This is a going to be a short one, yet somehow an interesting topic. It seems like Valve “forgot” to release the Deck with a low battery indicator that would actually let the end users know about the fact that their battery is about to die while playing a game. Even IGN clearly mentioned it in their early review:

It’s not that the battery could not be monitored - you can actually clearly see in the Mangohud screens that the battery’s status is clearly visible. It’s just that Valve did not implement any kind of notification - a notification that is fairly critical when you battery does not last much more than a few hours in the most demanding games.

Well, it’s now fixed apparently:

Great news, but I wonder how this feature was missed in the QA process until now - especially knowing that Valve is known to conduct a lot of iterations with gamers and testers.