"James Hahn" wrote in message
...
2) that same battery will alway be charged a little less than the other
because the charger measures the voltage across both batteries in
series.
Incorrect. The charger measures the voltage across both batteries in
series.
The voltage it sees is the result of both batteries being there, not the
one
with the highest or lowest charge. The charger will charge until the
voltage across the two batteries reaches the correct level.

There are two possible scenarios: one battery wears more quickly than the
other, eventually reaching a state where the cell voltages reach higher
values due to increasing internal resistance. Thus the charges shuts off too
early, leaving the better battery not topped up.

The other possibility is that due to a deeper discharge of one battery, it
takes longer for the total voltage to reach the point where the charger goes
from bulk to float. So while for the more empty battery the charge is still
ok (bulk), the other one still gets the full bulk current too while it
should receive a float charge.

Have you ever disassembled a broken battery pack of some device? In 99%
of
the cases you will find all cells in perfect condition (charged and all)
except one, which was obviously the weakest in the chain, showing a
reversed
voltage.
You have demolished our own argument. All cells were treated identically,
yet one has collapsed! You can't use that evidence to support a claim
that
cells treated differently are more liable to collapse (which I assume is
what you are trying to say).

There is always one cell that collapses first (the weakesrt), due to
tolerances in materials, production, etc. By introducing an extra load
(wear) on part of the cells in a string, they are more likely to collapse
first.

Collapse of one cell is the most common mode of failure for wet cell
batteries, but it is not associated with differential discharge rates.

We all know that a lead-acid battery suffers from deep discharge. And in
this setup, it is a plain fact that the battery "below" the tap is
discharged sooner then the one "above" the tap. And if the charger is on
"the outside" terminals, it can only see the charge state of the entire
string, which is in this case, wrong information.

For years battery manuafacturers have provided a 12v tap across the first
eight cells of high voltage industrial batteries. There has never been
any
evidence that using the tap shortens the battery life, and it has never
been
the experience of the users.

Uhm, 12V across 8 cells? You are talking about non-rechargable batteries
here?

Meindert