Suggestions for Victron in a country with unstable grid

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munga asked Jul 7, '23

Suggestions for Victron in a country with unstable grid

Hi Victron

Great systems you have but I have some suggestions. In South Africa we we have good rules for gridtie systems but the grid is unstable and this makes life difficult. In no particular order:

1) AC Coupled PV: It would be nice to differentiate the charging source and the limit of the charging source. When you have AC coupled PV you might want to charge harder from the AC coupled PV than you are allowed to from the grid. As an example the MP2 5kVA has 70A DC charger when paralleled for 10kVA you can charge with 140A DC. If connected to grid you must limit this in South Africa (depending on your municipality) to 3.5kW only. This limits your charging capacity from the AC coupled PV, maybe you can option that?

2) Dealing with SOC estimation errors: When running ESS on a managed BMS LiFePO4 battery you rely on SOC. Most BMSs are pretty bad at SOC estimation and they tend to drift with time. The only way to re-calibrate this is to charge them to their max voltage (~55V for 16s 48V system) and let the battery top balance and "reset to 100% SOC". This is all good an well when you have enough PV and the battery can get to 100% every 2nd day or so. When the battery is not fully charged for a week you can get situations where the battery is out by 25% on SOC in a week. One way is to force a scheduled charge once a week with ESS but the problem is you can only specify SOC in % and not voltage. It would be good to have the scheduled charge where you can say charge to 55v and stop once charge current is < 5A for example.

3) Delayed grid charge: I run the ESS and during night time I use stored power and set the min SOC for reserve. When the grid comes back it would be good to delay the charge on the battery from the grid somehow.

Thanks for all the great work!

MultiPlus Quattro Inverter Charger ESS AC PV Coupling charge current limit

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Answer 1 · 2023-07-07T15:39:57Z

Hi @munga

Not perfect solutions but half way solutions if you're desperate for these features:

1. Do the SA regs specify how the 3.5kw limit must be inplace? If it doesnt need to be in the multiplus and can be in GX device then you could use DVCC max charge current as the limit. And then write a script or use node red which adjusts the DVCC mac charge current limit when grid is disconnected?

2. A way around this might be to invest in a smart shunt (they're not cheap tbf, I used to use BMS communication but prefer smart shunt now over using BMS communications for the reasons you mentioned plus i like to have softer limits set in victron then the extreme ones in BMS. That way BMS limits will only be hit if victron system has a bug, i like that extra layer of safety. Two safety systems over one. And in my opinion especially for lifepo4 reducing charge current at higher voltage through BMS communication does minimal to help the battery life assuming you set reasonable absorption voltages, the 55v you mention i would say is definitely reasonable for 16 cell lifepo4. Current reduces in absorption phase anyway just naturally due to limited voltage). If you have smart shunt then settings absorption voltage to 55v and tail current to 5amp will do what you describe with schedule charging. However saying that I find smart shunt needs calibrating far less often than my BMS (although my BMS is not the best).

3. I'm not sure I understand the setup, so you have min SoC set overnight and you drop below that min SoC because grids down and then when grids back up you want to delay the grid from charging the battery to the min SoC because you're expecting solar soon, is that right?

A potential solution could be to use node red to adjust the ESS min SoC between certain hours dynamically. ie between 8am to 3am min SoC = 40%, but between 3am and 6am (when you're likely below 40% but expecting sun soon so want to delay grid charging if it turns back on) set min SoC to 20%. That will give you 20% worth of time delay if that makes sense. Assuming I've understood what you're after.

Answer 2 · 2023-07-07T17:46:43Z

nickdb answered · Jul 07, 2023 at 05:46 PM

These are workarounds for a hybrid system, one which is backup power focused but isn’t sized properly as an ESS.

You can automate around a lot of it with node red.

Scheduled charging will also deal with some of it, ultimately you can’t make a backup power system perform as a full fledged ESS without getting creative with developing your own automation.

The mechanics under the bonnet is a bit more complex than is often realised.

I’m in the same grid-unstable country. The only solution was to either reduce load or provide enough solar/battery. I eventually went with the latter.

Anything else comes with compromise.

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