Is 20 minutes the expected runtime for this trimmer (ST1502-S) and battery (2.5Ah)? It's good to get the taller grass trimmed away from the perimeter of the house and touch up a few areas, but seems like a rather short runtime.
Nathan, under normal use trimming the consensus seems to be people are getting somewhere between 30-60 minutes of use depending how hard they're pushing the tool and how continuous their workload is.
If you were using it continuously in taller thicker grass I can see a shorter run time.
Did you fully charge the battery fully before use? :-)
I'm not sure if there's any real proof to back this up or not, but I've heard claims that batteries tend to gain a little efficiency after a few charge/discharge cycles. I would try it a few times, especially after you have your taller grass under control, and make a judment call at that time.
Other than the short run time, what did you think of the trimmer?