Author Topic: Off Grid Battery Bank Voltage Monitoring and Control w/Load Switching?  (Read 759 times)

mrbrooker

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I have an off-grid system installed on my town home in downtown Houston.   It has a 3600 Watt PV array, 48V 2480 amp-hr battery bank, MidNite 200 Classic MPPT charge controller, and an AIMS 2000 Watt inverter charger.  My intent with this system is to reduce my overall electric bill and more importantly have essential power available in case of loss of grid power from events such as hurricanes (I lost grid power for 3 days following hurricane Ike in September of 2008).

I have three appliances powered with this system: (1) - home refrigerator, (2) - Water heater (converted from 220V to 110V), and (3) - 12,000 BTU window A/C.   All 3 of these are controlled with TP-Link Smart Plugs w/ energy monitoring.  The refrigerator is operated continuously, the water heater is powered so that hot water is available when needed, and the window A/C is operated during the day if there is sufficient battery bank voltage (my home has central A/C but the window A/C reduces the need for it).  All this works fairly well but to ensure the charge of the battery bank doesn't get so low that the inverter switches on to charge the bank from the grid requires me to constantly monitor battery voltage and adjust the loads.

My question is how can I (with instrumentation) monitor the voltage of the battery bank and shut off the power to the window A/C and/or water heater when certain voltages are reached?  This will reduce the need for constant manual adjustments and help to maximize the power I get from my system.

I've done a little research and now know I need to somehow set up my air conditioner as a diversion load.

Any insight?

Regards,

Mark Brooker
Houston, TX
« Last Edit: May 09, 2017, 04:45:31 PM by mrbrooker »

smanners

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I know this an old topic so maybe you have solved this.

I do a similar thing. I use a voltage sensitive relay from intervolt to switch on or off DC to an inverter. The inverter connects to an automatic transfer switch that switches Ac load. So inverter Ac out is connected to master in on ATS and slave is mains Ac. This way when the batteries start getting a bit low I can reduce load on the batteries by having the relay disconnect the dc power to the inverter. It’s a seamless switch as ATS switches load within 16ms. Victron sell a very nice little 16amp ATS.

When battery voltage rises to a set level the relay turns back on the dc load to the inverter and the inverter will switch back after 15 seconds or so. The intervolt relays are solid state mosfet based so there is no mechanical switching and so no spark.

This has worked perfectly for me and protects battery system from discharging too much.

As an alternative you can use good quality battery charger but use the relay on the positive lead going to the battery system. The enerdrive chargers have a special function to supply load only and not charge. So again it will supplement load at a set voltage thus again stopping batteries from discharging to far. In the this case you don’t need any ATS complexity.

Ron Swanson

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Would this not be a case for Waste Not or a similar relay output in the Classic?

smanners

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That’s one way of doing it. You still need something like an intervolt relay(high amperage), I use a 150amp relay. The classics itty bitty relay can connect to the intervolt as a signal relay and it uses that as the primary signal to open or close the intervolt’s relay. Good fun playing around with them.