Haven't had to get one recharged for yonks but I had the professionals do one once and of course they hadn't got a clue. So I took it to my out in the woods guy you'd sorted one out before. He pulled a vacuum on the system for 30 minutes and then recharged it. Any water vapour left will freeze when the system is operating blocking the pipes and can bugger the pumps.
Also find out why it needs recharging, can be a stone has hit the heat exchanger radiator and left an ever so minute hole