Get Sony A7R II at B&H Photo.
In the field, it is possible to charge the battery in the Sony A7R II (and other Sony cameras) using the supplied USB cable. Many battery packs on the market could be carried and plugged in while in the field for extra power.
Some of these charging options may be non-obvious but better choices than generic ones because they dovetail with other outdoor uses.
For example, your author is an avid cyclist and likes to hike (sometimes at night) and favors the best-of-breed bike lights and headlamps by Lupine: Lupine offers the diminutive USB One which lets any Lupine battery charge a USB device (even a totally depleted iPad, which is relatively high current). [See your author’s reviews of Lupine lights].
I used Lupine USB One to charge an iPad, an iPhone and the Sony A7R II battery. Works great, and is highly efficient, no AC-DC-AC conversion needed. With Lupine batteries from small to medium to large up to 20 amp-hours (140 watt hours), a lot of “juice” can be carried for outdoor uses (consider backpacking over multiple days, for example). And the batteries can be used for a headlamp or bike light too!