I’ve only tried this on a friend’s Xmax 300 battery that went flat down to 0.9V... and I’ve been able to get 12.8+V on the battery that I thought was way too low to revive!
Will let the battery stand for 2 weeks without use to test the voltage again. I thought I only read previously that batteries less than 2V were kind of a lost cause; but this one test case is really surprising to me.