Turns out you can download more battery as Valve's latest update for Steam Frame adds up to 12%

Valve has pushed yet another update for SteamOS on the Steam Frame. It includes power efficiency improvements, eye tracking accessibility, and improved streaming reliability. Also something about grabbing handles?

Improved power efficiency and battery lifetime during both streaming and local gameplay, with gains of up to 12%. Note: Result vary depending on framerate and content.

Added accessibility option for users who want to limit eye tracking to a single eye. VR Settings -> General -> Track Dominant Eye Only. (requires enabling "show advanced settings")

Fixed an issue that prevented controller bindings from loading for certain games, including Google Earth VR and Thumper.

Improved focus behavior when interacting with grab/resize handles

Further VR streaming reliability improvements, including additional fixes for “Error 17.” Note: On the PC, you must also opt into SteamVR 2.18.2 or later to reduce “Error 17” connection issues. At the time of this release, SteamVR 2.18.2 is available on the SteamVR [beta] branch.

Fixed steam frame wireless adapter …