Adding another report: 148 hours on the counter on the mower, 108 of those on a Trojan 31XHS battery pack purchased 3.5 years ago.
The good news is: battery capacity is still good, runtime is just a couple minutes less than 3 years ago and the battery "vitals" are solid (each charged battery is 12.74 V at rest with a s.g. of 1.3, variation between batteries is less than 0.1V). Here's a graph of a normal "mow-yard-until-deck-stops-and-then-charge" cycle:
This charge profile looks fine, a three stage charge pumping close to 85 Ah back into the battery pack. Trojan rates the 31XHS to be able to deliver 75A over 57 minutes so this discharge/charge balance is quite close. One improvement would be to raise the charge current in the first phase to the recommended 10-13% C20 value which would for this battery be 14 or 15 A instead of 7 A. I may have to experiment with connecting 2 stock chargers in parallel ...
The bad news is: the stock charger can NOT be trusted and will destroy batteries when charging them from a partial discharge. After blowing away some leaves and finishing the yard the next day using only 20% of the battery capacity before starting charge cycle 101 the graphs are:
The first charge stage is OK and the tapering down in the second stage starts fine as well but in this situation the charger does not detect the point where it should stop and switch to just a maintenance charge. Instead of finishing the charge at 9 hours with about 35 Ah total charge the charger does something more akin to an "equalizing cycle" cooking the batteries to 40 deg Celsius before finally giving up after 20 hours. As I had not equalized the batteries this season yet I didn't mind this particular charge cycle to turn out like this but if this is done during every charge then the batteries will run dry very quick.
So far for some battery lifetime experience and while on it more evidence about stock charger fail conditions. I'll continue to use this charger though as I rarely charge partially discharged batteries and at this point in time don't want to invest in a special 4 bank charger for lead-acid technology anymore. When this set of batteries is up it is time for a conversion to a Lithium chemistry. Not just to get rid of the battery maintenance having to check electrolyte levels 3 or 4 times per season (a chore) but also to get more capacity out of the same battery pack size at lower weight of that pack.