[RE-wrenches] For Big Bank Off Grid System Connoisseurs

[Mark Frye]

Here are details of a system I am working with:

2 hours south of Sacramento CA and west into the coast range for 5 miles and
30 minutes on a rough dirt road is an improvement. The improvement consists
of a large main house, 2 separate guest cabins, and a four car garage, all
built out of logs from British Columbia. Out back and up the hill is a power
house.  There is a well with bad water and a satellite telephone, no PG&E.
Most of the time no one is around, but a few times a year, folks show up and
have a party.

This off-grid power system consists of 16 - Enersys 1690 AH @ 20 hour HUP
12V forklift batteries. The batteries are arranged in 4 strings of 4
batteries each.

There are 10 - Outback VFX3648 inverters in a 2 phase stack.

There is a Kohler REGZ 30 KVA propane powered generator.

Also included is about 5 KW worth of Astropower 120 modules plugged into 2
MX60s.

A Trimetric TM-2020 watches the charge flowing in and out of the battery
bank.

The system has run without interruption or major problem for 6 years.
However, recently we have had our first major service on the generator, our
first smoked inverter, and we have a dead cell in one of the batteries.

Besides the occasionally high demand use events, the stand by conditions run
along two seasonal veins: In summer the main demand besides the usual
refrigerator and alarm system is pumping and treating irrigation water; In
the winter heating systems run to keep the buildings warm.

The contribution of the PV is secondary. On a recent visit the 2 MX's
generated about 30KWH of energy in a day. However, this significant
contribution to the system is generally not sufficient to completely offset
the summer standby loads and the generator will run according to it's AGS
setting about once every 2 - 3 weeks.  In the winter the generator runs more
often.

I have the AGS running on Volt Start and it appears that the generator will
run after somewhere between 2000 and 3000 Ah have been removed from the
bank. Once started the charge settings are bulk to 60V, absorb for 12
minutes and shut-off. In the winter the chargers can run at almost their
full power and deliver about 360 A during the bulk phase. In the summer the
chargers fold back significantly due to heat in the power house. This
folded-back charge rate is what has dictated the charge regime. Not wanting
to run the generator at reduce capacity for extended lengths of time means
getting the voltage up through the bulk phase as best as possible in the
summer and then bailing on the absorption phase. This has been the
one-size-fits-all setting that has been running all year round for 6 years.

Recently I checked the specific gravity of the batteries at the end of the
normal charge cycle and got 1260 in most cells. Interestingly enough I then
put in an additional 1200 Ah and the sp didn't budge.

My sense is that the bank, on average, has been getting charging up to about
80-85% of full and then discharging to about 50-55% of full.

So there you have it. Perhaps a lamentable state of affairs, but the state
of affairs none the less.

I know there are many many possibilities for this system. The owner would
prefer to manage on-going maintenance costs as opposed to making significant
capital expenditures on major system reconfigurations.

Here is what I am thinking of doing: I want to remove the string of
batteries with the bad cell completely from the system. That gives me an
increase in my bulk charge current for the remaining strings and reduces the
number of parallel strings. I also want 4 times a year to adjust the charge
parameters to match the seasonal condition including tacking on as much
absorption time as feasible.

So what to do with all those extra batteries? Can I cut them all out into
the individual cells, drain the acid and keep them in storage, available to
replace bad cells in the future?

Any advice?
 
Mark Frye
Berkeley Solar Electric Systems
303 Redbud Way
Nevada City,  CA 95959
(530) 401-8024
www.berkeleysolar.com 

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20100621/7579b05b/attachment-0002.html>