Yes and No.
There are two sets of rules, HOUSE and RAW. RAW is Rules As Written, meaning we look at how the structure of the rule is delivered. HOUSE rules are home-brewed or tweaked to the liking of the table.
The first statement is "While in battle", this indicates that YES you must be in battle to initiate this "as a bonus action". It clearly states the terms and as written must be upheld. Define battle in the physical sense, while you are physically engaged or physical engagement is imminent to induce such an adrenaline reaction as RAGE and cannot be done whimsically as the PC is not in battle. Therefore RAW dictates in a definitive manner that one cannot use RAGE outside of combat, it is not a meta concept as it is meant toward a trigger that only comes from battle itself, combat is not meta. If you're being punched in your face versus buying a chicken, everyone knows which scenario is in combat and which one isn't, you don't need to have a higher sense outside of your PC's knowledge to understand that. This is why some have come up with setting themselves on fire to induce longevity of RAGE while ENGAGED in battle. Meaning there is a threat imposed to induce the instance of battle. For example, sticking a fork in your hand doesn't cause you to RAGE because you're not in battle and no threat is upon you, you did it to yourself, therefore you are NOT in battle and cannot induce the RAGE effect.
HOUSE rules, aka 'home-brew' may deem it a bit strict to decide that a PC can only RAGE while in battle. Several deciding factors could occur to induce RAGE outside of physical combat. For instance if they lose at a card game, a distant memory of a time that causes adrenaline, or simply thinking about punching someone in the face to them, would mean, they could RAGE regardless if actual battle was triggered. The variables could be endless, and I personally allow my Players to RAGE without needing to be in physical combat or the threat thereof.
Hope this helps define a few things besides stating that something JUST IS without analyzing the entire context of the game. Ultimately it's up to your table how you want to define it, or rather, the DM should define the terms when going over your sheet if they are experienced. But if they're fairly new and make a judgement call, it would be something to discuss after the session or briefly state your reasons at that moment in a reasonable manner.