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Just for the record and FWIW: The "RG" as in RG batteries refers to
"recombinant-gas" technology. All RG batteries are SLAB's (sealed lead acid
batteries), but not all SLAB's are RG.

So, who cares? Well, you ought to if you're using, or considering using,
SLAB's in your airplane.

The RG technology is a development of SLAB's that allow them to be truly
sealed, though even RG's (the good ones, at least ) usually have an
over-pressure relief system in each cell - just for safety in case of
charging system failure.

During charging, the electro-chemistry of all lead-acid batteries causes
gaseous hydrogen and oxygen to be generated as part of the charging process.
That's why, in regular wet-cell batteries, you have to replenish the
electrolyte with pure water on occasion - the water is sacrificial during
charging and is the source of the hydrogen and oxygen (remember, water is
H2O) and the two gases do not recombine as there is nothing in a wet-cell to
catalyze that reaction. 'Swhy you need to provide a vent for these batteries
and also, why they have so much fluid in them in the first place - as a
reservoir. In an RG system, all the H and O is provided with a mechanism to
get back together as water and return to the active parts of the battery
cells.

Sealed batteries like gel-cells, do provide some gas recombination or they
would go dry (and therefore useless) in a short itme. However, they aren't
usually optimized in this area - exceed the recommended charging rate by
even a bit (common in stone-age chargers like we have on our airplanes,
i.e., they are not current limited) and excess gas is evolved that cannot be
recombined. As a result, the cell pressure then rises enough to open the
safety vent (or explode, if they don't have a vent!) releasing the
life-blood of the unit to the atmosphere. Too much of this and the gel will
dry out and the battery will fail.

In an RG SLAB, the chemistry and other components are designed to optimize
the recombinant process by orders of magnitude. These batteries start life
with very little water as it is designed to be 100% recycled within the
battery via recombination. And, as it is designed to handle very high gas
evolution rates (there are limits, but they are usually quite high) they are
unlikely to ever need to vent. I've never seen it, but the RG battery may
vent in the event of an unusual failure of your regulator where the voltage
runs away forcing the charging current to go really high. If you have an
over-voltage shut-off (like the OV Crowbar that Bob Nuckolls sells) damage
will be prevented (not only to the battery, but the rest of your electrical
equipment too).

In closing, here's a truism about any lead acid battery you must heed: They
do not survive if allowed to sit around long in an un-charged state.
According to most lead-acid battery manufacturers (and sad experience on the
B-1 fleet) they will be damaged, often irreparably, after sitting uncharged
for more than 72 hours. The culprit is a phenomenon known as sulfation (the
active plates are rendered useless) and usually can't be reversed.

Incidentally, for you LML readers on the other side of the pond, Hawker, in
Newport, Wales, makes a very good RG SLAB. Don't know how it is priced and
whether it's available in the States, but I used them on the B-1 when we
switched from NiCad's and they are performing very well, to date.

Dan Schaefer

LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.