The last time I checked (with B+B) the cost to rebuild the two NiCads was nearly the same as buying a whole new kit with Li batts.
I only have one large battery for my Milwaukee trimmer & leaf blower. It works good, but I can tell it's not what it used to be. My usual routine is to trim first until the batt dies, let it cool off and then put it on the charger while I mow the lawn. Usually by then it is fully charged and I can blow the driveway, walkway and my neighbors driveway off. It is usually about done after that. A second battery will be useful. The one I now have is 4-5 y