[smallscaleH2]

Ok, now some more number crunching:
We determined that HHO can have a energetic value between 0,0015 kWh and 0,00225 kWh, whereas hydrogen has an energetic value of 0,003.
RedDevil mentioned that HHO has 2/3 (66%) of the energy in hydrogen, so 0,00198 kWh. This is in line with this data.
That said, we won't need this data now as we'll calculate the range another way.

The range when using HHO in a Diesel engine:
A Diesel engine (running on Diesel) can have a fuel consumption of say 6l/100km.
Diesel has an energetic value of 37,95 kWh/gallon = 10 kWh/l
So it uses 60 kWh/100km or hence 0,6 kWh/km

A 1 kWh battery can generate (with a 50% efficient HHO electrolyser) 0,5 kWh on HHO gas
Using this in Diesel engine, you can hence cover: 0,83 km (0,5/0,6)

So if you would say have a 50 kWh battery, you'd be able to cover 41,66 km.
I don't think that's bad really. Seems practical if you consider just this data.
But obviously, if the energy isn't transferred suitably in the engine and blows up your engine instead, it's useless.

Next for the amount of gas the electrolyser would need to be able to supply to the engine per minute:
We assume using a diesel engine that consumes 6l/100 km and which drives at a speed of 100 km/h.
6l -> in 1 hour (=60 minutes)
6l -> 0,1 l/min (6/60)
Diesel has an energy content of 10 kWh/l, so 0,1 l = 1 kWh
So we need to supply it 0,1l/min
Those HHO electrolysers can generate up to 10-15 lpm, so that's more than plenty.