What he found, but may not always work, was to turn on the ignition but not start the car immediately. More often than not that allowed the battery voltage detection to work before the starter motor was used and therefore seemed gave a lower reading so stop/start didn't work.
I think that battery voltage check is only done when the engine is started and if the voltage is below the threshold then the Idle Stop is disabled for the whole trip. I recall that when I had a new Mk 3 Jazz the Idle Stop was disabled when leaving home but was fine after I had stopped to refuel at a petrol station about 2 miles down the road. I had similar frustrations with a Mk 2 HR-V and concluded that of the vehicle had been unused for more than about 3 days then the natural drop in battery voltage put it below the Idle Stop's threshold.The dealer tested the battery more than once and declared it healthy. My work-around was, if the car had been standing for more than 3 days, to put the battery on charge the day before making a trip.
Ultimately, it's a feature that would always work during the fuel economy testing but there was no obligation to ensure that it worked reliably in real life.
