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larry asked

Keeping batteries in a specified range without ignoring AC IN

I mainly use my system for grid backup, but I have some solar and want to keep the batteries near full, but still above 80% or so, so that the MPPT can charge them but still have most of the battery for backup. I need to dynamically balance power from grid and battery to keep the battery in that range, but never ignore AC IN unless it can be quickly re-enabled to handle higher loads as they occur. I am open to a combination of settings, assistants, etc, but I don't want to use ESS because I don't ever want to feed power to the grid.

Multiplus-II
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charlie-d answered ·

Hi Larry

The easiest would be to use ESS and set it to not feed in excess to the grid, then just set the state of charge in ESS to 80% or whatever you want. You will get small overruns to the grid but it is a small percentage of usage. I have a lot of systems like this that are not allowed to feed to the grid (set by the service provider) They don't seem to have a problem with the minuscule amounts that go back.

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larry avatar image larry commented ·

Thanks. I have played with ESS, and I was able to get it to work just fine as you outlined, but I'm not comfortable with feeding back any energy to the grid. After some testing, I also determined that the Multiplus II can feed the grid even without ESS if conditions are right. The hardware design is such that it is capable of back-feeding the grid any time, and only the software prevents that. How do the "off-grid" inverters prevent this? Is it the basic design, or are they also capable of feeding the grid, but just not certified to do so? Is there a third-party device that can prevent feeding the grid? I read that the rules for anti-islanding devices allow up the 2 seconds of back-feeding, so is this just accepted? I am concerned about insurance if something goes wrong, or alerting the utility.

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