[RE-wrenches] AC coupling

[mick abraham]

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 
Voice: 970-731-4675
            

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