Morgan Blakely, Are you saying that measuring the battery 1 way then you get 19.8Vthen revering the probes you get 0V? I the only thing I can think of right now that would cause that, is if your meter has a built in “protection” that makes it so you can’t measure “negative voltage” (ie positive probe on negative point and negative probe on positive point).
Also, 21.6V lithium battery seems a tad strange as lithium batteries usually go in increments of 3.7V (which would be 22.2V).
Testing the charger for output voltage is always a good first test. That way you know whether it outputs the correct voltage.