Low Voltage Disconnect and Auto Switch to Grid

EarthImage · May 2016

Howdy, I've been mucking about with 2 x 250 watt panels and 24 volt battery setup to run my office computer and printers. Works great while I have enough juice in my batteries! What I'd like to do is have a device that will monitor my battery voltage so that when it drops to say 24.2 volts that my inverter is shut down and my system automatically switches to grid power. I would then run off of grid power until my solar panels charge my batteries to a specified voltage (25.3 volts?) and then automatically my system would switch back to using the inverter. Is there an affordable (ha!) device that can do this? Am I thinking about this correctly? Yes, I am new to the solar game.

Thanks!
EarthImage

Raj174 · May 2016

Hello Earthimage and welcome to the forum,

Higher end inverters (more expensive) have this function built in, however an automatic transfer switch with an appropriate logic circuit triggering a low and high voltage relay at the inverter input might work. Google automatic transfer switches,
Google this to control the inverter on and off at set voltage.

JLD404AH Intelligent AH meter

The control relay will have to be big enough to handle the max amperage you expect the inverter to draw. This equipment is not cheap, maybe 500 dollars.

Rick

EarthImage · May 2016

Hi Rick,

Thanks for the tips. Ya, the more I research this topic the more I'm realizing that I am going to have to make something. Strange, it seems that a device like this would be mandatory for small non tie-in installations. The conspiracist in me believes this is a concerted effort by the solar industry to push folks to expensive tie-in solutions instead of smaller homeowner independent solutions!

EarthImage

westbranch · May 2016

Look at the Outback inverters, those with G in the code name are for grid tie but don't have to be... but you are up > $1000...
hth

Raj174 · May 2016

I think the problem is that the kind of features that you are looking for are not economically practical for such small systems. I was originally going to put a couple of appliances (fridge and freezer) on solar power as a backup for grid failure, but ran into the same problem you did. Wanted automatic control.
Decided to spend about 15K to put the whole house on solar (except range and dryer) with grid as source or backup. I used an Outback Radian GS4048 hybrid inverter. This is one of the models that Westbranch spoke of above. Our house is not large, about 850 sq/ft not counting the garage.

EarthImage · May 2016

I've ordered a simple 12v LVD from England (http://www.reuk.co.uk/buy-REUK-PROGRAMMABLE-LOW-VOLTAGE-DISCONNECT-WITH-DISPLAY.htm) and combined with 120v relay (with 12v switching) might do the trick. I'm thinking of using a simple UPS to cover the switching from battery to mains. This solution is sub $100, whereas the cheapest hybrid solution is still WAY WAY WAY more expensive. Cool if it works.

littleharbor2 · May 2016

Can you tell us what charge controller and what output specs your panels. When you say 2 x 250 watt panels and 24 volt battery this throws up a red flag. Most 250 watt solar panels are not 24 volt nominal and will not properly charge a 24 volt battery bank without , at minimum, 2 panels, series wired and running this output through a true MPPT charge controller. Also your charging voltage can be up to 28.8 volts and resting voltage of a full bank more like 26.4 to 26.8 volts.

Low Voltage Disconnect and Auto Switch to Grid

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