Expanding Battery Storage in a Rental — What's the Best Approach?

Inverters
82

I’m on board with this answer. Giving you the reasons why it is not normally advised was just the obligatory thing you will always get from the support guy who may need to wash his hands of it later :grimacing:

I do something similar, but again, it is totally DIY and somewhat manual at this point. I think TTT did something similar in the past. I have a second Victron system, a 3kVA 12V system, which is normally a test bench. It allows easily testing things without messing with the main house system. It has about 2kWh of battery storage.

Sometimes, not at the moment since we have the weirdest weather in Cape Town in a long time, but when there is lots of sun, I will instruct that system to feed energy into the grid over night. Since there is always a fan running somewhere, or an AC maybe, and I use slightly more over night than I want to discharge the main house battery with, this makes sense. Tomorrow, I recharge that second system again when there is a lot of solar power.

In this way, the second system is in parallel with the first one, but they know nothing about each other. The main system just sees a lower load over night, because the smaller system is injecting a bit of power.

This can be automated with things like Home Assistant, or Node Red, or even with the GX device itself (scheduled charging combined with a negative grid setpoint).

It works for me because I have slightly more solar than battery storage, and instead of buying more battery for the main system, the battery on the test bench is used. Very similar to what you want.

Edit: Also, something has to be said about the efficiency of doing it this way. I have about a third of my total solar production AC-coupled, ie through PV inverters. So the overall efficiency of recharging the 2kWh pack is about 92% (on the PV inverter) and then another 90% on the inverter/charger. If all my PV was DC-coupled, and the main unit was a Multi or Quattro (it is an RS in my case), the round trip efficiency of this second system would have been high 70’s…

With the Dye/Sunsynk being an HF unit though, and the PV to grid part probably quite efficient, the efficiency of this method should not be too bad.

83

What Plonk said.

I had a small 12v 800va Multiplus on a Frankenstein battery bank connected to a Raspberry PI (the Venus “brain”) because the parts where laying around.

Same as he is doing, I fed the banks charge back to the house at night sommer via a normal plug.
Batts where charged daytime using some of the main systems power into the house.
Had the batts, so use them.

Was going good for weeks till the wife told me “Enough now, cleanup!”.
Sold it all on. :slight_smile:

Buying wisely and carefully, you can build same, until you have decided how to use your DIY bank optimally.

The NEEDS, WANTS or HOBBY thing comes to mind.

84

There we go :smiley: - so that last point, feeding this small unit’s excess back to the house @plonkster and @TheTerribleTriplet - how exactly is that done with the primary system still running the house’s electricity? And yes, it also works for me if the house system and this one doesn’t know about each other, as long as I can use the capacity.

I think in my head it is as simple as having the inverter’s AC output plug wired to a wall socket and the system works itself out.

85

That’s precisely how mine is connected. Now make no mistake, this is not really compliant. At the small power levels I use (less than 2 amps) it’s probably fine, but if you’re feeding in a large amount of energy on the wrong side of the breaker, you’re messing with overcurrent protection. Ideally I would advise doing this on a dedicated circuit…

After that, it is essentially a system that charges during the day, and discharges into the grid at night, with a power level set that matches the battery. If you are using the GX device, you’d set the AC power setpoint to -350W (for example) so that the system constantly feeds 350W into the grid, but then you will also set up a charge schedule to charge it to 100% around noon or something, and hold it there until later (say, 7PM).

The effect is that the system will feed into the grid for about 5 to 6 hours, then stop when it hits the minsoc, and then when the scheduled recharge time arrives later the next day, it recharges (hopefully from solar, which should be plenty then).

Keep in mind the inverter needs about 35W or so just to run itself, so really low levels of feed-in, below the example 350W, will result in low efficiency, less than 90%. So better to discharge a bit higher for a shorter period. Some balance will have to be found.

1 Like
86

What Plonkster said.

The detail you need to pick up on … need a Venus/Cerbo or a Rpi with the Venus OS on with a MK3 cable connected the the inverter.

All new, a Rpi could cost more or less, all parts needed, similar as the price of a new Venus/Cerbo if you shopped really hard.

Or someone may want to sell their Venus/Cerbo.
Or a 2nd hand MK3 cable.

87

Screenshot 2024-12-17 at 12.56.35
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Is this what we have been discussing? I’ll add the Cerbo/monitoring bits in next iteration.

88

Yes for sure. Fortunately I’ve got a RPi to start on before I go the Cerbo route - as long as I can find a MK3. Also noticed GX vs non GX price difference is quite small on MPII. So for starters it might be worth pursuing.

But, can I set up the inverter completely without any GX bits? Like with front panel, etc?

89

Not 100% sure. There are dipswitches, but who does that?

I always used a MK3 and since the Venus/Cerbo, via VRM.

With a Cerbo/Venus you don’t need a MK3 cable.
With Rpi you do.

I know someone out there, on this forum, has a MK3 to sell or loan.

EDIT: That Multiplus 800va, was so old, for it to be programmed, I had to go the old way with a MK3 cable and the Windows software.

So if one dabbles with older Victron inverters, a MK3 in a drawer can be a lifesaver once i a while.

90

Alright I’ve put out a wanted listing for the MK3.

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91

Ah so as I understand this - the MP will then send power back to the house wiring over the AC input plug? Not the AC output plug as I drew in the picture.

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92

Ideally you want it on a double pole breaker in the DB, just like the Deye.

But at a push, a plug will have the same result. T&C’s apply, it being called a “suicide plug”.

Difference being, Victron has more software controls, and safety checks (being grid tied), than the DIY’er who got an idea around a braai from his wanna-be electrician buddy who had, yes had, a friend who had a cousin who told him the cheap way to have a grid-tied inverter using off the shelf el-cheepo “here hold my beer” contraption connected to a plug outlet.

93

T&Cs noted on “suicide plug”

The Victron point being that because it has better controls than a cheapo random inverter doing that same thing. In both instances on the “suicide plug”.?

1 Like
94

As the longer term exploratory route: This would then be in parallel with the Deye? They both feed the same essentials circuit, except that the Victron feeds in both directions to/fro?. So during the day with excess solar, the Victron just looks like a load to the Deye. And at night, it is a negative load, whereby it then reduces what the Deye draws from battery/grid. Still then leaves all the other circuit breakers (lights, plugs, etc.) able to do their job?

Am I making any sense?

95

Yes.

96

What were the chances :rofl:

97

Alright a different facet of this, connecting battery to inverter

Maths being assuming 5kW coming off a 51.2V battery - That’s just below 100A. So cabling to battery bank should be 100A rated? Would then work in future too when battery added.

Then, inside the battery pack right now the 50Ah one - at 0.5C that’s 25A - so then 25A/50A rated cable for that? Then the appropriate lugs and heat shrinks and all that on the ends. Correct?

98

Me, being tired of running on bare bone specs, have opted to default all 5kva inverters to 50mm2 wire, and if longer than 5m, from batt to inverter back to batt length, 75mm2 wires.

Recently got silicone 75mm2 cables with lugs.
More bendable.

Fuses, for cable protection:
I have 2 x 125a fuses. One on Neg and one on Pos.
Fuses will go loooong before the cable.

This is a very neat safe way to disconnect the battery under pressure, drama, quick and safely:

Do it once and never worry think about it again when you upgrade the battery.

Others may disagree or have other ideas.

Use what was there before.
Lengths of cables inside are shorter than short.

99

Another thought:
From battery pole to busbars.
From busbars to fuse holder.
From fuse holder to inverter.

MPPT, in the future, also connects to busbars.

100

Eventually, agreed.

101

Good lord i forgot about the fuses, but i knew about it last month :rofl: thanks for the reminder!

Then i guess when there are multiple batteries in parallel, each battery gets one of these?