I don't understand this Multiplus 12v/charger and 120v overload behavior

DIY
, ,
1

I have a 12/3000 120-50 Multiplus (old style, not II). I am new to it all still, and it is not behaving as I would have expected.

It is part of a rv/travel trailer system.

Currently the rv is plugged in to 30 amp, 120v shore power, the app shows ~121v for input L1, so power is of decent quality? Shore power runs directly and only into the Multiplus, and then to L1 output which goes to the rv power distribution center.

First thing I don’t understand:
Right now I am running the electric water heater, and my draw is 1540 watts. It is early enough in the day that the solar panels are not producing power yet. When the water pump runs, which runs on 12v, the batteries show they are discharging. Why wouldn’t the Multiplus supply the 12v here?

Second thing I don’t understand:
Last night, plugged into 30 amp shore power, there was a moment where the air conditioner and microwave were both running at the same time. Draw was ~2700-2800 watts. There was an alert in the app for overload, it looks like the batteries kicked in to supply the extra, which is good. But also, doesn’t the Multiplus pass input power through? I suppose how much power gets passed through depends on the setting, right now mine is at the 50 amp default. So 30 amps at 120v is 3600 watts, why did it overload? Yes, I see the data sheet says 2400 watts max for the inverter, so if I weren’t plugged in I would expect to overload and shut down in this situation, but I was on shore power.

Third thing I don’t understand:
Last night with the air conditioner set to run, the batteries were discharged to about 75%. The rv was plugged in. When the air conditioner is running it draws ~1500 watts. Of course, the air conditioner does not run the compressor continuously, so sometimes it is only drawing ~150 watts. In any case, the batteries were not charging, despite there being plenty of power to charge them. Interestingly, it wasn’t until I also ran the microwave and the app showed the overload alert that the batteries started to charge with the extra power. What could be going on there?

Thanks for any insight you might have.

2

Also, Dynamic current limit is enabled.

3

The inverter does not know about DC loads so won’t supply them, when the battery voltage drops it will step in and top it up,

The inverter always leans on the batteries, with current limiting even more so, if you introduce a transient load or surge it would be easy to overload the inverter.
Often an overload can actually be the result of the battery being unable to cope with the transient.
If you disable dynamic limiting (traditionally used for generators) it may improve things. Perhaps rather set a fixed lower limit.

It would help if you share your settings and what batteries you have so others can help with guidance.

4

So, sure, the inverter doesn’t know about 12v, but the charger does.

When dynamic limiting is OFF and the charger is on, I can’t run the air conditioner from the generator (and possibly 30A shore, haven’t tried yet). My understanding is that what dynamic current limiting does is instead of just making all 90A (or whatever charging current is set to) available to the batteries (which would take it), it dials down what the converter is doing so as not to overwhelm the AC.

I have seen it happen with this Multiplus with the generator running that the charger makes current available to the battery with whatever excess power the 120v isn’t using. At one point on the generator, the batteries were full to the point where the BMS shut off charging (I have a lot to learn about batteries yet). But anyway, with the batteries off, the charger supplied the 8 amps the fridge pulls. And then no amps supplied when the fridge compressor wasn’t running.

… which happened on shore power last night, except not at first, only after there was a fault. Why would that be?

Batteries are 2x280Ah Watt Cycle. Inexpensive, yes, but they work so far.

Anyway, I guess I am expecting consistent behavior. Maybe I don’t have the variables lined up as nicely as need be, but so far I wonder why sometimes the Multiplus isn’t working the same under what seem to be effectively similar conditions.

Thanks again.

5

The multi is a converter, it can either charge or invert, not both.
The charger is unaware of any load, it has no measurements to go on, if the battery voltage does not dip, it won’t pull any power. So you tend to see a seesaw as the battery discharges and is topped up again.

Some add DC shunts as load meters, but I am not sure if this solves the issue. There are a few posts on this topic already, lurking in this forum.

6

Do you happen to know under what conditions the multi will STOP inverting?

The answer might be an explanation to what I experienced.

So yes, it makes sense that the multi could either be inverting or charging (or neither), but never both. I had not rigorously thought about that. Using that as a way to think about things, here is what was going on when I expected it would stop inverting and start charging:

About an hour away from the destination, I turned on the air conditioner in the travel trailer to cool it down so it was comfortable in the rv when we arrived. The multi was inverting, running off batteries and some solar yield. When we arrived to the campground and plugged into 30 amp shore power, I guess I was expecting without thinking strictly about it that the inverter would shut off and the multi would charge once I was connected to shore power. But this seems not to have happened … at first.

After the alert, the multi seems to have turned off the inverter and gone to charging.

Also, I still don’t know why the multi had an alert at 2800 watts draw since I am expecting it to pass up to 3600 watts through on AC input. Maybe the startup of the microwave had a spike that, added to the 1500 watt draw of the air conditioner put it over how much power the AC input was actually delivering?

Anyway, any thoughts on how and when the multi decides to stop inverting, and why it might have continued to run the inverter even after plugging into what should have been sufficient shore power?

Thanks!

7

It is difficult to diagnose behaviour from the internet, we just lack the access and information to work out what is going on.

The system has to synchronise to the grid, so when you connect there is always a delay while this happens, before closing the relays.
Your grid code also specifies what a suitable connection is, if the quality of your grid connection is poor, the system may not accept the grid connection at all (even though it sees power) or it may delay the process.

The multi also has a ramp time where power is phased onto the grid, it isn’t instantaneous.
Do you have any settings enabled like the dynamic current limiter or wide frequency range?

8

Is there a quick way to dump settings from the Android VE connect to a file? That would make it simple to share all the settings.

In any case, I believe I am running mostly default settings at the moment, with the exception of dynamic current limit being enabled, and absorption and float current turned down a little (14.00 / 13.8) for some initial charging of the batteries (which was a recommendation I received on another forum, and whether it was a good suggestion or not, I don’t think would impact much here).

Accept 45-65 hertz is on. The campground was delivering 60 Hz at 120v.

Interestingly, right now I was just checking things, and I have the same situation: I am expecting the multi to be making charging current available on the bus (which would run the DC loads) but the batteries are discharging to run the furnace fans.

I am pulling 120v loads (a couple of laptops and a computer monitor) at the moment. So maybe the inverter is on? The light on the multi box would tell me, but it is in the closed compartment I am sitting on. But I am on 30 amps of shore power at 124v and 60.3 Hz

Is there a way to tell if the inverter is on from VE connect? Can the inverter be manually shut down?

If the batteries are discharging to handle 12v loads, does it mean the inverter must be on?

9

Hmmm, well yesterday evening and this morning the multi functioned exactly like I expected it would. The batteries are full and neither charging nor discharging, the multi charger is supplying the 12v bus for the fridge and water pump as required, as well as the 120v loads.

Not sure what the missing variable was from when I first posted that things weren’t working as expected.

Anyway, thanks for the input.