On 17/11/2017 20:26, alan_m wrote:
On 17/11/2017 17:18, newshound wrote:

Does anyone have an explanation?

Left out in a sunny sheltered location or indoors behind a window and
the water is just getting hot and evaporating?

Good thought, but no. It's inside a shed which never gets particularly
warm because it is under trees, and in a valley orientated so that it
doesn't get much direct sun at the best of times. And its quite cool at
the moment. And it is only this one of about four batteries which
behaves like this. And you don't get anything like this evaporation in a
car battery even in the summer, with the combination of direct sun and
underbonnet heating from the engine.