I only need to use my powerbank for a few days once or twice a year and was hoping somebody would know what is the best state to leave the powerbank in? ie fully charged or 50% or 75% charged? Thanks.
Best practice when not using your powerbank?
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Your best storing in a 40-60% range when not using for a long period of time, it is normally agreed to be the sweet spot by most. Also store in a cool environment to prevent wear and tear on the batteries.
Anker manuals do normally recommend a complete charge cycle every 3-4 months from what I remember to ensure longevity of your PowerCore.
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When you have to store a lithium battery for a long time you have to consider two factors: charging and location (temperature).
Storage Temperature-----------40% Charge------------------100% Charge
0 ° C (32 ° F)--------------------2% loss after 1 year----------6% loss after 1 year
25 ° C (77 ° F)------------------4% loss after 1 year----------20% loss after 1 year
40 ° C (104 ° F)----------------15% loss after 1 year---------35% loss after 1 year
60 ° C (140 ° F)----------------25% loss after 1 year---------40% loss after 3 months
From wikipedia.it 
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Hey, i suggest that you flush it out after a few months of not using it.
try reducing it to 0% and charging it maybe midway if youre not gonna be touching it for a while, and then repeat the process every few months.
Pssst @ndalby & @AnkerOfficial : Hence, yet another reason for a forum/ thread dedicated to the Maintenance of Anker products (like this one: Cleaning/Maintaining/Servicing Anker products)! Just saying … 
It’s a great question, with an answer I needed to know. Especially since I’ve just been tossing mine in a box until whenever, paying no attention or even thinking of the need to monitor their charge. Thanks for posting @David_Adams3 and thanks for the prompt response @ndalby 