Offgrid System Components & Sizing Sanity Check

[riewest14]

Hey Everyone!

I'm helping my parents build out a battery backup system for their off grid cabin and wanted to see if I could get a sanity check on components and a few questions answered if anyone has the time. The first round will just be a battery/inverter charger and then we will add solar later (I know that's probably blasphemy here but gotta start somewhere ). But this will get them to the point where they don't need the generator running all the time to keep their fridge cold.

Background/Goal
Their cabin has been a slow project that they started about a decade ago so all the appliances are relatively new and pretty much any that can run on propane. The biggest critical loads that I can think of are the refrigerator, well pump, washer/dryer(propane), and keeping the gas fireplace online and running (Probably not that much its just a fan and some circuit boards). Other loads would be microwave/coffee maker/power tools for short periods or relatively small loads like lights/phone chargers/etc.... For reference they have been using a gas generator (8 or 9 kw I think) and it has been sufficient other than when they have a lot of loads like power tools and appliances all go at the same time but that will be happening less as most of the bigger building tasks are done by this point.

The goal is to have a reliable system that will work for lots of years and be simple for them to use when they arrive. I don't want them to get fed up with it and regret the purchase because turning it on/off becomes a hassle. Also has to be simple enough that other people who visit the cabin when my parents aren't there can either follow written instructions on how to turn it on can.

Main Components
Generator: 10kw Generac Propane Generator (My dad has already purchased this)
Battery: EG4 Indoor Wall Mount (280ah)
Inverter/Charger:
Option 1: 2 x Multiplus II 48v 3kVA
Option 2: 2 x Multiplus II 48v 5kVA (This one and then Option 1 are the primary options)
Option 3: EG4 6000xp

I would also be adding a Cerbo GX and getting that setup to autostart the generator if that matters.

Questions/Concerns

1. Would one EG4 Indoor Battery be enough to sustain the 2 5kVA Option 2 above?
   • I initially though yes but then my brain has been nagging at me that it wouldn't be able to sustain the surge amps that the 2 inverters would be capable of with an appropriately sized battery and I'm wondering if there is any safety issues if that is the case or if I just would have less surge capacity but be fine on continuous output power.
2. Do I need a DC Breaker/Fuse in between the single battery and each Inverter or is the 2x125A dc breaker on the side of the battery sufficient?
3. Anything else I'm missing?

 

[Rocketman]

I would highly recommend making a written plan. I use google slides and post pictures of the components and red/blk lines for the wires, fuses, etc. Then that becomes my build plan, and is changed to “as built” and the plan is saved for future troubleshooting.

That battery can do 200a discharge (correct?).

(I only know Victron stuff - so that’s my comments).

Is your generator a 240/120v generator or a 120v generator? Do you have any 240v needs?

That battery can fully run a Multiplus 48/5000. (Get the manual on Victron’s website and look in the install section). If you need 240v and want the 48/5000’s you will probably need a second battery to fully power the inverters.
If a pair of 48/3000’s will be enough, then you could probably get by with one battery.

A lot depends on how many big things you run at once. You can only run one big item at a time on a Multiplus 3000 - so if you have two in split phase - put things that run at the same time on different phases.

In a cabin or a house I would have two 48/5000’s.

The one item I think you may be missing is a Smart Shunt or BMV712 - especially if you are getting the Cerbo. (And a Victron solar charge controller- but that’s just down the road - and I agree inverter and battery first). Because all negative power needs to flow through the shunt - you will need a bus bar for the negative wire.

Good Luck - answer the questions above about 240v - and ask a few more - that will focus the discussion…

 

[riewest14]

@Rocketman Thanks for the response!

I use google slides and post pictures of the components and red/blk lines for the wires, fuses, etc. Then that becomes my build plan, and is changed to “as built” and the plan is saved for future troubleshooting.

I have something similar that I used draw.io to make. I'll post below the quotes to keep the info at the top of my post. That is a good idea about saving it as built for future documentation I will do that! (I'll probably even print it for at the cabin itself since there's no good cell signal there)

That battery can do 200a discharge (correct?).

That is correct

Is your generator a 240/120v generator or a 120v generator? Do you have any 240v needs?

Yes & Yes. My dad specifically asked for 240v and it is currently wired up for 240v in the Load Center from the current generator. So the plan is to just move the 240v coming from from the generator to the inverters and then hook up the inverters where the generator used to be hooked up.

The one item I think you may be missing is a Smart Shunt or BMV712

I initially had one of these in my plan but I took it out since the battery supports Victron Communication and can be connected via Ethernet Cable (I think that's the COM Port/CAN Bus) Is there a huge benefit to the smart shunt over closed loop communication?

and ask a few more - that will focus the discussion

I guess I'm still wondering about the questions I posted towards the bottom of my main post.

1. Will under sizing the battery on the two 48/5000s cause issues/safety concerns or will they just not produce their full surge capacity?
   • I won't necessarily do this I just want to know if its an option
2. Do I need a Fuse/DC Breaker in between the battery and each multiplus or is the breaker that is included in the battery sufficient in that regard? (The battery has 2x125A DC breakers in parallel which disconnects the built in bus bar)

My Build Plan Drawing (Thin Lines == AC, Thick Lines == DC) (Still strongly considering the 5000VA despite what the drawing says)

 

[Rocketman]

That battery said it can do 200a (it also said it preferred to run at 160a discharge).

200a * 48v = 9600watt available (at lower states of charge)
9600w /2 =4,800 W per inverter.

160a * 48v = 7680 watts available
7680w /2 =3,840 W per inverter.

Ok - here is my opinion. (I would also ask these questions to your Victron dealer (and your battery dealer)- just to make sure there is not something else to factor in). I think one battery can handle both 48/5000 inverters (barely). Obviously you will lose the startup surge ability if the inverters are both running hard and you can’t run them very long fully loaded. 280ah / 200a = 1.4 hours of fully loaded runtime. But I would not expect that to happen very often (if at all). If you use a lot of power - you will be much happier with 2 batteries- longer run-time between generation running.

In my motorhome occasionally I overload my Multiplus 12/3000. If we have one large item on (Microwave) and we turn on another large item (hairdryer). What happens is the DC voltage goes low - the cerbo beeps and the Multiplus shuts down - we lose all 120v. After 1 minute it automatically comes back on. Now my battery does not shutdown- I don’t know in your case if the BMS would shutdown first or the Multiplus. In commissioning you will want to test that - so you can write up the steps to get the battery running again.

Fuses: My opinion is that fuses should always be used. The owners manual calls for a 200a fuse for the inverter (and 2/0 wire - if under 5 meters) for the 48/5000. I would use Blue Sea MBRF holder and BUSS brand fuses (just not the cheap Chinese ones - spend the few extra dollars here - it’s not many dollars). Also you could use a Class T fuses.

Inverter capacitors. One issues you may have - because everything will be shutdown when cabin is not in use- is the inverter capacitors will be empty. If you just turn on the battery there will be a big spark. I believe the capacitors will charge if the 120v is applied first. So your startup sequence may be start the generator, turn on the battery, turn on the inverters. Otherwise, you will need to figure out some kind of automatic pre-charge unit.

Shunt: Unfortunately I don’t have my battery connected to my Cerbo, so I don’t know any of the issues there. But I understand it will work ok. I believe (from what I have read) that the shunts are more accurate than the BMS state of charges are - but I have also heard lately that they are very close now.

Plans: I would plan now for everything you plan to add - put the solar charge controller(s) in now and a panel. That way you don’t overlook something simple - like the bus bar being too small. Also put the Cerbo in (with a different color of wire for the data wires) - no use forgetting something.

Generator starting… can your generator start/ stop with 2-wire lead? Or do you need an auto start module? Atkinson electronics makes modules that do that- also check the generator manufacturer.

Good Luck