[RE-wrenches] AC Backfeed Battery Over-Charging

[Darryl Thayer]

Jeff I agree with Kent, 
The OB is in the 'no grid mode' and as such is regulating the AC to be a perfect grid signal.  The thing to do is to use a second controlled load.  On smaller DGI inverters I use a Tristar 60 set for diversion load or use the DAvid Katz method and let the Outback Aux relay or the FNDC relay open a power relay that breaks the connection between the DGI inverter and the Bimodal inverter. 

DT  

--- On Mon, 8/3/09, Kent Osterberg <kent at coveoregon.com> wrote:

> From: Kent Osterberg <kent at coveoregon.com>
> Subject: Re: [RE-wrenches] AC Backfeed Battery Over-Charging
> To: jryago at netscape.com, "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
> Date: Monday, August 3, 2009, 9:23 AM
> 
> 
> 
>   
>   
> 
>  
> Jeff,
> 
> 
> 
> The Outback inverters have no battery charge regulation
> while operating
> in ac coupled mode because the inverter is regulating the
> ac output
> voltage (preventing it from going too high) by charging the
> batteries. 
> The SMA Sunny Island can regulate the battery voltage if
> the other
> inverters are SMA products (Sunny Boy, Windy Boy ...) by
> shifting the
> ac output frequency to tell the inverters to shift off the
> max power
> point.
> 
> 
> 
> Usually the input voltage to a grid-tie inverter is higher
> than 240
> volts so, even with ac coupling, it is more efficient for
> the long
> feeder to be on the dc side.  The dc side also has a
> wider operating
> window too.
> 
> 
> 
> Kent Osterberg
> 
> Blue Mountain Solar
> 
> 
> 
> 
> 
> Jeff Yago wrote:
> 
>   
>   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
>   
> 
>  
> 
>   
> 
>  
> 
> 
> -----Inline Attachment Follows-----
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