Potential Bug / Feature Request: Inverter Low DC Voltage Shutdown Prevents Partial-System Charging

Q&A and troubleshooting
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1

I believe there may be a firmware issue with the DC input low-shutdown and DC input low restart settings on systems with multiple inverter/chargers. I have tested this with several different Multiplus-II Systems but pretty sure it also applies to Quatro Inverter Systems and if needed I could test it.

The issue occurs when a system has 2 or 3 inverter/chargers, but only some of them are available to charge the battery.

For example, the settings shown in the screenshot are:

  • DC input low-shutdown: 18.60 V
  • DC input low restart: 21.80 V

If the battery voltage falls below the low-shutdown voltage, the inverter shuts down as expected. However, if only one of the inverter/chargers is subsequently connected to a charging source, the inverter will not restart and begin charging until the battery voltage has already reached the low restart voltage through some other charging source.

This creates a problem in several common configurations.

Example: 120/240 V Split-Phase System

We have two Multiplus-II inverter/chargers configured as a 120/240 V split-phase system, but we want to use a 120 V generator to charge the battery through only one of the inverter/chargers.

If the battery is charged before reaching the low-shutdown voltage, everything works normally.

However, if the battery reaches the DC input low-shutdown voltage, the inverter shuts down. Once that happens, connecting the 120 V generator to one inverter does not allow it to restart and charge the battery. The battery must first be brought back above the DC input low restart voltage by another charging source.

The same issue occurs with a 3-phase system if only one or two of the inverter/chargers are being used for charging. I have an existing customer with this issue on a 3 phase 120/208v system.

When all of the inverter/chargers are capable of charging, the system behaves as expected. It can even wake up and begin charging a lithium battery that has completely shut down due to low voltage. (I really like this feature!)

The problem occurs specifically when only a subset of the inverter/chargers is available for charging.

I have a system at our shop that I can use for testing purposes and can help with the testing if helpful. If Victron gets this fixed I believe it could be a great benefit.

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4

Has anyone else noticed this issue? Did you figure out a workaround?
We have a customer that is connected to VRM with this issue and they don’t have an autostart generator and about 2 or 3 times a year they don’t start their generator in time then we connect remotely and temporarily lower the low battery volt cutout until it starts charging and then raise it again once the batteries have charged for a little bit. In some different systems we have also just installed a separate small charger which slowly charges the battery until the voltage raises high enough and the inverter starts charging again.

5

Have you disabled “switch as a group”?
Usually multi-phase systems have this enabled by default so will power off if one system loses an AC supply.
Disabling it does prevent using ESS.
The 120V systems are outside of my wheelhouse and have their own quirks on occasion, but the above is how it works by default, so expected behaviour.

To disable it, this can be done when the units are initially paired using vebus quick configure/system configure.

6

Yes I have disabled “switch as group” in VE.Bus System Configurator. It works fine charging with only one out of 2 inverters as long as the battery voltage doesn’t hit Inverter “DC input low-shutdown”

7

Thanks for clarifying. That is an interesting limitation.

8

Would this be connected to L1 or a different phase?

10

Typically L1 but I’m pretty sure it doesn’t make a difference if it’s L1 or L2. I have also tested it with applying the same 120v source to both inverters and it will just connect to it with one inverter and start charging as long as the battery voltage is above the inverter low battery cutout. (This would be with the system setup in a split phase 120/240v US system) I also have a 3 phase system where I’m charging from 2 legs with the same issue. So I believe it would also apply to 3 phase European systems.

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11

Here’s an example of the 3 phase system that also has the same issue. This system also works perfectly fine with charging using 2 of the 3 legs as long as the battery stays above the low battery cutout. If the customer doesn’t start his generator in time before it hits the inverter low battery cutout then it won’t start charging.

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12

It is interesting, i was wondering if it was a leader follower related issue.

Black starting is awesome.

I usually have the systems shut down on SOC rather than low voltage, and then not actually restart until a certain SOC (variable there -particularly if i know the system will start under a pretty hefty load it will be higher.)
That way the system is starting from inverter off not whole battery bank off.

I have also never had one phase have a low battery before others. On a three phase typically all the DC cables are the same length etc for that reason.

Does the issue happen even if the system is switched into charger only to recover as well?

13

I don’t typically use the shut down on SOC, I use the low voltage but I always set it higher than the voltage the battery shuts down. I don’t have the issue with one phase shutting down earlier. The issue is just if the battery hits the Inverter Low battery cutout voltage and you want to charge with only one of the legs in a 120/ 240v system or 2 of 3 legs in a 3 phase system.

Does the issue happen even if the system is switched into charger only to recover as well? I thought it does but I’m not 100% sure but I will actually go test that on our Demo System.

14

Keen to hear what you discover.

15

“Does the issue happen even if the system is switched into charger only to recover as well?” Just got done testing it and it won’t start charging if you put it in charger only mode. As soon as you connect the second leg it starts charging or if you raise the low voltage cutout on the inverter. I attached some screenshots.

FYI I increased the low battery cutout to 52v for testing purposes since my batteries were to full. This setting is not our typical setting but it still acts the same if you set it lower and the battery drops below that.

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16

I also tested it by lowering the low battery cutout to 49.2v where we would typically set it but then enabled shut down on SOC and set it a little higher than my current battery SOC and it still won’t start charging on just one leg of ac power. As soon as I connect the second leg it will start charging. The led code on the inverter shows the Low Battery Light solid red (Low-battery - alarm) then when you connect one leg to ac power that inverter still shows the low battery light solid but then the float light starts blinking. Victron Toolkit says “LED combination does not exist”.

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17

Decent as you can eliminate battery as the issue. Just the wake up on AC is tested. I don’t have experience with 120 either here. So its obviously a different beast

18

I think it’s just an issue that Victron may not even be aware of.

Some different applications that this could also affect would be a RV or Boat environment if you have a 120/ 240v setup using 2 inverters but then need to charge from 120v shore power if you charge before it hits the low battery alarm your good and it starts charging. If you don’t charge in time and you only have 120v charging source then you would need a different external charging source to recover. The other option would be using VE Configure or maybe Victron connect and temporarily lowering the low battery cutout until it starts charging. If you do lower it then you better raise it again so the battery can’t completely die because you won’t have the black start option either.

19

We just got this information back from Victron through our Distributor. The Victron Sales Rep took a while to get back to us.

Victron Answer

**Yes , if multi phase and only one phase is fed it will do this

What i have done is connect all loads to ac 2 out and have ac 2 out turn off based on soc say 15%

This way the inverters can stay on and be charged from a single phase after.

It is a limitation as the system wont make half power on the output .**

20

And this is specifically for 120/240v systems?

21

It should also work the same for a 3 phase system (Voltage and Hertz agnostic) with 3 inverter and only charging with 1 or 2 inverters. You would not have black start capabilities though.

I would really like to get the attention of the Victron Engineers because it shouldn’t be that difficult to fix the firmware. If it would even just automatically put them in charger only mode until you are above the Low Battery cutout that would be great. I don’t know what the best way is to try and get it on there list.