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Latitude 7480: charge with USB-C
Hi
I have a Dell Latitude 7480 and I've been loving it. At work I always use it connected a docking station that is connected to the laptop with USB-C. It's amazing what goes through just a single port: it charges, gives graphical throughput for two monitors, and so on. Because of this experience, I had expected the laptop to charge with a normal USB-C charger as well, e.g. a phone charger. I have tried two of these and they don't work, meaning that the laptop doesn't register any power input. Their specs:
- Input: 100-240V 50-60Hz 0.6A
- Output: 5V 4A
What can be the reason for this? Is the output of the chargers too low?
jphughan
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December 23rd, 2018 07:00
Sorry if I’m stating the obvious here, but laptops require more power to run than phones. And chargers don’t just handle charging the battery; they’re meant to also provide enough power to run the laptop itself, on top of charging the battery, meaning you can’t connect a low-capacity charger and have the laptop draw whatever it can to charge the battery whenever it can and continue drawing on the battery for the power it needs to run. Some laptops will do that in situations of extreme load and/or seriously undersized chargers, but even there, below a certain level the laptop doesn’t even bother trying to work with the charger. And even before attempting the operation mode I described, laptops will first throttle their performance as much as possible to avoid drawing on the battery while plugged in, so using an undersized charger isn’t a good experience.
So, your charger offers 5V 4A, which is 20W. That’s actually quite a lot for a phone charger. Most phone chargers are 5V 1-2A, or 5-10W, although some Qualcomm Quick Charge 3.0 devices go higher by increasing voltage to 9V. Although if that’s a multi-port charger, it probably can’t supply all 4A out of a single port, fyi. But either way, if you look at the AC adapter included with the 7480, it delivers 65W. The other possible issue is that the USB-C charger must support the USB Power Delivery spec to be usable, and most phone chargers don’t, even some USB-C phone chargers. But that spec also defines a variety of power output levels, so even among USB-PD chargers, they’re not all the same. I have a Nekteck 65W USB-C wall charger that works well with my 7480 and other laptops that charge over USB-C. It’s USB-IF and UL-certified, which I appreciated seeing given that some USB-C chargers on the market have proven to be unreliable at best and equipment damage or even fire hazards at worst.
Also note that to go over 60W, you’ll also need USB-C cables rated for up to 100W. There are two levels of USB Power Delivery support for cables: all USB-C cables can carry up to 60W, and the next step up is 100W. When I connect a “base” USB-C cable to a 65W charger, I only get 60W. The USB-C cable included with that Nekteck charger supports up to 100W, but it’s only 6ft, and my AC adapters all had about 9ft of total cord length, so I wanted something longer. I ended up finding a 10ft USB-C cable also from Nekteck that was rated for 100W.
Windblows
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March 19th, 2019 04:00
One would think a 20 [W] charger should suffice given the following settings in BIOS Setup 1.6.4 of Latitude 7480:
jphughan
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March 19th, 2019 10:00
@Windblows, that refers to the amount of power the system will provide to an attached USB-C peripheral. You can limit that in order to keep more power available for other aspects of the system (remember, the entire system is designed around a 65W power budget), with the caveat that some USB-C devices won't perform as well or at all if they support 15W USB Power Delivery. I believe much of that is indicated in the description box below that setting in the BIOS interface. Either way, the amount of power it needs to draw in order to run properly from AC power rather than off the battery is entirely separate.
Bill987
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December 15th, 2020 15:00
I have a 7480 and it charges fine with a USB C charger that is designed for delivering power for laptops, namely I used the power supply from a Lenovo Yoga. This power supply is capable of delivering 65W at 20v, 45W at 15v, or 18W at 9v. The key here is the 20v capability. Unlike USB 2 & 3, USB C has the ability to negotiate the power delivery voltage to be used. Most laptop charging circuitry is designed for the 19-20V range. Although some may be able to operate or charge at the lower 15V or 9V settings, at 5V most will not. It is more difficult to step up voltages than it is to step them down and most charging arrangements for batteries are set for higher input voltages so the 5v and even 9v are probably too low. It's not always about the wattage capability, but often a minimum voltage level may be required for the laptop to accept a charger as adequate.
jphughan
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December 15th, 2020 19:00
@Bill987 That's all true, but fyi under the current USB PD spec, all voltage levels below 20V are restricted to 3A output. Only 20V is allowed to go higher, up to 5A. The next step down from 20V is 15V, so the 3A limit means that a USB-C charger that only supported up to 15V would be limited to at most 45W output. And consequently, any charger that supports more than 45W output is guaranteed to support 20V. But Dell does make some laptops designed for 45W that support receiving that as either 15V or 20V, including at least some XPS 13 systems.