All great advice but: Anecdotaly speaking (is that even a word)
We have been storing our batteries at full charge for several years and as of last season we were still using three year old batteries in competition (fully tested of course.) without a single battery related failure.
Treat your batteries well and they will return in kind.
Buy one of these
CBAIV for load testing (it's a lot of money but they can be shared among a couple teams in your area)
Also purchase one of these
Battery Beak, they are invaluable for testing things like internal resistance and overall health, and knowing the state of charge and battery health of a battery before it goes into your robot at competition is great peace of mind.