I got a battery-operated mower for this season: I needed a new mower, wanted to try out the technology, and given the hour at which I mow on a Texas Sunday—before it gets too hot—I wanted something quieter than a traditional gasoline-powered machine. However, unlike modern gasoline-powered mowers, this one is human propelled.
The mower's design facilitates dismounting the battery for recharging between mowings; for me, this amounts to a walk of some 30 feet from my wife's garden shed where I store the mower to the garage, where I recharge the battery.
That's a long introduction to get to the weird physics matter. I've noticed, now, some 3+ months into the mowing season, that the battery is quite light when it's fully charged and I'm carrying it out to the mower. However, after it's given up all those electrons to the mowing operation, the battery has gotten quite heavy for the carry back to be recharged.