Ideally you’d own two or three smaller Powercore, use one then cycle through the next then the next, so each get used a little over time. Charged, used, recharged, stored, then used a month or two later, repeat, with one being actively used now and then again later as you swap between them. That way you always have a fully charged backup and they are never left alone too long.
That verification your backup actually works is thus continual, every few weeks you’d be swapping and so knowing it worked. If your backup were to fail you’d know while the working one exists pending the postie to replace backup. As such the backup concept disappears as you have two or three and each is used alternatively, you don’t have a dedicated backup.
If you didn’t want to do the ideal then at least try to swap between them as often as reasonable.
The issue is not long term storage, the issue is the false assumption, which grows over time, that your backup is actually still functional. The longer you leave it, the longer there is for it to fail, such as it got cooked in a hot place without you realising. So the idea of a backup causes more chances the backup fails.
If you did want to ignore the best advice, then store it roughly 2/3rd full in a cool, not cold, not hot, place like in a drawer in main part of house away from sunshine and worst of heat and cold. If it’s not shocked by cold or heat it should last month’s between uses and years before it’s totally dead.
I do also recommend people avoid having one big Powercore as it’s more likely to fail. Two or ideally 3 smaller ones are much harder to fail. Smaller Powercore can be kept with you all the time, useful for you or anyone else.
The larger Powercore use multiple cells which all have to be working for the Powercore to work. So larger Powercore fail faster. There is a voltage balancer inside which can cope only so far with one weak cell.
I have 3 10000 and use as above.