* navti wrote, On 5/31/2007 4:31 AM:

I am curious as to how the battery capacity can be 50 % diminished and
still deliver 12.6 volts (or thereabouts.

Are the individual cells in the battery wired in series or parallel ?

series


If I had four 12 volt cells wired in parrallel each with a capacity of
10 amp hours then I would have 40 amp hours at 12 volts,

yup


If one cell went dead then I would have 30 amp hours at 12 volts,

Actually, in this case the dead cell might kill the whole bank. If
one cell of a parallel setup is shorted, then all are shorted.


If I had four 3 volt cells wired in series each with a capacity of 10
amp hours then I would have 10 amp hours at 12 volts,

yup



If one cell went dead then I would have 10 amp hours at 9 volts,

only if it died "shorted," which is often the case.


How does it go with my battery ? Is it a combination of series and
parallel ?


You have 6 2.1 Volt cells in series. This is the case with all "12
Volt" lead acid batteries. If you had a catastrophic failure of one
cell, the battery is trash. This is most often an internal short,
sometimes its a cracked case. There are some high end batteries that
have replaceable cells, but they are expensive and heavy.

The issue you have is probably simply deep discharge, which can cause
several problems. One is a loss of lead or "shedding" - the lead
comes loose from the substrate that holds it. In a flooded cell, this
tends to fall to the bottom. The battery acts the same but with
diminished capacity, until the debris builds up and shorts the plates.
Most of us have encountered this if we tend to leave the lights on
in the car every now and then - with each episode some of the lead
falls off and after a dozen or so times, its toast. This is
particularly the case in starting batteries, because the the plates
are so thin. Slow shedding is a normal process, which is why a car
battery often dies the month after the warranty expires.

The other common problem is sulphation. During the normal discharge,
the lead and lead oxide plates are converted to lead sulphate.
Charging reverses this. If the cells are left discharged, the
sulphate can harden and resist reverting back. This can sometimes be
reversed by "equalizing" which is charging at a higher rate which
"loosens" the sulphate. The problem with AGMs is that you have to be
careful not to overcharge, since you can't add water. Severe
sulphation creates a thick layer and cannot be reversed.

In both of the situations, the only definitive test is a load test -
it will be difficult to determine the health simply by charging. Each
manufacturer has a different construction and formulation of the
plates, so detailed experience gained in one brand does not
necessarily apply in another. I've been using the same batteries
(Trojan T105) for my house banks for the last dozen years, and I check
the load, voltage, and amp-hours several times a day, so if they're
off by a tenth of a Volt I start to get suspicious. But on a
different boat, the normal reading could be a tad higher or lower.

Frankly, my experience with AGMs is limited to a pair of starting
batteries that have worked flawlessly for several years, including one
severe winter that killed the flooded house bank. But the basic
chemistry and issues are the same. However, AGM's are able to
withstand long periods of inattention. The Optima site, for example,
claims theirs can sit for a year and still start a car. There is
reason to hope that yours have survived, but only a load test will
tell for sure. You may be able to resurrect them with an equalization
cycle, but I would only try that if its impossible to return them.