What causes a battery to become dry?

[cat007]

Both my deep cycle battery and my main cranking battery in my campervan have the liquid levels below the cells. What causes this? Overcharging?

[matt dunn]

Both my deep cycle battery and my main cranking battery in my campervan have the liquid levels below the cells. What causes this? Overcharging?

possibly but not always.

Old batterys will do it with normal charge,

and goo batterys with the correct charge will do it if they are on charge all the time, i.e campervan is always plugged into 230v when not used,
or a decent solar panel that is always charging the battery's.

Would be worth putting a voltmeter on and making sure it is not overcharging too though.