I have several Milwaukee M18 and M12 tools. For the most part, I charge the batteries as needed (I always have a few on-hand that are fully charged to swap if needed to complete a job). For my ice auger drill (2804), I charge the battery (or batteries) after each outing. I let them warm up to room temp first, then charge. Definitely want the batteries at full charge before hitting the ice the next time.
Lithium-Ion batteries are very Private-proof when it comes to charging and today's chargers are sophisticated. One would have to work hard to trash one by charging practices.
I heard something once about keeping these batteries at 50%-ish charge to prolong their life, BUT that was just for prolonged storage situations. Every couple months or so, I round up my M12 and M18 batteries and cycle them thru a charge to prevent one from sitting around too long - likely unnecessary, but it is easy to do. Most of the time in these cases, the charger goes Green Light right away b/c the battery did not need to be charged.