, rather than the amount it always sends to whatever is connected?