Actually, the liquid in a charged battery *is* a sulphuric acid solution.
In a fully charged battery, it's a strong solution. As the battery
the acid solution becomes weaker and weaker, approaching that of
water. This is how a hygrometer is able to measure the condition of
a battery. Sulphuric acid is denser than plain water, so the float level
changes according to the battery charge.
The chemical reaction in a battery converts lead + sulphuric acid into
lead sulphate + water. Charging a battery reverses this process, just
as long as the process hasn't progressed too far and you have an
unchargeable 'sulphated' battery.
There's "dead" (battery below 10 volts and unable to crank the starter)
and then there's "really dead" (sulphated). A dead, partially discharged
battery still has a strong enough acid solution to prevent freezing.
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