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mark-chewins avatar image
mark-chewins asked

help with system config

I am running a 3 x single phase system each with own solar bank (5kw), MPPT controller (250/100), Multiplus 48/5000 and Venus GX unit. The system has a single 48V battery bank and BMV unit.

My plan is to be able to run the house off solar during the day and certain appliances at night off the battery until the battery reaches a certain SOC, then switch over the grid supply. I do have a reasonably stable grid supply so the idea is to reduce electricity costs as well as provide a backup for essential appliances during a power failure.

I would like inverter 1 to run down to 75% before switching over and the other 2 to switch over at 95% - any ideas how I can achieve this? The problem I have is that the built in SOC readings from the inverters is unreliable with a single battery bank as they have no idea what the other inverters are doing.

Ideas would be appreciated.

Thanks, Mark

system design
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ben answered ·

Inverter 1 can have its SOC value overridden by the one BMV you have installed. You can then set up your inverter 1 to switch over as needed.

For inverters 2 and 3, the simplest way is to install a dedicated BMV for each independent inverter. You can wire them all up to share the single shunt you already have installed if you like. Feed each BMV output to your other two Venus GXs.

Or, you could write your own Venus driver to propagate the SOC from your Venus #1 (which was received from your single BMV) over a network to your other Venus devices.

Or, in your special case where the other two inverters need to switch modes at exactly the same point, you might be able to configure your single BMV's alarm relay to close/open at the 95% SOC point, wire that relay to your two inverters, and add an assistant in VEconfigure that does the correct thing using one of the AUX inputs on each device.

There might even be other ways, but these come to mind off hand. If it were me, I'd probably just buy two more BMVs. Moreso if you have reasonably frequent sync events that bring your battery back to 100%, because each BMV will drift over time as they accumulate error.

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mark-chewins avatar image mark-chewins commented ·

Thanks very much Ben - very comprehensive response :).

I am going to go with the extra BMV units.

Regards, Mark

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