We are often asked if it is okay to leave a notebook computer connected to one of our USB-C docking stations with Power Delivery for extended periods of time.  The short answer is yes, it is no different from leaving the laptop connected to the manufacturer's original USB-C power supply for the same time.  The long answer is yes for modern laptops, and maybe for older (1990s-early 2000s laptops) and involves going into the different battery technologies used in consumer electronics devices.

Another common question is if it is possible to use the docking station but to disable powering and charging the computer.  When a modern notebook computer runs on battery power it will often set the system to a reduced power state which may impact performance, or connected devices and we recommend always powering the computer when using a desktop docking station.  For all of our docking stations that provide power to the host computer this will not affect the lifespan of the computer's battery.

Modern Laptop Batteries: Lithium-Ion

Lithium-ion (li-ion) batteries are found in a wide range of consumer electronics from notebook computers and cell phones, to electric cars, power tools, and wearable electronics like wireless earbuds.  Li-ion offers fast charging, high-current discharging, fairly long service live compared to other rechargeable battery technologies and are relatively inexpensive.

The life-span of a rechargeable battery depends on many factors including age, temperature history, charging patterns, the chemical composition of the specific battery, and usage.  For example batteries stored at 100% charge will degrade faster than batteries stored at 50% charge, this is why most consumer electronics devices arrive from the manufacturer with between 25% to 75% charge.

Lithium-ion batteries are consumable components, however in most modern computers, cell phones, and tablets these are not user serviceable components.  To help maintain the battery all modern computers and most consumer electronics will include battery charge and protection circuits.  These can be fairly simple, charging up the battery at preset rates depending on the charge level to help maintain the battery life, or complex software controlled charging that monitors battery temperature, voltage and current draw to maintain the fastest charging while maintaining the battery longevity.

Modern notebook computers can be left connected to the original power cable or a docking station with charging capability for extended periods, and do not benefit from regular discharge/recharge cycles.  Our docking stations with charging capability rely on USB Type-C Power Delivery to power and charge compatible computers.  USB Type-C Power Delivery is a negotiated charging protocol between the host computer and the docking station or USB Type-C power supply, this allows the computer to draw only the power it requires, and even select the best voltage level for powering the computer.  In combination with a computer's built-in battery charging controller the computer is capable of maintaining the battery's optimal state even when left connected to a power source for an extended period of time.

Legacy Laptop Batteries: NiCad and NiMH

Older laptops, from the 1990s and some early 2000s, as well as some consumer electronics, and most rechargeable AA or AAA battery replacements use Nickel-Cadmium (NiCad) or Nickel-Metal Hydride (NiMH) batteries.  These batteries are slower to charge and discharge than li-ion batteries, and require very simple charge controllers, and in some cases can even be trickle-charged ( very low-current continuous charging ) if desired.

These batteries generally don't have smart charging controllers and to prolong the life of the battery required "training" or fully discharging and recharging the battery every so often.  Many laptop manufacturers recommended fully charging and discharging a new laptop 2-3 times to train the battery, this is not necessary with modern laptops.

Conclusion

Modern notebook batteries are managed by the computer's built-in battery charging circuit, and require little to no user intervention to maintain optimal battery health.  It does not harm the battery to leave the computer connected to an external power supply, so long as the computer is being used regularly.  If the computer is to be stored for a prolonged period then discharging the battery to between 50-75% can help to maintain the battery life.

Batteries are consumable components and degrade over time, however modern notebook computers can extend the battery life generally to meet or exceed the life of the computer's other electronic components.

The humble Universal Serial Bus, more commonly known as USB, has evolved to be much more than just a way to connect a mouse or keyboard to your computer. With the proliferation of power-hungry portable devices that charge over USB, we’ve all come to expect USB to operate as a Universal Source of Battery charging but, unfortunately, it’s just not that easy.

Modern Charging Standards

USB-C and USB Power Delivery (PD) Standard Power Range (up to 100W)

The USB-C connector and USB-PD specification up to revision 3.0 enable charging at up to 100W (typically 96W to 98W real-world maximums per IEC regulations). However, the protocol for negotiation between host and charger can be complex and hard to understand. Early USB-C PD implementations (starting in 2015) could be unreliable, and many manufacturers of devices used proprietary implementations to block universal charging solutions (docking stations, third party chargers, etc). With time and several updates to the PD spec there have been vast improvements in the consumer experience, though we do still encounter scenarios where there may be compatibility issues that are beyond our control (3rd party devices).

Generally speaking, USB-C PD is an intelligent charging standard with the ability to deliver variable voltage and amperage charging rates (equating to the final charge wattage). USB-C PD defaults to 5V and can adjust dynamically depending on the implementation of charger and device up to 9V, 12V, 15V, and 20V up to 5A. Not all USB-C ports on devices (like a hub or dock) offer PD charging, some USB-C ports may only offer standard USB 3.0 power at 900mA. Others may offer more at commonly found values of 1.5A or 3A at 5V (7.5W and 15W). Likewise, not all USB-C devices (like a laptop or tablet) will have the ability to charge over this connection, some still require a dedicated DC barrel jack charger that shipped with the device.

Without going into overly complex detail, USB-C PD can either use a set signal on two communication lines through a USB-C connector and cable (CC1 and CC2) which is determined by either a set resistance value and subsequent voltage reading, or two-way data communication can be established where the charger can tell the attached device how much power it can supply and/or the device can request how much power it needs. Once a contract has been negotiated and established, charging may then commence.

As an example, a laptop may require 65W of power to charge, this would most likely need 20V at 3.25A. When connecting this laptop to a USB-C charger (or dock, etc) a series of events as outlined above will occur. If connecting to a lower wattage charger, say 60W for this example, the laptop and charger will try to establish the fastest possible common charging rate which would be 20V 3A. In most cases this will work perfectly fine and the laptop will charge slightly slower than it would with the original 65W charger it shipped with. 

But what would happen if connecting to a higher wattage charger, like 96W or 98W? Many would expect their laptop would be damaged, however, because USB-C PD is intelligent some two-way communication will occur, and the charger should negotiate the fastest possible rate that is requested to deliver 65W. It is also important to note that chargers do not "push" power to a device, rather, devices "pull" power as needed. As long as the charger does not output a voltage higher than what the device is expecting damage won't be possible. Because USB-C defaults to 5V, a higher voltage will not be output unless that higher voltage has been requested and properly negotiated.

What about a laptop that needs more power, like 135W or higher (which is above the USB-C spec)? If the laptop supports USB-C PD through its USB-C connector despite needing more power than can technically be provided, it will try to charge as fast as the charger can offer. Due to the higher power requirements charging may be slower but still gain battery percentage, or the system will be "treading water", only able maintain a stable battery charge without gaining. In some situations like this, the system when under extreme CPU or GPU load may supplement more power by draining the battery while also drawing power from the charger. This may lead to a slow drain and would eventually run out of power until the load is reduced. In some rare situations like this, the laptop may only be able to gain battery while in sleep mode or off. 

USB Power Delivery (PD) Extended Power Range (up to 240W)

The new USB Power Delivery revision 3.1 specifications allow for the delivery of up to 240W via a USB Type-C cable (an increase from the previous 100W max). The previous Power Delivery range has been relabeled as Standard Power Range (SPR), and the new specifications (between 100W and 240W), have been labeled as Extended Power Range (EPR).

For a more in-depth look feel free to check out our blog post on this topic here: https://plugable.com/blogs/news/what-is-240w-usb-extended-power-range-epr

Thunderbolt 3 / Thunderbolt 4 / USB4

Thunderbolt 3, Thunderbolt 4, and USB4 all share the USB-C connector specification and utilize the aforementioned power and charging abilities introduced by USB-C.

Qualcomm Quick Charge (QC) 4/4+/5

Qualcomm QC 4, 4+, and 5 are cross-compatible with both USB PD and USB-C specifications, falling back to PD in the event of a compatibility issue. Currently, Plugable does not offer any QC devices.

Legacy Device Charging