Redundant Off-Grid Residential System?

[earthvu]

I have an existing fully off-grid home in rural Colorado: lead-acid batteries, Conext XW+ 5548 inverter, AGS, 3200w solar panels across 2 controllers, and 10kW backup genset. I'd like to replace my propane fridge with an electric one, so looking to expand the system. I have 5100w of solar panels ready to expand this spring, but most of this new capacity will be "wasted" unless I increase the size of the battery bank. Problem is, the existing lead-acid battery bank is 6 years old, so I don't want to mix new lead-acid into it -- and the batteries still have 4-6 years of life, so I'd rather not throw them away.

Is it possible to keep my existing system as-is and add a "redundant" system in parallel? I'm thinking along the lines of a new lithium battery bank and 2nd Conext XW+ 5548 inverter connected with the new solar panels. Ideally I'd like to connect the two inverters on the AC side, but I think phase mis-match is an issue? A friend has recommended using a transfer switch that would allow switching (loads) between the two systems, but I think this would temporarily disrupt power to the house, which probably wouldn't be good for electronics -- not to mention resetting clocks, etc. The other idea I'm kicking around is to use the existing system to supply non-critical circuits and a new system to supply non-critical circuits.

Apologies if this has been addressed elsewhere. It seems like it'd be a common question, but I couldn't find anything relevant in the forums under search terms like "redundant systems", "mixing batteries", "parallel inverters", etc.

 

[OffGridInTheCity]

Ideally I'd like to connect the two inverters on the AC side, but I think phase mis-match is an issue?

You don't want to hook the AC output of independent inverters to shared circuits simultaneously. Has to be 1 or the other for common equipment.

A friend has recommended using a transfer switch that would allow switching (loads) between the two systems, but I think this would temporarily disrupt power to the house, which probably wouldn't be good for electronics -- not to mention resetting clocks, etc.

I use ATSs between grid and off-grid solar (so I have automatic grid-assist) and you're right, some circuits - such as computers, TV - are sensitive. To smooth this out I use APC UPSs - works great.

 

[v_green57]

Since the new fridge is the additional "nut" you need to crack, put up all your new stuff and just run that on its own ckt. Leave everything else as-is since it's working.

Then run out the clock on your FLA battery bank.

When it's time to replace that, re-configure for the "whole house" approach with your new hardware.

 

[earthvu]

Since the new fridge is the additional "nut" you need to crack, put up all your new stuff and just run that on its own ckt. Leave everything else as-is since it's working.

Then run out the clock on your FLA battery bank.

When it's time to replace that, re-configure for the "whole house" approach with your new hardware.

The new fridge is what's going to tip the existing system over, but we're already on the edge. 2 days/3nights of blizzard will deplete the existing batteries below 50% if we didn't run the genset, so it'd be nice to have more capacity anyway. 5 day blizzards are a common thing in a normal year. I like this idea -- "critical" and "non-critical" circuits. Simplicity rules!

 

[v_green57]

The new fridge is what's going to tip the existing system over, but we're already on the edge. 2 days/3nights of blizzard will deplete the existing batteries below 50% if we didn't run the genset, so it'd be nice to have more capacity anyway. 5 day blizzards are a common thing in a normal year. I like this idea -- "critical" and "non-critical" circuits. Simplicity rules!

A-yup.

As you've probably learned from living off-grid, the more "points of failure" you have, the more points of failure you have the potential to encounter. And these always seem to happen during one of the "5 day blizzards" you mentioned.

Running the fridge separately can be as simple as a little 2-breaker box downstream of the new inverter, some 12/2 from inverter to the locale of the fridge, and a duplex outlet there.

Maybe you can "live large" and put a couple other things on this ckt as well. Woo-Hoo!!

By-product of this approach is it gives a "shakedown" to the new hardware while leaving you with backup.

 

[mciholas]

Idea: have two systems, three panels.

Panel 1 is served exclusively by system 1.

Panel 2 is served exclusively by system 2.

Panel 3 has an ATS that chooses system 1 or 2 based on who is operating. Like this:

Amazon.com

If one system is just for backup (say the old FLA system), then ditch its panel entirely and run it through the ATS so it only powers the critical panel should system 1 fail. The old system would not be used until system 1 is depleted.

Another way to do it is have each system run separate panels, but the old system panel has the ATS to switch to the new system when the old system fails. Put the critical things on the old system panel.

Mike C.

 

[SouthernSolar]

You can run lifePo4e and lead acid parallel together . Search this forum.

I have done with SLA. I just used the lifePo4e settings on my controller

There's some YouTube videos on it also.

Only problems the lead acid won't kick in until the voltage drops below 12.7

 

[earthvu]

I use ATSs between grid and off-grid solar (so I have automatic grid-assist) and you're right, some circuits - such as computers, TV - are sensitive. To smooth this out I use APC UPSs - works great.

@OffGridInTheCity I don't mind using UPSs for a few sensitive electronics, but not sure how far I need to go with that. 1) Would a modern refrigerator qualify? (I'd have to assume it was running while switching) 2) On-demand hot water heaters? 3) Does your ATS switch quickly enough to prevent appliance clocks from resetting?

 

[Q-Dog]

If the battery is the only limitation ... why not just supplement the old battery? How about a new solar charger to a new LiFePO4 battery bank, then use a DC to DC charger from that new battery bank to dump power into the old battery?

 

[earthvu]

If the battery is the only limitation ... why not just supplement the old battery? How about a new solar charger to a new LiFePO4 battery bank, then use a DC to DC charger from that new battery bank to dump power into the old battery?

Interesting idea @Q-Dog. It might not be the most efficient, but it's simple. Any recommendations on a DC-DC charger appropriate for a 48v 30kWh battery bank?

 

[Q-Dog]

Interesting idea @Q-Dog. It might not be the most efficient, but it's simple. Any recommendations on a DC-DC charger appropriate for a 48v 30kWh battery bank?

Victron makes 48-48 volt DC chargers, like this one.

Victron Energy ORI484841110 Orion-Tr 48/48-8A Isolated DC-DC converter

 

[earthvu]

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[OffGridInTheCity]

@OffGridInTheCity I don't mind using UPSs for a few sensitive electronics, but not sure how far I need to go with that. 1) Would a modern refrigerator qualify? (I'd have to assume it was running while switching) 2) On-demand hot water heaters? 3) Does your ATS switch quickly enough to prevent appliance clocks from resetting?

Appliance clocks are 50/50 but we dont care about this. Refrigerator, appliances, furnace/thermostat, hot water heater, all good with switchovers. I use UPS for computer+routers, tv/tivos. I have easy access under house and got an APC 3000 and wired plugs to key locations so I just have 1 UPS. Each UPS has a parasitic draw of 30-60w’ish depending on size.