Large Battery Banks [RE-wrenches]
Brad Bassett
bsbassett at earthlink.net
Tue Oct 8 20:30:43 PDT 2002
Jeff,
We at Schott/Applied/SES and I personally have experience with the
Absolyte IIP battery in solar applications for the pas 12 years or so.
The experience has been mostly positive if certain measures are taken in
the design of the system. We've seen life times in the 5 to 10 year
range depending on use, with 7 yeas being common under fairly heavy use
in hybrid systems with generators.
The caveats are that the temperature needs to kept down and it needs to
be recharged well and often. High temps kill any lead acid battery
eventually, but the IIP's seem extra sensitive. This may be due in part
to the potential drying out of the electrolyte. If the system does not
have temperature compensation on the charging control and the battery
goes into thermal runaway they're usually toasted. Also they need to be
kept cool if being charged heavily by a generator.
They are tolerant of higher charging voltages than you would expect of a
sealed battery when charged daily instead of continuously (as in a pv
system). We've been using 2.45v/cell for on/off controllers, and about
2.4v/cell for PWM controllers. the warranty spells out the acceptable
voltage for cycle service and I think it's close to the 2.4v/cell for
deep cycling, maybe a little less.
The tendency to dry out is more pronounced when in float service where
they actually do not have a very good reputation.
They are advertised as being able to recover from being frozen also, but
don't believe it! Maybe in lab conditions, but not in the field.
The other condition is that they need to be brought to a full state of
charge fairly often, all Pb acid batteries do, but I've seen the results
of seasonal charge cycles first hand on my own set. After the first
winter without a full charge they were reduced to about half capacity,
after the second winter about half again, and after the third winter my
750 Ahr battery had less than 100 Ahrs capacity. The cause, or at least
a symptom, was the positive plate growth, which has just about broken
through the tops of the cell cases.
So given a good charging regime and temperature compensated charging
control, they will give very good service for a sealed battery, even
with fairly heavy service.
On to other batteries. I spoke with an engineer at East Penn (Deka) once
(years ago) about their sealed batteries in solar, and he strongly urged
me to use their industrial traction based gel cells, the "Dominator" or
"G75" series. He said the Unigy II is optimized for float application
not for cycling and that the Absolyte was better for cycle life. Sandia
Labs chose these batteries (G75) for their nearby telecom system. There
are other battery companies making traction gel cells also; Enersys
(General or Exide based). These have names you would expect for fork
lift batteries also (like Renegade). We've sold a few of these but do
not have any reports yet on their service, maybe no news is good news.
The biggest problem with traction batteries is that they don't come with
the nice built in racking like the telecom batteries, but on the other
hand they are often less expensive.
Brad
Jeff Yago wrote:
>
> I have a client that due to downgrade of his health wants to remove
> large battery bank of 2 volt wet cells on his off grid home, and
> install a battery bank with absolutely no maintenance. Cost not a
> primary consideration (within reason).
>
> I am considering stacked battery modules using sealed valve regulated
> 2 volt Daka Unigy II cells. This would be two (2) stacks wired in
> parallel, 2000 amp-hours @ 48 Volt total, with a stack weight of 4100
> pounds each. Existing loads on old system indicate there would only
> be a 20% daily draw down maximum, which should give him the very long
> no maintenance life he wants.
>
> We have also considered gel cells, C & D VRLA batteries, and GNB
> Absolyte IIP batteries. Has anybody used any of these brands or types
> for a system this large? Good, Bad, Other?
>
> Jeff Yago
>
>
>
Brad Bassett
Schott Applied Power
Tumwater, WA office
bsbassett at earthlink.net
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