[RedDevil]

The amount of HHO produced by charging lead acid batteries is minimal. And it need be; otherwise the batteries would lose their water way too quickly to maintain them.
The generation is not free. It essentially wastes electricity, just like it would if you deliberately generate HHO.

As a gas HHO is over a thousand times less dense than water. You'd need a big balloon to store just enough HHO to have a noticable effect on fuel consumption for even a short period. You need to make sure it cannot escape to any source of heat or sparking; it is violently explosive. My highschool chemistry teacher ignited about 2 liter of HHO foam in a plastic tub. We were impressed. So were about 10 other teachers from the neighbouring classrooms, who came in to investigate. We were lucky not to have broken any windows...