What your Not Understanding is that 0.8 Volts is the Voltage under Load of the Battery.
On a Really Badly desulfated battery that Voltage might be 3 volt or more.
It Depends on the Battery's INTERNAL RESISTANCE.
The Lower this Internal Resistance is, the "CURRENT of this Spike" in a Desulfator must be Much Greater to create a Higher Voltage.
And a Battery that is Already BAD, Definately Can't deliver HIGH CURRENT to a Desulfator, to create a Higher Voltage Spike.
A NEW, Large 12 Volt Car Battery has a Very LOW Internal Resistance.
That is Why they can deliver Hundreds of Amps into a 12 Volt Starter.
Smaller, but good Batteries have Somewhat Higher Internal Resistance, so they Can't delive you expkained everything quite welll.. ty :r nearly as much current.
A Battery with a BAD CELL or a battery that is badly sulfated has even a Higher Internal Resistance.
If a Desulfator was made for Direct Mains Power, Than a VERY HIGH CURRENT SPIKE, with a Very High Voltage could be Created.
But That Spike would Probably do MUCH MORE DAMAGE than Good.
Everything has its LIMITATIONS.
Sorry I don't know How to explain this any better to you.