Digest for RE-wrenches at topica.com, issue 1307 [RE-wrenches]

Steve Willey, Backwoods Solar steve at backwoodssolar.com
Mon Dec 8 08:56:14 PST 2003


Geoff:

I have seen a problem like that before.  Essentially the Solar Converters EQ
is limited to its rated transfer amps, and the fuse should not blow.  But
there is another factor. Used in a system where the inverter draws on the
lower tapped voltage, these units often get quite warm and fuses can blow.
We decided this is caused by the very pulsed manner in which the inverter
draws current, causing measurable pulsations in battery voltage of the lower
half ONLY.  The equalizer unit tries to transfer power to follow the needs
of the lower battery, resulting in equalization current turning on and off
at 60 Hz rate.

In a more common application, with a 24 volt inverter and 12 volt DC loads
being tapped and equalized this stress on the equalizer does not occur.  The
24 volt inverter still pulses the battery voltage, but the half voltage tap
is pulsed half as much, being part of the total inverter supply.  So the
ratio of the voltage tap is not changing in relation to overall battery
voltage and the equalizer does not respond to those pulsations at all.  With
an inverter pulsing half the battery only, the voltage ratio of battery bank
halves does change and drives the equalizer to very hard work.  I advise
people not to use them in that manner.
Steve Willey,
Backwoods Solar Electric Systems
steve at backwoodssolar.com

Date: Fri, 5 Dec 2003 10:35:28 -0500
From: "Geoff Greenfield" <Geoff at frognet.net>
Subject: 24 volt wind into 12 volt bank???

OK - here is a good problem to sink your teeth into, and it involves no
trigonometry!

A year ago I was asked to install a wind genny into a poorly designed and
poorly functioning system. (Graham has also helped this customer with
advice). Off grid, one 240 load (well pump) someone sold her a stacked power
panel and set up a 12 volt system!  undersized solar, big line losses, no
design work, no breakers, no metering, no load analysis, 16 T-105 batts...
I should have advised her to start over...

She has a huge diesel genny on auto that starts about 6 times a day
(typically when well pump surge brings down voltage) and slams 200 amps into
that small bank!  (oh - terrible corrosion at bank interconnects and it
wasn't cabled from the corners...).  So... I have corrected all these
problems (still need to turn down the charge amperage though, the voltage
climbs quickly and the thing runs for 1/2 hour max).

So... the wind...  In hindsight, perhaps I should have gone for a 12V unit,
but I like the Bergey XL-1  (run one myself, installed plenty) and the
distance to the tower is about 215'.  So - with discussions with Bergey
folks and Solar Converters folks (yes, I speak Canadian) I split the battery
bank into a high low 12/24 bank with a solar converters EQ 12/24-50 set up
as an equalizer.

In other words:  the bank is 24 volt, the Bergey feeds the bank, the
inverters and solar are hooked up to the bottom 1/2 of the bank (12 volt).
The "battery equalizer" is supposed to balance the two halves of the bank.

The problem:  repeated blown 25 amp fuse on the Equalizer (it has 25 amp to
the high positive and 50 amp fuse to the 12 v positive).  The Bergey will
put out up to 60 amps into my 24 volt bank (at 29 volts!), so this may be
the culprit, but the batteries should be absorbing the energy and the EQ (as
I understand it) will move power over a long period of time to "catch up"
and keep the banks even.  The other culprit could be the high 12 volt
charging regime, which I think is too high anyway.  (C20 of 1760 ah IS 88
amps).

Solutions....?  Parallel a second equalizer in the same set up?  Step down
inverter charging amps (still have surge loads pulling huge current
though)... OR  rewiring the bank to simple 12 volt and using the EQ 12/24-50
as a "DC auto transformer" stepping the 24 V wind input down to 12. This
would also help keep the trimetric more useful, as the split bank approach
does not allow the Tri Met to log wind amps.  Amps is the reason we didn't
do it this way the first time, but I am considering paralleling a second
unit and trying this.

Sorry for the long winded post... I share this as it is a good question, I
need help/input, and maybe this will help future retrofitters avoid these
pitfalls.

Sincerely,

Geoff Greenfield
NABCEP Certified Solar PV Installer TM
GLREA Certified Photovoltaic Systems Integrator/Installer License Number
0211-01

THIRD SUN SOLAR AND WIND POWER
340 West State Street, Unit 25
Athens, OH 45701

Phone (740) 597-3111
Fax   (740) 597-1548

www.third-sun.com

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