Deep discharging
The acid density in a deeply discharged battery is very low (nearly all the sulfuric 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.
| 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 sulfuric acid will be consumed and only water will remain in the electrolyte.
Where lead sulfate is more soluble in water than in sulfuric acid, some of the lead sulfate 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.