> From: John Hasler [mailto:[EMAIL PROTECTED]
> Sent: Tuesday, September 20, 2005 11:16 AM
>
>
> Josh Battles writes:
> > I would think that a marine deep cycle would be better suited to the
> > task...
>
> Only if you expect to have a power failure every day. Deep cycle
> batteries are designed to be deeply discharged with great frequency.
> Car batteries are designed to frequently delivery large amounts of
> current and be either on float charge or disconnected most of the time.
> The ideal is a standby power battery, but car batteries are close enough
> and readily available.
John is right, there is a large difference in design of batteries for
different applications. There are basically two physical cell types based
on the form of electrolyte: flooded liquid cells and gel cells. Car and
marine batteries have flooded liquid cells. These release hydrogen and
water vapor when overcharged, though the newer ones ("no maintenance")
manage to decrease the loss of water enough that the battery usually dies of
other causes by the time that becomes a problem.
Gel cells are designed to trap the hydrogen before it gets out, which they
are mostly successful at. They can also be mounted in any position and lose
very little water vapor with a properly designed charger. When originally
introduced, we all thought gel cells would have incredibly long lifetimes,
but that has turned out to be untrue. Their useful life is not very
different from flooded liquid cells. None of the lead-acid constructions
can deliver much of their stored charge at low temperatures and all are
difficult to charge without damaging them at high temperatures.
Both physical types of cell can be optimized for short high-rate discharges
(typical UPS), long low-rate discharges (emergency lighting), very deep
discharges (marine batteries) or other criteria. One problem for the
consumer is that gel cells optimized for completely different applications
still have the same external package size and nameplate rating. Thus, you
can go to the battery store and get a 12V, 7Amp-hour battery and get any one
of a number of constructions that may not function properly for a given
application.
Here are a few generalizations of my own, from having designed a lot of
battery backup circuitry.
- old-style car or marine batteries with removable cell caps are somewhat
hazardous and are best suited for facilities that are set up to handle them
(i.e. telecom offices, data centers)
- newer "no maintenance" car batteries are still hazardous, though somewhat
less than the older style
- gel cells are very safe, but cost more for the same capacity compared to
flooded liquid cells
- pick a battery that is optimized for the operating condition you want;
that is, do you want to run from batteries for ten minutes or two hours?
it's not the same battery
- if you greatly oversize the battery, you have more flexibility in what it
will do, though you paid for it
- pay attention to the design of the charging system; a good charging system
will greatly extend (or at least not shorten) the useful life of the
battery; poorly designed chargers can cause any battery to dry out and
prematurely fail
- battery chargers need to know the temperature of the _battery_ to properly
terminate charging; that means don't keep the battery outside and the
charger inside, unless the charger is a special design with a remote
temperature sensor
- recharge any disconnected lead-acid batteries every few months; they
slowly self-discharge and need to be "topped off" periodically
- never let a lead-acid battery sit around discharged, as it will gradually
suffer a permanent loss of capacity
--
Seth Goodman
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