You’ve upgraded, but you want to keep your old device just in case you need it. Here’s how to do that.

Five phones stacked together on a wooden surface

Key seems to be charging it to only 50% and turning it off and storing it in that state, with a recharge back to 50% every 6 months (you do remember to set an alarm for that?).

But, as the article suggests, if you are going to store it for a year or more without using it, maybe you should consider giving it away or selling it rather.

See https://www.zdnet.com/home-and-office/how-to-store-your-old-smartphones-and-tablets