End of season update, 1 year later:
Another year of happy electric mowing. (Well, I'm not exactly
happy to be mowing, but you get the idea.)
The mower had less work to do this year than last, since I
installed a driveway and did some landscaping that exposed a good amount of granite bedrock in the front yard. Which means: yay, less grass to cut! Also, we had a severe drought which caused the grass to go dormant for much of the summer.
The old battery pack is holding up: it's still enough to do my small-ish property on one charge if I hustle (~15-20 mins), or over 2 charges (over 2 days) if I take my sweet time (usually how I did it).