question

Marcus Fosker avatar image

AC Current Limit when using ESS as a grid parallel system

I have a Multiplus II GX system which is installed with the ESS application. It's connected to the UK grid via a single phase 100A supply, connected to AC IN of the Multiplus.

All loads and an AC coupled solar inverter are also connected to AC IN of the Multiplus.

The Multiplus is able to balance the grid supply close to zero, charging the batteries off of solar and discharging them as required. I also have time of use tariff which enables the Multiplus to charge the batteries over night.

The issue I have is that I run a lot of loads over night in the cheap period of my tariff including a 32A electric car charger. This regularly take my load over 50A.

The Multiplus needlessly regulates its own charger down in this period and on occasion supplements my grid supply from batteries. This is needless and results in me not having a full battery bank at the end of the cheap time-of-use period.

I can't increase the current limit beyond 50A on the Multiplus or seem to disable it. It is not needed in a parallel grid connection as I have it, as far as I can see.

Can somebody advise me as to whether I have missed a setting or if this is something Victron need to consider in a future firmware update. I want to be able to heat my water, charge my car, batter bank, run my dish and clothes washers during my off peak period!


Multiplus-IIESScurrent limit
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I am not sure if I understand correctly.

You are using the scheduled charge feature in the ESS menu to disable discharging from the battery during the off peak time period you want.

Setting the target charge to 0% (so the battery doesn’t charge from the grid), and the set the time it begins, and for how many hours.

During that period the battery should not discharge to power loads.

But you are saying that it is discharging the battery during this period?

Marcus Fosker avatar image Marcus Fosker Guy Stewart (Victron Community Manager) ♦♦ ·

Thanks for getting back.

I'm using the scheduled charging to attempt to charge the battery during the off-peak period of my time-of-use tariff. I want to exit this period with as a high state of charge as possible.

I am also using my grid connection for other loads at that time. e.g. charging my car, doing clothes washing, heating water.

All loads, including the Multiplus are connected to AC IN, so the Multiplus is parallel to the loads.

When the total load of my grid connection reaches about 50A the Multiplus starts to limit the power of its charger and also supplements the grid connection. I do not want it to do either as I have a 100A grid connection.

Marcus Fosker avatar image Marcus Fosker Guy Stewart (Victron Community Manager) ♦♦ ·

It is as though both the PowerAssist and PowerControl functions are enabled and are functioning when they should not be.

Please could you advise how I disable them.

Hi

You disable them by using VE configure configuration software.

Thanks for getting back to me. In the Inverter Tab, I have PowerAssist "unticked". In the General Tab, I have AC Input Current Limit set to 50 Amps. I cannot set it to 100 Amps, nor set it to disabled.


Is this how it should be?

Marcus Fosker avatar image Marcus Fosker Guy Stewart (Victron Community Manager) ♦♦ ·

I've taken a video of the Multiplus performing a schedule charge with no SOC limit set. When I start the car to charge, you will see the Multiplus throttle down its charging, then start up the inverter, all during the scheduled charge. This isn't what I was expecting from the ESS Assistant.

The Multiplus is installed with the grid and loads all connected to AC IN on a 100AMP feed to the grid with the intent of using cheap electricity stored from grid over night and excess solar produced in the day from a grid tie AC inverter, so there's no need to perform this PowerControl and PowerAssist type function.


Video of VRM console

Can't offer any help but just wanted to say thanks, really useful to read that. I'm in the UK and have the same set up (though not the electric car yet!). Hope you get it sorted.

3 Answers
Rob Duthie avatar image
Rob Duthie answered ·

Hi

The current is the maximum of the inverter type model 3kw is 32 amps the 5kw is 50 amp.

What sort of battery do you have in your system?

And how is the power wall integrated?

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I have the 32A Multiplus II GX. The battery is Lithium Ion with a CanBus BMS.

The Powerwall (Multiplus) is connected to Grid on its AC IN. The current sensor is plugged in and connected to the grid meter tail so it can measure import/export.

I've used the ESS Assistant.

My layout is as per this reference model from Victron with the exception I am not using the No break loads option on AC OUT 1. Given the reliability of the grid in the UK, there's little point.

Victron ESS has a scheduled charging function to allow you to top up the batteries during off peak rates on a time of use tariff. They say "

It allows you to set up to five scheduled periods, during which the system will take power from the grid to charge the battery. This is typically used to charge the battery during off-peak tariff time windows (TOU). For each schedule, configure a start-time, duration, and optionally set the percentage up to which you want the battery to be charged.

If the target state of charge is reached, and it is still within the period of time set, the battery will stop charging but will not discharge (unless there is grid outage). This optimises battery cycling and still allows room in the battery for PV charging."

My system is not only failing to charge as it limits its charger based on the current limit setting but also discharges the battery during the charging time, contrary to what's stated above.

Hopefully Victron will change their software such that when installed as per below, one can set the current limit as per their grid connection, rather than artificially constraining it and discharging the battery when there is no need.


1570661495198.png (124.7 KiB)
Rob Duthie avatar image
Rob Duthie answered ·

The Lithium BMS has charge control setting parameters as well?

The battery discharging could be the BMS thinks that the batteries are full so will discharge a bit off the top as mine does also the setting for the lithium battery,

Need to check these settings for the lithium battery type Absorption setting and float setting and SOC when bulk finished when i had these set wrong the battery would charge up an then discharge down to these settings odd behavior, once change the battery now charges fully up now and doesn't discharge like it use to.

Also check the battery ESS mode is set to Optimized (without battery life) Note there are so many setting etc which is convoluted

in operation i have found.

I have a very similar system to yours, i don't use charge scheduling at all, i use to but had some issues so changed to a normal ESS control.

And all works as normal.


Regards
Rob D

NZ

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My batteries cells are set out in a 16S arrangement and I currently charge them to 64 volts. Experience now tells me I should have gone with 15S, but that is not relevant here and I shall change them later. The settings I use on the Multi are:

Absorbtion voltage: 64 volts

Float voltage: 63.8 volts

The Multi very rarely reduces its charge current with these settings, unless there has been little load and a very long, sunny day. I do not believe this is contributory to the issue I'm experiencing now with the Multiplus II throttling charging when 50 AMP grid load is reached and then discharging during the Scheduled Charging session.

The BMS is a Batrium system connected via CAN BUS. The parameters shared with the Mulitplus from the BMS are:

Charge Current Limit: 90 Amps

Battery Low voltage: 48 volts

Discharge Current Limit: 180 Amps

Max Charge Voltage: 64.8 volts

You can see the BMS often limits the discharge current when the battery reaches a low SOC or a particular cells reaches a low voltage, however charge voltage and charge current are rock steady.


I also use the Optimized (without battery life) so as not to have the low SOC point changing.

I use the Scheduled Charging function from 00.30 to 04.30 each day as my electricity supplier has a low rate to encourage electric vehicle charging and day use off setting through home storage solutions.

Thanks for your suggestions and looking into my issue - I appreciate it. I hope Victron are also able to come back with some advice/a view too.

1570701998827.png (27.2 KiB)
Sean avatar image
Sean answered ·

I have an almost identical setup, 250kWh of leaf cells, Batrium, parallel MPIIs etc etc

I use a ET112 for metering, which is rated for 100A max

The MP CT (which I believe is being used by the OP ?) is rated at 32A - it will likely be saturated at 50A and give rather erroneous results.




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You're correct that although it's a 100A/50ma CT, Victron do rate it at 32A max. If I can get the behaviour to work as I'd like, I should replace this with the ET112 meter.

I'm not sure it's the root cause of the issue I'm experiencing though and I think my issues are caused by an error in my config or my misunderstanding of Victron's manual.

My issues are two fold:

1) When the input current (loads in house plus MP-II charger) exceeds the level set in the Venus GX (or via VEConfig in the General Tab), the MPII inverter starts up and assists the grid during a ESS schedule charge session. I've overcome this by using Node Red to switch off the inverter during the schedule charge window, using TCP Modbus.

2) When the input current exceeds the level set (as above) the charger reduces its current to attempt to keep the input AC current below the value set. In VEConfig I can set this to a max of 32A, in the console of the Venus GX, I can set this to 50A. Ideally I'd like to turn the function off or set it to the 100A limit of my breaker.

Victron's manual says that you "can" set a value for this Current Limiter, but in my case so far, it's more of a "must". I can't find a way to not set a value.

Please would you share your config with me?

PS. 250kWh! I am envious - I have 12kWh of cells from a Mitsubishi Outlander PHEV.