[RE-wrenches] low voltage disconnect

Travis Creswell tcreswell at ozarkenergyservices.com
Mon Dec 21 07:12:03 PST 2009


You didn’t mention if you had a Trimetric but I’ve experienced numerous
instances of them sapping a bank.

 

In one case, as part of winterizing his cabin a customer turned everything
off, including the solar but did not remove the fuse for the Trimetric.  He
called in the spring to let me know the inverter was dead.  On a side note
this same customer’s well pump shorted its internal windings and took out a
set of Outback control boards before we figured out what was going on so I
assumed he was correct.  I arrived to find the 24v nominal system at 8v.
Even though we had a generator we couldn’t even get the inverters to turn on
so we had no charger and had to bring the batteries back to the shop to
recharge them.  The MX60 wasn’t even able to turn on at that low of a
voltage.  Once the fully charged batteries were returned everything was fine
and customer was instructed to leave solar on at all times.

 

At a forest service campground a charge controller failed and even the
though the LVD turned off the loads the Trimetric was still on.  I arrived
to a battery voltage that was about ¼ of the system voltage.

 

Milliamps x 24 hrs/day will get deplete even a large battery bank especially
if the bank was severely discharged to begin with.  I could go on but the
rest of the examples are similar.  In all of the instances the batteries
were healthy to start.  Of course self discharge factors in but none of
these banks would have self discharged that severely in the time period.

 

Maybe that helped.  Best, Travis Creswell

 

Ozark Energy Services

 

 

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Solar Plexus
Sent: Sunday, December 20, 2009 5:23 PM
To: re-wrenches at lists.re-wrenches.org
Subject: [RE-wrenches] low voltage disconnect

 

Wrenches.

 

            I have a customer with a stand-alone system with an XW inverter,
XW controller,  pole mounted array of four 235-watt panels, and 4 of the 8D
gel batteries.   He uses the system mostly on weekends.   

            Winter came, the solar array  was still set at about 30°,  a
snowstorm covered  the panels while no one was there, and 4 days later the
battery voltage was 6.

The week-end use had been substantial, so I assume the batteries were low
prior to the snowstorm.  With no solar coming in, and loads still on, the
battery voltage presumably went to Low Battery Cut Out  voltage of 44 before
the inverter disconnected  the AC loads.  With the AC loads disconnected,
and no DC loads in the system, what caused the battery voltage to go to 6? 

             I assume the XW controller used 2.5 watts continuous or 60
watt-hrs per day as battery voltage went from  44 volts to 10 volts when the
controller would have shut off completely.  What happens with the inverter
when the battery goes below 44 volts?   Does the inverter shut off and there
is no more draw on the batteries, or does the inverter continue to draw the
no-load draw of 28 watts, or does the inverter draw the search-load draw of
8 watts?  At what voltage does the inverter stop drawing power? 

            At 44 volts, the battery is presumed to be pretty dead. How many
amp-hours are available in a 100 amp-hour 48v battery  (4 12-v batteries in
series) while drawing the voltage from  44 volts and 6 volts?  Is there
anything there or will the voltage just drop from 44 to 6 with very little
power draw?

            The Low Voltage Disconnect is set for 44 volts,  but if the
inverter itself does not shut off, then the practical low voltage becomes
the voltage the inverter stops working at.  To protect the batteries, is
there any advantage to limiting the low voltage to 44 volts as opposed to 6
volts.  If so, is there some way to have the inverter shut itself off at
low voltage?

 

We have had this same thing happen with an Outback inverter, again taking
the batteries to near zero.  Is there any way that either the Outback or
Sunny Island would operate differently than described above?

 

Looking forward to  any comments on these issues. 

 

Thank you.

 

Lee  Tavenner

Solar Plexus

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