Q: Can the PowerPack 20000 (QB820) charge my Lenovo ThinkPad X1 Extreme?
A: The Lenovo X1 Extreme is a powerful computer that can potentially consume a lot of power. This is evidenced by the 135-watt OEM AC power brick that comes with it. In comparison, the PowerPack 20k offers up to 45 watts at 15 volts and 40 watts at 20 volts. Regardless of 40 or 45 watts, that's a long way from the peak power consumption of the Lenovo X1 Extreme. What this means is that if you rev up your laptop's power use by running energy-intensive applications like serious number crunching and graphics rendering, the battery pack won't be able to keep up with that, and some additional power will have to come from the laptop's built-in battery.
However, the PowerPack 20k might be able to charge the Lenovo X1 Extreme when the laptop is shut down, in idle, or in light use. Even if the laptop revs up during charging, the additional power from the PowerPack 20k could extend the battery life of the laptop. But, we don't know this for sure because we haven't tested it using a Lenovo X1 Extreme and can only base this answer on experiences with various other high-powered Lenovo laptop models. A previous customer had partial success using an Apple 87-watt power brick to charge the Lenovo X1 Extreme. In that case, he got a warning saying “warning slow USB charger detected." That is not surprising because the Apple charger did not provide full wattage for the laptop.
Bottom line: We are not sure whether the ZMI PowerPack 20k can charge the Lenovo X1 Extreme at all or to the extent of your expectation. Therefore, we cannot recommend it for your intended usage.