I have a number of used but still functional car batteries which I use
for electric fencers and lighting at a stables with no power.

Some are sealed, but for the ones with removable caps I always remove
the cap and check the acid level before putting them on the charger.

I have just one battery which always has low fluid in every cell when I
bring it back for recharge. It takes an exceptional amount of demin
water each time, at least 50 ml per cell if not more. All the cells are
similar. But, when recharged, it reaches a reasonable voltage and
certainly holds a useful amount of charge.

The cells can't be leaking significantly, firstly there is no sign of it
and secondly this must have done a dozen cycles like this over the past
year or two. I havn't actually checked the acid concentration, I
probably should, and top it up if necessary.

Does anyone have an explanation?