Deep discharging

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The acid density in a deeply discharged battery is very low (nearly all the sulphuric acid has been consumed and almost pure water remains. There is a high risk that the battery will be damaged beyond repair by freezing at relatively mild temperatures.

The table shows the freezing point of electrolyte in relation to the degree of charge of the battery.

The freezing point of the battery in relation to the degree of charge

Degree of charge	Freezing point
100 %	approximately -70°C (-94°F)
40 %	approx. -25°C (-13°F)
10 %	approx. -10°C (+14°F)

A deeply discharged battery can also hydrogenate .

Hydrogenation

If the discharge is extremely deep, ultimately all the sulphuric acid will be consumed and only water will remain in the electrolyte.

Where lead sulphate is more soluble in water than in sulphuric acid, some of the lead sulphate in the plates will fall into the electrolyte. When the battery is charged, lead will fall on to the negative plates and separators. Lead gathers in small spots on the surface. This can cause short-circuits. This is known as hydrogenation.