External Batteries for PCs Do Not Work as Expected
by Paul Teich, Principal Analyst
Dell demonstrated its house-branded laptop power bank at CES in January and recently sent me an XPS 13 with their production 43Wh / 12,000 mAh Power Companion (model PW7015M) for evaluation. I am in the process of reviewing the XPS 13 user experience (UX) and usability, I will publish that in the next few weeks. While I’ve been abusing the XPS 13 and its Power Companion, I’ve noticed a few behaviors that don’t belong in a review of these products; they are more general challenges with recharging PCs using external power banks instead of alternating current (AC) power converters.
Most of us recharge our smartphones on-the-go with portable “power bank” external USB batteries. Most of these USB batteries are not designed to recharge laptops and productivity tablets such as Microsoft’s Surface Pro 3 (SP3) and Surface Pro 4 (SP4) models. A handful of third-parties, such as Lizone, have designed specialty USB power banks that also output power specifically for laptops and x86-based tablets. Dell’s Power Companion is one of these specialty power banks.
Power banks are designed to deliver the same power spec that AC power converters deliver. Smartphones, tablets and laptops cannot determine the difference between an AC power supply and an external battery based on volts, amps, or the type of power connector.
I like Dell’s Power Companion. It enables me to partially recharge my XPS 13 without having to find a wall socket. But, I expected the Power Companion to behave like my smartphone power banks. I forgot one key fact: Android does not have an AC-powered high performance mode and Windows does. Android always runs in the same mobile power profile (iOS has the same behavior). Not so with Microsoft Windows. That is a challenge for benchmarking and using power banks for Windows based devices.
Recent versions of Windows (including Windows 8 and 10) offer two “Power Options” control panel menus that affect power usage when plugged into a wall and when on battery power:
- “Create a power plan” defaults to a “balanced” profile that weighs power consumption against performance. This setting automatically changes between AC and battery only modes. Users can choose from other modes or create their own, but most do not.
- “Choose when to turn off the display” allows users to set different profile settings for “on battery” and “AC” use. These settings include display brightness and time-out settings, as well as a computer sleep time-out setting.
Both of these configurations change the way a Windows based mobile device uses power, and Windows automatically sets these two configurations differently. Windows will use more power to get more performance and offer a better user experience when plugged in than when it is operating on battery only.
There are three situations for which I wanted to test my XPS 13 and Power Companion that are normal for smartphone owners, so I’ll describe them generically:
- Run the device’s battery down, shut off the device, connect an external battery, drain the external battery completely or charge the device’s battery completely (whichever comes first) and then boot the device again.
- Run the device’s battery down to a warning (say 10% or 7%), connect an external battery and then recharge while working, all without shutting down.
- Connect the battery while the device still has plenty of battery power and keep the internal battery topped off until the external battery is drained completely.
Smartphone owners try to avoid the first situation, because then the smartphone is of no use at all, it is an inert “brick” that cannot even place or receive calls. For Android and iOS devices there doesn’t seem to be a noticeable difference in battery life between the second and third situations, because there are no alternate power modes on a smartphone until the internal battery is drained very low and “power save mode” kicks-in. But Android’s power usage configuration reverts to normal when an external battery or AC charger is connected.
Here’s my challenge for Windows-based devices in situations two and three — as soon as an external battery is plugged into the power socket, the device bumps up into a higher power consumption model, which drains the battery power at a faster rate than when it is not plugged in. The obvious winning situation for the best battery power usage is the first situation — shut off the device, recharge it, unplug the external battery and reboot the PC. Note that depending on the Power Options settings a Windows device might wake from sleep when plugged into an external battery.
I would like to automate testing the Power Companion, but every battery rundown app available simply shuts off when I connect the external battery, because the battery looks just like AC power to the app. There is no way for the app to tell the difference between AC power and battery power. I am effectively stuck trying to document my usage and power consumption the slow way, as it happens.
Dell says that the XPS 13 can be recharged via its USB-C port, but that recharging behavior will mirror recharging the XPS 13 through its standard coaxial power port. This is because the first generation of USB-C power management does not provide any hints that the recharging source is infinite (plugged into a wall) or finite (drains a battery). I’m singling USB-C out because it is a new spec and is guaranteed to evolve over the next few years. USB-C also spans vendors and is the single most likely replacement for all of those proprietary power supply bricks and cables in the laptop market today.
My USB-C specification request to the folks at the USB-IF: please enable USB-C rechargers to report what type of recharger they are — wall-powered or battery-powered — and then enable batteries to report their instantaneous charge remaining to a device being recharged. With this simple addition to the USB-C power delivery spec, a Microsoft and/or a laptop manufacturer can make intelligent decisions about how to best use an external battery.
Every USB battery that has a charge status indicator already contains a small microcontroller, even simple LED lights. The USB-C specification includes low-speed signal wires separate from the high-speed data transfer lanes, and they can already report that a “weak battery” threshold has been reached and the remaining charge time (in minutes). Adding an AC/battery charge source descriptor requires a little additional code in the laptop’s power management system (and in Windows), but should not add any cost to USB-C batteries (likewise for reporting instantaneous battery charge remaining).
Over time, I believe that smart high-voltage power delivery over USB-C will enable more efficient battery recharging and device independent docking stations that span smartphones to Windows laptops and will be a win for long flights and power users.
— Disclaimer: We are not a testing lab — we occasionally evaluate products by using them in our high tech road warrior lifestyles. Dell sent me a demo unit to evaluate.
