[RE-wrenches] AC Coupled Re-visited

[Jeff Yago]

There have been some good advice related to battery bank sizing and a better description of how Sunny Islands work with SunnyBoys, but I am afraid we are getting away from my original post that started this and that was:

Since a battery based inverter has a battery charging section and all kinds of software control over the charging process when connected to the grid, why does the battery charging process "go wild" when the grid is down and the AC power is being backfed from a separate AC coupled inverter.  Also, why do we keep having to field wire and program an added power relay or a separate diversion load relay to prevent over-charging and destroying an AGM battery bank because the battery charging process suddenly has no clue how to charge the battery bank.

Yes, the Sunny Island varies frequency which will cause an AC coupled SunnyBoy to be faked out and drop off line to avoid overcharging the battery bank, but it seems to me this is all just software programming, so why can't any inverter manufacturer simply control battery charging no matter where the AC power is coming from?  Why can't grid tie inverters used in an AC coupled application have the software to recognize a full battery condition and stop charging without having to drop off line on an out of limit grid condition and then cycle through the 5 minute wait? 

I am not an electronics engineer, but I don't see where any hardware changes are required, just some added software to select during initial setup.  Many of you may not deal with battery based systems, but half of our systems are and AC coupled is a great way to avoid long low voltage DC wire run losses.   We select generator type from a setup menu, how about "Press 1 for standard inverter setup, press 2 for AC coupling".

Jeff Yago

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