charging a phone via a laptop which is also being charged

Question

Suppose my laptop and my smartphone are both at low battery level and I want to charge up both. I start charging my laptop, and I also connect the phone to the laptop using the usb cable. Is this going to slow down charging of both devices?

Answer 1

The Oversimplified Principle Of Battery Charging

Assuming you know the following:

• The input power, in Watts, that your power supply / charger is capable of supplying to your laptop
• The power, in Watts, that your laptop is draining while it is running (assuming you have it running; otherwise this is very close to or equal to 0 Watts)
• The power, in Watts, that your phone is draining while it is running
• The minimum power, in Watts, that your laptop's battery requires to charge it at all
• The maximum power, in Watts, that your laptop's battery is capable of taking to charge it at the maximum allowed rate
• The minimum power, in Watts, that your phone's battery requires to charge it at all
• The maximum power, in Watts, that your phone's battery is capable of taking to charge it at the maximum rate

And discounting the following possible impairments for the purposes of simplicity:

• Voltage conversion inefficiency
• Heat loss
• Cable length current loss
• PSU circuitry current loss
• Noise, ripple, etc. which can slow or stop charging depending on severity
• USB protocol problems, which can occasionally occur, where the USB port will draw less power than needed for the client device (the phone) to charge

Then you can make the following conclusions:

1. If the total input power from the wall socket, in Watts, is less than the "desired" maximum charging power of the device system, then one of the following is true:

   (a) If the total input power is less than the power required to keep the device system operating, then the battery will discharge.

   (b) If the total input power is greater than the power required to keep the device system operating, then the battery will charge at less than its maximum possible speed.

where "the device system" includes the sum of the power, in Watts, drawn by:

• The continuous operation of the phone; AND
• The continuous operation of the laptop; AND
• The charging of the laptop's battery; AND
• The charging of the phone's battery.

2. If the total input power from the wall socket, in Watts, is greater than the "desired" maximum charging power of the device system, then the laptop and smartphone should charge at maximum speed (unless one of the impairments listed above is causing a problem).

---

In Practice

In practice, you should only encounter situations where this will slow down your charging in cases like the following:

• When you purchased your laptop, you had a choice of multiple different power supply units, any of which will work, but you chose the one with less Watts output (because it was cheaper, smaller, etc.)
• You have a laptop with a lot of accessories and are currently using it, and doing energy-intensive things (disk I/O and high CPU usage and GPU usage, like playing a 3D simulation game for example; with maximum screen brightness, a DVD playing, and other USB peripherals like external mouse and keyboard).
• Your laptop's battery is very low. Laptop batteries draw a "non-linear" amount of power depending on their charge level: most batteries charge extremely quickly from the 20% to 80% level, but while doing so, they draw a huge amount of power. This amount drops off as it "trickle charges" the last 10% or so to fill it up.
• You have a modern smartphone that draws a larger amount of power from the USB socket.

In a situation like this, you could see your laptop battery charging slower. In all likelihood, the smartphone battery charge will get priority over the laptop battery, because the USB port, being fully powered by A/C power (the PSU), will strive to meet the desired power demand of the USB device. The laptop battery will take whatever scraps are left over after all the core system functions are done (GPU, CPU, screen, peripherals, USB, disks, etc.)

You see, the USB subsystem of your laptop does not know that the device connected to its port is asking for X number of Watts because it wants to charge a battery. All it knows is that it needs that much. It thinks that, if it can't provide the requested charging power, the device will turn off because it's not powered up correctly. This certainly takes precedence over charging the laptop battery, because the laptop does not want your peripheral devices to randomly turn off if it can avoid that.

---

Overall Advice

• The specifics of whether your phone and laptop battery are charging at the maximum possible rate will depend on your specific hardware, the capacity of your power supply, the amount of energy your devices are drawing just to operate (which depends on how hard you're pushing them at the moment), and the quality of the power (high amounts of noise, ripple and load variability in the power can impede charging for any type of sensitive battery system).
  
  
• In general, for most systems, if you notice that there is a slowdown and you would like to correct the issue, putting the laptop into "Standby" or "Sleep" mode should resolve the problem: the primary power consumer of this device system is the laptop while it's running, so if you eliminate that power draw, there will be enough "slack" power to charge both the laptop battery and the phone at the full rate. Note that you may need to enable a setting in your BIOS to tell the system to keep the USB bus on when in standby.

Answer 2

Yes, but minimally. USB does not draw very much power compared to what the laptop can draw from the wall.

Answer 3

The phone will be charging normally, as it will be getting its full draw from the USB port. The laptop would charge ever so slightly slower because of that.