[RE-wrenches] AC coupling

Wed Nov 5 09:49:23 PST 2008

The method Dave showed is the simplest and least expensive solution -
other approaches are possible as well, such as diversion regulation with
a dump load via a solid-state relay for example.  

In most utility back-up applications, this simple "on-off" approach is
acceptable as the loads will be consuming most or all of the output of
the grid-tie inverter and some of the batteries power during a power
outage.  The control of the relay is simply to prevent an over-voltage
condition for the batteries - not precise recharging regulation.  This
is acceptable on an AC coupled back-up system since the battery will be
properly recharged by the OutBack's built-in battery charger once the
grid has returned.  

For off-grid AC coupled applications - I would advise using a more
sophisticated approach to controlling the battery recharging process by
using a dump control system.  This also puts the excess power produced
by the system to a good use - such as space or water heating.  

If you would like more information or assistance with these types of
applications,  contact myself or OutBack's application engineers &
technical support people.  

Christopher

Christopher Freitas

Director of Research and Development 

OutBack Power Systems, Inc.

cfreitas at outbackpower.com <mailto:cfreitas at outbackpower.com> 

Tel 360 435 6030

Cell 360 202 4239

19009 62nd Ave NE 

Arlington WA 98223 USA

www.outbackpower.com <http://www.outbackpower.com/> 

________________________________

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of mick
abraham
Sent: Wednesday, November 05, 2008 9:39 AM
To: dkatz at aeesolar.com; RE-wrenches
Subject: Re: [RE-wrenches] AC coupling

Dave Katz sent a drawing which breaks the AC connection between grid-tie
inverter and battery based inverters when the battery voltage is high.
I've seen similar sketches from others. 

Thanks as always, Dave...but it seems that in this arrangement the
amperage to the battery is either 100% on or 100% off, with no finely
tapering pulse width modulation as is commonly preferred for a good
finish charge. On/off hysteresis would be relatively slow just as in the
days of Trace C-30 type charge controllers. 

Is there a "shift to float voltage" with this simple setup? What about
temperature compensation? Those are not accusations, just questions
because I'm unsure whether the Outback auxiliary control output manages
using those factors. 

If the battery lifetime suffers because of rough management at the end
of charge, that's a hidden cost which reduces the appeal of using a
simple AC relay. "Pulse width modulated" diversion into a dump load may
be better for the battery (especially with float mode and temperature
compensation)...but that requires a controller, dump resistor, & circuit
for relatively high amperage DC at the battery voltage, so there goes
part of the cost benefit of AC coupling. 

One must weigh the cost of fancy charge control methods against the cost
of possible battery degradation using the simple method in Dave's
drawing. The battery degradation cost (if any) is difficult to predict,
and may vary from one installation to another. If the AC loads during
the daytime are big relative to the array size, the battery may only be
receiving a trickle charge anyway.   

Mick Abraham, Proprietor 
www.abrahamsolar.com <http://www.abrahamsolar.com/>  
Voice: 970-731-4675 

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