Multiplus 48/5000/70-100 in island mode shutting down under load

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jwr asked Sep 18, '23

Multiplus 48/5000/70-100 in island mode shutting down under load

I have an installation with a Multiplus 48/5000/70-100 and 4x100Ah VRLA AGM batteries, operating mostly in island mode. There is some solar, there is also a permanent AC connection, limited to 16A.

This is a workshop and I have the Multiplus configured to operate in island mode whenever possible, and conditionally switch to AC input: when load is high and when battery voltage is low. It's a workaround for a deficiency of the Multiplus: I'd love to self-consume whatever I can from solar and *then* use as much AC as I need to supplement that, but Multiplus can't do that. So I have it set to auto-switch to island when batteries are charged and switch back to AC when they are low.

I use PowerAssist (it's the main reason why I bought the Multiplus!), because I really can't exceed the 16A on my AC connection: if I do, I trip a breaker that I do not have access to. Very annoying.

This installation has worked fine for a year or so. But I installed a larger machine recently, with a 3kW motor. The machine is on a circuit with a C16 breaker, which should really be C20 (rated max current at startup is 19A).

When I start the machine, if conditional AC input is enabled, lights dim, voltage in the installation drops to 140V, and the inverter shuts down after a second or so. I would have expected the inverter to immediately switch to AC and enable PowerAssist (if needed) instead.

If I disable conditional AC input and run the inverter from AC, I get an "overload" warning every time I start the motor, but otherwise things run fine.

Any advice is appreciated: I'd like to keep operating in island mode as much as possible and have conditional AC input enabled, without the inverter shutting down when I want to start the 3kW motor.

multiplus-2023-09-16.vcsf.zip

powerassist island mode

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Answer 1 · 2023-09-18T10:35:17Z

nickdb answered · Sep 18, 2023 at 10:35 AM

The startup power of a 3kW motor is orders of magnitude higher than that, without a soft start it is way too much for your inverter and by extension, batteries.

Hence ripple warnings, overload etc.

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jwr commented · Sep 18, 2023 at 05:50 PM

Hmm. First, the motor has a whole complex soft-start system, it starts up to a slow speed first, stabilizes that, then after about 5-6s ramps up to full speed.

And it's on a C16 breaker.

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Craig Chamberlain jwr commented · Sep 20, 2023 at 09:22 PM

A Type C MCB will trip at a current between 5x and 10x the rated current. So 80A-160A in this case. Most domestic circuits use Type B MCBs which trip at 3x - 5x rated current. The Type C is recommended for circuits with small motors to allow for the startup current and even with a soft start mechanism it will still pull extra current on start.

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Answer 2 · 2023-09-18T12:15:22Z

JohnC answered · Sep 18, 2023 at 12:15 PM

@jwr

You have a 5s delay set when the load comes on. It's all over by then. Try 0s.

It's surely a big load for a 48/5000. The overloads aren't a real issue, but you also don't have a lot of battery. So the shutdowns may even be coming from low battery V, especially if the batts are cold.

There may be other ways of dealing with this (outside of VictronConnect), but some complexity involved, so I won't go there yet.

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jwr commented · Sep 18, 2023 at 06:01 PM

Ok, I can try a shorter delay and see if that helps. The problem with a short delay is that then it trips often even on short loads (such as turning on a vacuum cleaner). But perhaps I need to use a higher power setting and lower trip time there.

As for big load, doing some back-of-the-envelope math, let's say I want to deliver instantaneous 5kW. That translates into (very roughly) slightly more than 100A at 48V. A VRLA AGM battery has internal resistance of <8.5mΩ, so with 4 of those in series I can expect a voltage drop of less than 3.4V when pulling 100A. I thought I should be able to pull 100A from these (fully charged) batteries. The temperature is 22°C by the way.

And that's assuming my solar doesn't deliver anything — when I was testing, I had around 1kW in from solar. So 20A@48V right there.

In other words, I'm puzzled as to why 1) I can't deliver the power, 2) why the inverter doesn't immediately switch into AC+PowerAssist, which is why I thought I'd ask if I'm doing something wrong.

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Matthias Lange - DE ♦ jwr commented · Sep 18, 2023 at 06:29 PM

why the inverter doesn't immediately switch into AC+PowerAssist

It doesn't switch immediately because you set a delay of 5s.

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Answer 3 · 2023-09-20T16:17:34Z

jwr answered · Sep 20, 2023 at 04:17 PM

Ok, so, after setting the conditional AC input delay to 0, things are slightly better, but still pretty bad overall. The net result is that the inverter sometimes manages to switch to AC in time, but not always. And every time it fails to do so quickly, it results in a shutdown because of low battery voltage warning. Suboptimal, to say the least.

Anything else I can try to make it switch faster?

What makes things worse is that right now I have to bring a laptop, connect it to the MK3 USB interface, run Victron Connect, enter the settings, and switch off conditional AC input — all this every time I want to use the machine. I thought this could be done via the Venus interface, but it seems this is not possible. In other words, switching this setting is a pain, and I can't leave it completely off, as it nullifies my solar installation then.

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JohnC ♦ commented · Sep 21, 2023 at 12:19 PM

@jwr

This 'Conditional AC' function is available elsewhere, usually called 'Ignore AC'. Perhaps look beyond VictronConnect to VEConfigure, for access to the more serious Multiplus functions. Virtual Switch contains it, but Assistants are more fully optioned.

For convenience, maybe wire in a toggle switch across the Multi's Aux port, and/or even a wire pair across from the GX relay (if yours has one), then it's selectable from Remote Console.

That Aux port status can then be used to Ignore AC by setting up Assistants to handle the logic. And might even be able to improve upon your your other ac logic.

This stuff comes with a learning curve, but it's not so hard. Your Multi can do lots of things you haven't imagined yet.

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jwr JohnC ♦ commented · Oct 05, 2023 at 08:22 AM

Thanks, that's very helpful! I guess I need to start using VEConfigure. The problem there is that it's Windows-only, and all I have is Mac and Linux machines. I can't even run Windows in VMware anymore, since the switch to Apple Silicon.

Oh well, I guess it's time to buy a windows laptop just to run this kind of legacy software…

Once again, thanks for the great ideas and suggestions!

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jwr JohnC ♦ commented · Nov 27, 2023 at 12:51 PM

@JohnC — so I bought a Windows laptop (no other way to run VEConfigure, it's all MacOS and Linux around me) and tried VEConfigure. It doesn't seem there are many more options, and the assistants look *very* interesting, but it seems I can only pick from a list of pre-built assistants. I couldn't find a way to write my own, and I have no idea how I can make use of a switch toggled to the AUX port.

I do have the skills (programming, electronics), but it doesn't seem that writing my own assistants is a thing?

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JohnC ♦ jwr commented · Nov 27, 2023 at 01:53 PM

@jwr

Yeh, I did mention a learning curve. Once you get your head around how they work it'll come together.

Some are very simple, like Programmable Relay. which does one thing. But you can have many in a sequence - and the order is important. Others like ESS are complex, like a finished app.

https://www.victronenergy.com/live/assistants:start

There's some links there too which may help.

You can play around with them on a pc disconnected from your Multi. Doing that will help you get a feel for how they work.

Searching for 'Assistants' on this forum too might find you some examples and clues.

There's even a training module on Victron Professional..

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jwr JohnC ♦ commented · Nov 28, 2023 at 12:31 PM

Yes, I specifically looked at ESS, because that seemed to be close to my use case. But assistants seemed to be closed apps, with no way to write a new one.

I'll follow your advice and take another look at the ones that are available, thanks.

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Answer 4 · 2023-10-05T08:25:53Z

jwr answered · Oct 05, 2023 at 08:25 AM

To summarize: I've gotten things to somewhat work. If conditional AC input is off, things are reasonably fine — there is still a voltage drop, but the inverter manages to assist. It worries me, however, that it exceeds the input current limit (I have it set to 12A and it trips a B16 circuit breaker sometimes, not good).

With conditional AC input ON, if I set the delay to 0, I can avoid an inverter shutdown, but only if my batteries are fully charged. Otherwise it is not capable of switching to AC and into assist quickly enough and shuts down because of low battery voltage.

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Answer 5 · 2023-10-05T14:03:14Z

chrichri answered · Oct 05, 2023 at 02:03 PM

What are the cable square mm and length to the batteries?

The takeover of such a load has a very high current peak on the batteries. If you use less than 90mm square cables you should go very low with dc low shut down. 9 V per battery = 36V The AGMs have no problem with that. But ensure that you have set a second shut down via assistant if the voltage drops longer than 10 seconds. otherwise you will kill your batteries on a long therm.

And on a better long term, double the battery pack...

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jwr commented · Nov 27, 2023 at 12:55 PM

My cables are 32mm² and the length is less than 1m. I don't have the system at hand right now, but I think that low DC shutdown value was not configurable below a certain threshold, as I remember I did try to modify it. Also, I think the inverter was shutting down because of battery ripple and "overload", not low DC?

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