All of what Brent writes is good data, but as the pitchmen on TV say
(yell?): "But wait, there's more"!

One of my last projects on the B1-B before retiring was the replacement of
the 28 volt  NiCad primary batteries and chargers with Valve Regulated,
Recombinant-Gas, Sealed Lead Acid batteries (VR/SLAB). Up to then, my
education included the familiar statement "batteries not included" but I was
lucky enough to work on the project with a PhD at the Battelle Memorial
Institute whose specialty was batteries. As a result, I learned that there
is a lot to learn about something we tend to take so much for granted.

First and foremost (IMHO) is that batteries from different manufacturers can
be very different in their internal construction and chemistry, performance,
cycle-life, charge acceptance, capacity discharge characteristics and deep
discharge recovery, etc., etc. In fact, major battery manufacturers usually
are quite secretive about just when to spit in the brew when mixing the
stuff that goes into their products. Qualification testing on some candidate
B1 batteries showed even some big name mfg's couldn't cut it.

Another lesson learned was that letting a fully discharged lead-acid battery
sit for a relatively short period (could be as little as a week) meant a
chemical process (sulfation - see Brent's note regarding this) could result
in a battery that wouldn't accept a charge and became a candidate for the
recycle pile. (There is a process, called "Hotshotting", that can possibly
bring a badly sulfated battery back to life but it isn't guaranteed to work
every time and requires a special Hotshot charger and up to 25 to 50 hours
to work - if it does - and besides, you're unlikely to have one handy).

VR/SLABs are generally of the "starved electrolyte" type, meaning that the
electrolyte is completely absorbed into the plate separator matting (looks
like TP to the untutored - me) so there's little likelihood of the plates
shedding pieces that can fall to the bottom of a cell and cause a short.
Doesn't mean they can't develop a short, only that it's going to have to
come from a different mechanism.

One more thing, if you're to believe the ads, the recombinant-gas batteries
are more or less immune to drying out, being able to recombine the gasses
produced during charging (Hydrogen and Oxygen) and return the water to the
cells. The implication is that this immunity extends to an over charge too.
If this were true, the cells wouldn't have to be "Valve Regulated", e.g.,
crack when the internal pressure gets too high. When this happens, like when
your regulator is set too high or you leave your unregulated shop charger on
when you go for a beer and forget it, the recombination mechanism can be
overwhelmed by excessive over-charge, the internal pressure will rise and
the valve will vent a lot of H's and O2's and the battery will eventually
dry out.

Flooded cell batteries are a black horse of different color and vent just
about all the time they are being charged - s'why you need to add water and
provide a vent for the hydrolysis products to the outside world so the VERY
explosive gasses won't accumulate in your fuselage just waiting for a spark
to ruin your day. I personally won't use one, but that's just my hang-up -
many others do and are happy.

Since, on a VR/SLAB you can't tell if and when the valve vents and you can't
measure how much moisture the battery retains, unless you are equipped to
measure your battery's capacity Brent's recommendation to change them out
every so often is right on.

This is too long but hope it helps someone.

Dan Schaefer