The eye on the battery is there for predelivery purposes only. In reality.
it is simply two coloured balls that migrate up and down in an angled
tube.... once the battery has been in service for a while, the tube can
gather debris trapping the balls immobile. It is first a poor mans
hydrometer and second, only measures one cell.

It may, in fact require the addition of distilled water though this will
usually buy you a years use at best. New batteries are genrally of such
light construction that they can't handle much abuse. Before we have the
nay-sayers, yes, I have had batteries that have lasted for a long, log time
but usually opt to replace them before they leave me stranded. I have little
patience for broken machinery and even less free time to deal with the
results... I much prefer to replace or repair at MY liesure than when my car
demands it....

Sorry to digress.... what I meant was that the eye is a poor way to judge
the batteries condition.

I service my battery twice a year - in the fall and in the spirng.  I
disconnect the battery cables and remove the battery cover and top off
the cells with distilled water (about 1/2" from the top of the cell).
I then clean off my battery post and cables with a wire battery post
cleaner.  At the first sign of the battery slow cranking the starter,
I replace the battery.  I usually get 4-5 years out of my battery.

I have to agree with Jim (and yes this is a little OT). Any more, when my
batteries get to the end of their projected life, usually 4-6 years, I start
looking for sales and pick up a replacement when it's convenient, as opposed
to scrounging around and paying the going rate when it dies unexpectedly.
This is based on too much personal experience, after which I finally
realized that delaying the $60 expenditure and trying to get another 6
months out of a battery wasn't worth the headache of getting stranded or
delayed when it finally gives up the ghost, usually without warning. I've
seen almost as many batteries die during summer heat as of winter cold,
usually of an internal short caused by mechanical failure. No warning, just
a small pop when you turn the key to start.  Have even seen where there's
enough left in it to run the headlights, but the higher current of the
starter is just too much for it to handle.  YMMV.

Steve