if the mowers are anything like any other battery powered tool, it would be cheaper to buy a whole new set up and get your extra battery and charger that way, then it would to buy a separate extra battery
Hence "which I might", lol..
If anybody's interested, Tractor Supply has a 2pack of 60v Max flex-volt 6ah for $200, that's 1/2 off/BOGO. They aren't compatible w/the mower, but got them to back-up the 9ah batt that came with the chainsaw.....had to get a raincheck...wonder why? Lol!