Looking for a UPS Design That Doesn't Overheat Batteries

Our company has had a long-standing problem where UPS batteries will at
various points in their lifetime suddenly overheat, sometimes
catastrophically to the point where the battery casing starts to melt and
you can actually smell the gases from the battery leaking.   So far we
have
been lucky to catch such thermal events with temperature sensors but it
has
always been a goal of mine to better understand why this happens, and to
find some UPS system where it can be avoided entirely.   To date, we have
seen these problems with APC Symmetra tower, Symmetra rackmount, and
SmartUPS.

After working with an electrician, I have a theory about why this is
happening, and if correct, the theory suggests a different design for UPS
systems that would avoid the problem.   I am hoping some manufacturer has
already implemented this idea and someone can refer me to their products.

On all of the UPS systems we use generic "brick" batteries are joined
together in a series, then the leads from the ends of these battery chains
are connected to the UPS.   The problem is that batteries rarely fail all
together.   If a 12V battery should be considered discharged and not
useful
at around 10V, and you have two 12V batteries joined in series, what
happens
when one of the batteries maintains a full charge at 12V but the other
battery in the series starts to lose its ability to hold charge and slips
below some critical level?      From the point of view of the UPS, it
doesn't see anything about the state of individual batteries.   The UPS
only
sees that the total voltage of the two 12V batteries in series has fallen
from 24V to 22V.

Maybe an electrical engineer can step in here and explain what is
happening,
but my pure guess is that to maintain the same power output, an increased
amount of current probably has to flow through the batteries.    That
creates problems with heating for the "good" battery, which is still
measuring 12V.   Now that 12V is receiving too much current, overcharges,
overheats, and at some point the casing of the battery starts to melt.   
I
haven't done enough experimentation to determine if it is the good battery
or bad battery that is overheating.    To be honest, in such situations I
have often seen evidence that both batteries start to melt.   Perhaps this
is nothing more than one battery being in physical proximity to the
overheating battery and therefore gaining heat from its physical contact.
The only thing that is common to all cases is that one of the two
batteries
has discharged and should have been replaced before the overheating event
took place.

Regardless of the actual mechanism for the overheating we are observing,
it
seems to me that the obvious solution is to design UPS systems to
physically
connect to each 12V battery individually.   Forget connecting multiple
batteries in series, at least don't do that at the battery itself.    By
connecting to and monitoring individual batteries, now the UPS can see
when
an individual battery falls below some critical voltage threshold and put
it
into a special recharge state (not put any load on it).   If the battery
fails to recharge, the UPS can declare the battery defective and can
signal
the condition by an LED on the battery's compartment.   If there is a
network attached monitoring system, the UPS can send an e-mail.

Aside from increasing safety and utility of the monitoring system, such a
design would allow much easier re-use of off-the-shelf batteries,
improving
ease-of-use in making battery changes and lowering cost.   While I realize
that APC in particular has no desire to make anything regarding batteries
non proprietary, maybe some other vendor has a UPS design that puts a
direct
monitoring circuit on each individual 12V brick battery, thus avoiding the
overheating problem I have described?

Any information on why this overheating takes place, how to avoid it, and
any referrals to third party UPS products that employ a more robust design
are appreciated.

-- 
Will