The main thing I am wondering is how long will they hold their charge...Any idea? If I charge it today and put it away, will it be able to charge my device after a month or 6?
What is your planned usage for the battery pack?
-Quick top-off for a long day outside? (lightweight keychain or thin pocket pack around 2k-4k mah)
-Something to toss in a briefcase or backpack to top off things during the week when you need to? (10k-20k name brand pack)
-To put in luggage for trips when you may be away from an outlet? (10-20k pack for your day backpack, do you need an international wall charger to recharge the pack in the hotel room?)
-Something for the car glove box for emergency power? (can it take the extreme heat or cold of a car's interior?)
-To put in an prepper 'go bag' in case of a natural disaster? (will the weight of a huge pack be better used for other items?)
A good Li-ion pack should hold (most of) it's charge for a couple of months. Less so if it has had many recharge cycles. (like my 5 year old packs)
If you need power when away from a power outlet for a log time, consider getting a good battery pack and a solar charger. Companies like GoalZero make their business doing solar and portable charging.
For 50 bucks, here's an Anker solar cell that (says it) is weatherproof, 12 ounces, and can be unfolded and strapped to your backpack to charge your phone when you hike.
https://www.amazon.com/Anker-Charge...ag=hawk-future-20&ascsubtag=UUacUvbUpU5400886