The point I get here is that the weak nature of the charging systems and these small batteries dictate a battery tender is key to maintaining your battery in the best state possible prior to use. If you can't use a cord, I saw this solar charger for around $20.00 bucks and they make versions that go from 1.5w up to 8W.
https://www.amazon.com/SUNAPEX-maint...xpY2s9dHJ1ZQ==