As the title says, I have low-power sources (100-200W) available to slowly top up the B300.
The AC300 only refills the battery when the conditions are rights.
Is it safe to keep the B300’s MC4 input plugged in (an active) when AC300 recharges it?
Given the question in your title, the answer is yes, B300 can.
For the last question, the answer is safe, don’t worry about it.
I have a similar situation with an AC500 and B300S batteries.
The MC4 input of the batteries can stay plugged in but I noticed that in this case the battery is not being charged by the AC500 as long as current is flowing trough the MC4 input.
interesting, I’ll make a test asap and see if it’s similar with AC300.