There does not exist such a mythical animal as a "smart" battery charger; may as well snipe hunt a unicorn. Like a wheelchair battery, specific batteries need specific chargers that have lots of time in charge mode to use all the time in the world to do an AGM battery.
By those standards, you would be running your generator all night. A manual charger is tricky at best to guesstimate runtime.
So batteries are guesstimated and so as to NEVER violate the no-gassing rule. You CAN tell a slight bubbling from Mt Saint Helens can't you?