Charging suppressed well before absorption voltage with AC-coupled system

[adamreynolds]

Overview

I have recently upgraded a 5-year old off-grid setup to increase the solar production, as we’ve started carrying a much larger load than it was initially designed for.

We went the route of getting new panels and a PV inverter and turned our setup into an AC-coupled system.

The problems?

All charging - from both the new PV inverter and the MPPTs - seems to be at least partially suppressed (via the frequency shift) far earlier than I’d expect, with the various chargers going into absorption mode quite early. For example, with the battery SoC at about 40% and the battery voltage still under 50 V (with an absorption voltage of 52 V).

Probably as a result, we often max out at about 80-90% SoC (with 88% being a very common), even on very sunny days with low loads. We haven’t gotten to 100% SoC once on the new setup. If I delete the ESS assistant and just manually manage things, I can see the MPPTs only move into absorption mode at 52 V, the PV inverter is able to produce what I’d expect for bulk charging, and it’s possible to get to 100% SoC.

Even in the late afternoon, the output frequency generally stays shifted (e.g. +3.1 Hz), and the system continues to idle along when it could be charging.

Setup

Location: South Africa, which has 230 V AC at 50 Hz.

Existing setup from 2020

• 2 strings of 2 x 430 Wp Canadian Solar panels on two different roof faces (1720 Wp total)
• 2 x MPPT 150 V | 35 A Victron SmartSolar charge controllers
• 3 kVA, 48 V Victron Multiplus II inverter
• 2 x 3.55 kWh, 48 V Pylontech US3000 LiFePO₄ batteries in parallel (so 7.1 kWh or 148 Ah nominal capacity)
• Cerbo GX

New components

• 5 x 570 Wp LONGi Hi-MO X6 solar panels (2850 Wp total)
• 4.6 kW Solis S6-Grip Single Phase PV Inverter
• Carlo Gavazzi ET112 energy meter

PV array summary

• 1720 Wp on the two MPPTs
• 2850 Wp on the new PV inverter
• ∴ 4570 Wp nominal total solar power

Firmware

• All Victron components are up to date.
  • Cerbo GX
  • Multiplus II
  • Both MPPTs
• I haven’t updated the firmware on the Pylontechs since I got them.
• The Solis PV inverter is as it came out the box.

Connections

The following diagrams describe all the connections. In South Africa, red is live, black is neutral, and I’ve shown earth in green.

The connections show that there is a grid connection, and a CT cable for the Solis inverter to prevent feeding into the grid. These can be ignored because, at the moment, we have no grid connection at all, and we aren’t likely to have one restored in the foreseeable future.

Settings

I didn’t want to go into too much detail here, but I honestly don’t know what’s relevant. Please let me know if I’ve missed a crucial piece of information!

Victron Multiplus

As a starting point, I followed Victron’s recommended settings for Pylontech batteries, set using VE Configure 3.

ESS Settings

• Sustain voltage: 48 V
• Dynamic low-cut of voltages: all set to 46 V
• Inverting allowed again after 1.2 V above low-voltage cut-off
• After high-voltage cut-off, restart PV inverters when the DC voltage is lower than 50 V
• Frequency shifts:
  • Start reducing output at 50.2 Hz
  • Reduce output power to minimum at 52.7 Hz
  • Disconnect if frequency is higher than 53 Hz

Inverter settings:

• DC input low shutdown 44 V
• DC input low restart 48 V

Charger settings:

• Float voltage: 51 V
• Absorption voltage: 52 V

Via the Cerbo

• DVCC:
  • DVCC = Forced on
  • Limit charge current = ON
  • Maximum charge current = 50 A
  • Limit managed battery charge voltage = OFF
  • SVS Shared Voltage Sense = Forced off
  • STS Shared Temperature Sense = Forced off
  • SCS Shared Current Sense = ON
  • SCS status = Disabled (External control)
  • Controlling BMS = Automatic selection (Auto selected: Pylontech battery)
• Energy meter:
  • Role = PV inverter
  • Position = AC output
  • Phase type = Single phase
• ESS:
  • Mode = Keep batteries charged
  • Grid metering = External meter
  • Inverter AC output in use = ON
  • Self-consumption from battery = All system loads

Solis PV Inverter

I followed the following instructions from my supplier:

• Grid code set to Gen 50
• Mode 4
• Frequency derating settings set to match 50.2 Hz and 52.7 Hz in the ESS

What I’ve tried

• Using the PV Inverter Support assistant gives the same behaviour as the ESS assistant.
• I’ve tried increasing the float voltage from 51 V to 52 V.

Questions

Over and above, “What am I doing wrong?” / “Where is the problem?”:

1. In the absence of the grid, should I be using the PV Inverter Support assistant rather than ESS? Or something else?
2. Also in the absence of the grid, could I connect the PV inverter to the AC input of the Multiplus?
3. Is there a problem, or is this just how it is with ESS setups? (ChatGPT has tried to convince me that this is the case, but I’m sceptical.)
4. AC-coupling and ESS are very new to me. If I appear to have any misunderstandings, please correct me and any relevant reading would be useful (I’ve read all the Victron manuals etc.).
5. Could it be a battery cell balance issue? The’ve seemed okay prior to the new setup, but we’ve been struggling a lot with the increased load, which we’ve mostly put down to just not having a large enough PV array. But all I really know about their state of health is the “94%” reported via the Cerbo.

[MikeD (Mike)]

Query: Is the PV inverter assistant the last assistant to be loaded? I’ve been told that this Must be the last one loaded to work properly.
Your questions:

1. In the absence of the grid, ESS should default to an off grid state. The PV inverter assistant should be loaded in either case, as the last assistant.
2. If the PV inverter is connected to ACin, then this will not function in the absence of the grid. If connected to AC out on the inverter, then the 1:1 rule must be observed, and the inverter will function when the grid is down.
3. I’d also be skeptical of anything from chatGPT. Any problem caused by cell imbalance may cause early reduction in the rate of charge. This will show up in VRM with the Max/Min cell voltages, and the DVCC Limit widgets.
4. -Only thing to wory about here is the 1:1 rule, and to make your decision on whether to couple on ACin or out, this is sometimes mandated by the grid regulator.
5. Sometimes Pylontech batteries need to be balanced, again the min and max cell information can be obtained both from the Cerbo and VRM widget. If the difference is more than 100 - 150mV, then balancing is needed.

[adamreynolds]

Thanks for your reply!

On the ordering of the assistants

With the initial setup for the upgrade, and with my experimenting, I’ve only had one assistant at a time: either the ESS assistant or the PV inverter assistant. And I don’t have any others.

1:1 Rule

Thanks for highlighting this. My supplier wasn’t able to find a smaller PV inverter, hence having a 4.6 kW inverter despite the 3 kVA Multiplus. We have limited the PV array on the PV inverter to be 2850 W, so I’m hoping we won’t run into any issues.

AC Out vs AC In

Thanks for clarifying that AC In wouldn’t work, and I understand why now.

Potential Cell Imbalances

Unfortunately, the version of the firmware on my Pylontech batteries doesn’t allow me to see individual cell voltages, and I’ve been avoiding updating the firmware until I really needed to, as it seems like quite a formidable process and I didn’t want to take the risk of bricking the batteries unless it was really necessary. I’ve sent an email to Pylontech requesting the necessary files, as it seems like this is the best way to go about it.