[RE-wrenches] AC Backfeed Battery Over-Charging

[Tump]

Hey Jeff this was something that Greg @ OB was working on summer of 06.....
Should have it tump, almost got it little problem .... Ahhh sorry we are no
longer working on that! I was in the middle of a large AC Coupled system &
had been waiting for 4 months for this "trick" soft ware to come on line.
Perhaps CF might be able to update us on this software. Kinda of a no
brainier perhaps, John B & the boys @ SMA patented this "trick" software.
 
 
     <mailto:Tump at swnl.net> Tump at swnl.net         <http://www.swnl.net/>
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-----Original Message-----
From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jeff Yago
Sent: Monday, August 03, 2009 9:22 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] AC Backfeed Battery Over-Charging


I have a question that may be one of those that everyone wants to know the
answer but feel they are the only one that doesn't know the answer, or
perhaps I am the only one that doesn't know!.  
 
In an AC-coupled system, i.e. the 240 VAC output solar inverter mounted at a
distant array (240 VAC out to reduce long line loss) is wired to the 120/240
VAC output emergency panel supplied from a battery based inverter.  This
allows the solar inverter to stay up when the grid goes down since it thinks
the grid is still up on the load side of the battery based inverter.  The
battery based inverter will use any excess backfed power from the solar
inverter that is in excess of the other supplied loads to re-charge the
battery.  However, when this happens the battery charging process can boil
the batteries when there are no other loads running.  
 
I have been told this is because the battery based inverter does not
regulate its charging of the battery when using backfed power from its
output panel, but I still don't understand why.  If the battery charger in
the inverter can provide regulated 3-stage battery charging when the power
is supplied from the grid or a generator, why does it suddenly become a full
output un-regulated battery charger when power is supplied from backfeed and
will keep charging regardless of reaching a high battery voltage?    
 
I would have assumed the battery charger section would charge the same way
regardless of where the power is being supplied from, or perhaps just
shutdown on some kind of backfeed safety.   As far as I know, this
over-charging problem in an AC coupled dual inverter system is true for all
battery-based inverters.   Any inverter techs out there that can explain
why?   Perhaps this would be a nice feature in future high-end battery-based
inverters so we no longer need the custom power relay field wiring to
provide this high voltage cut-out?.
 
Thanks,
 
Jeff Yago

 

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