Please please link to these top quality iron phosphate 20ah batteries.Usually for storing for periods of 3 months or less the battery can be fully charged and left. For longer periods the usual recommendation is to take the state of charge down to 50% SOC with the battery. But there are at least three different types of "lithium" batteries being used on e-bikes. There are lithium-ion, lithium cobalt manganese, and lithium nickel cobalt aluminum. There are also lithium iron phosphate that tend to be much higher capacity batteries although far less expensive than what the bike manufacturers are selling theirs for to customers.
A top quality lithium iron phosphate 20 Ah battery sells for less than $200 whereas a Rad Power 14 Ah battery sells for more than $500.