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 connectivity issues that affected users in some countries.

It's the battery life improvements that most will feel the benefit of. A 12% improvement is pretty impressive for a single software update—the Steam Frame runs for around two hours while streaming, so you're looking at another 15 minutes or so from that bump alone. Though, as Valve notes, that's dependent on what you're playing and the power setting.

Valve also recently improved charging on the Steam Frame, allowing it to fast charge even when the headset is turned off. That dramatically decreases charging times in my testing.

Details

SteamOS for Steam Frame is a whole new project for Valve. A version of its Arch-based Linux distro, used on the Steam Machine and Steam Deck, except built for virtual reality and Arm-based processors. Specifically, the Qualcomm Snapdragon 8 Gen 3 in the Frame, though laying the foundations for any further Arm-powered projects. I'd love to see an Arm-powered Steam Deck, though a recent rumour suggests the Steam Deck 2 will use another AMD chip instead.