question

It is important to remember that the LOAD terminals on the MPPT have a maximum rating of 15A. A fridge compressor can briefly have a high startup current initially.
Now, when the MPPT is in BULK / ABSORB, the battery voltage is higher than when it is in FLOAT. I am not entirely sure how the National Luna fridges work, but I did read that they have a wide input voltage range which suggests to me that they have a DC/DC converter within. I have discovered that the National Luna fridge uses a Danfoss BD35 DC compressor. This means that the fridge's brief inrush or startup current will be higher at lower battery voltage (FLOAT) than when the battery voltage is high (BULK or ABSORB). The BD35 compressor has maximum running current of 6.5A (I don't know what speed the motor is set to run at). If the inrush current, which could well exceed 15A. is being limited (to 15A) then it may well cause the symptoms that you describe. Do you get any error code flashing LED on the fridge?

Another thing unrelated to the MPPT its self is the size of the cable going to the fridge. The National Luna manual (and the BD35 datasheet) goes into some detail to ensure that your cable is correctly sized according to its length.
I should add that the size requirements for the cable between the MPPT and the battery will need to be included as it forms part of the circuit to the fridge.

Ja, so those algorithms pertain to the battery voltage sagging due to inrush currents, and will allow the low voltage condition to be present for 2 minutes before disconnecting the load.
You are not experiencing this, I think you are experiencing the MPPT load output being current limited at 15A and this does not have a time condition. The fridge wants more than 15A to start when battery voltages are at FLOAT voltage or less.
Although I don't know the circuit designs for MPPT or PWM, I'm sure they are different and I'm sure the MPPT responds to over current events quicker than the PWM does.

If I am right in my thinking, then the inrush current of the fridge must be only just above 15A because, like you said, you do not seem to have a problem when battery voltages are at the high end.
I'm sorry, I don't really have any more ideas for you. Hopefully someone else can chip in and maybe prove me wrong.

HI Wkirby

Thanks for getting back to me, I am glad i am not the only one with the issue now .

I have also noticed that my controller is doing the same in "Bulk Mode" when there is very little solar energy or Amps been generated from Solar .

I also measured the inrush Current on the fridge and it never seems to go above 6.02 Amps

Screen shot is of the controller later in the afternoon when the fridge restarted . " Very little" energy been created .

I am even more convinced this is a software / Programming issue on the controller

@WKirby

I managed to get hold of a second Victron 75/15 Mppt controller and ran the exact same tests.

This second controller seems to be better . It only drops the load after every 4 to 5 compressors start ups.

How can the second controller , same Hardware / same firmware etc on it perform much better ? My Unit drops the load almost every time the compressor starts up

I have Bought and invested in Multiple Victron Equipment and looking at more , from this i don't know if i want to continue with Victron any more , this is an issue that needs to get resolved as i bought this controller to be able to see my Solar consumption vs Solar generation and i don't feel running my fridge directly off the battery is the correct way as i wont be able to use the controller for what i bought it for. I am going on a 21 day road trip so i need to be able to monitor my solar system is creating enough energy .

Please can you assist in getting some sort of answer to my problem , And as you have seen i am not the only one with this issue .

It is a surprise to me that either of the units load output is able to run a fridge compressor. That is typically a very high current load.

The load terminal on those units is a great and low cost convenience, but it is very limited by design.

The robust, reliable and accurate solution is a BMV, and MPPT connected to a CCGX.

This is a lot more expense, probably more than you want to spend and in someways not as good (as the VictronConnect app makes everything very handy). But your application falls just on the line, too big for that little MPPT to be 100% and too small to justify full system.

I hope it doesn’t sour you on Victron as a whole though. One of the big strengths is a product range that spans tiny applications to massive power demands.

@Guy Stewart (Victron Energy Staff)

The strange thing , Maybe everyone is missing the point here

When the solar charger is been charged , Either in the " Bulk" or ' Abortion" and there is for example 6 Amps power been created by the solar system the fridge works Perfectly , No issues , It runs all day , Multiple compressors restarts , i can even bypass motor speed control and run fridge at maximum speed. The controller is then able to handle the inrush currents perfectly all day.

The issue only really comes up when there is very little to no power been Generated from the solar panels , this can be in "Bulk" or " Float" the problem is even worse in "Float" when the solar energy been created is very little to "Float" the battery .

This proves to me that the inrush current is not the issue , if inrush was an issue it would do it all the time , Regardless of how much solar energy is been created .

If the fridge requires 280W to start, and the batteries are nice and full, the voltage will be high 14V, requiring 20A.

If the weather has been bad and the batteries are not charged, getting low at 12.2V you will require 23A to make the 280W required.

That additional 3A current may be all the difference necessary.

With fast test equipment, you may see the battery voltage dip even lower during high current draw (this will not happen if the solar is also powering the load keeping voltage high). The lower the voltage, the higher the current required for the same power demand and the higher likelihood of overload.

@ray_pridham@bigpond.com

@Guy Stewart (Victron Energy Staff)

Hi Ray_pridham

That is exactly the same issue i have also seen with my Fridge . My battery was also near full charge or fully Charged i even had it on a Battery charger the night before to make sure it was 100 % fully charged . I get the issue with the Compressor not starting the first time in Float

So the fridge Compressor will start , It will stop for a Second and then it i will start and work okay until the fridge stops the compressor again

So My question if in rush current was an issue the fridge would then never Start ??

And then why does the fridge start a second latter , Surly there would be two inrush currents then ??

I will try upload a video of the fridge and how the Compressor starts up, then switches Off , then starts up and runs okay .

One option:

connect a relay between battery and loads, drive the relay coil through the load-output of the MPPT. Don't forget to put a diode over the relay-coil (some relays have this build in)

Don't forget to fuse according to wire-size and relay current - and please choose a relay a couple of sizes bigger than maximum current you expect. 70A automotive relays are cheap and good, but make sure you have a spare on board.

I got the NTC which I ordered.
I have connected it up in series with the load which consists of both 55W filaments of a H4 car lamp. Remember, just one filament is enough to trip the Load Output.

The NTC does its job and starts both filaments without any problem.
The inrush current is limited to about 18A which is quite reasonable.

The possible downside is the heat that comes off the NTC. It is a resistor after all, so depending upon how much current is going through it, it might get hot.
It got up to about 85°C with a continuous 9A load.
It gets up to about 40°C with a 4.5A load.

I'm pretty certain that this will solve the fridge startup problem.
The thermistor is an Epcos B57127P0508M301 - 0.5Ω @25°C
If you are in SA then you could get it from RS Components. https://za.rs-online.com/web/c/passive-components/variable-resistors/thermistors/?searchTerm=B57127P0508M301

I hope it works with a fridge. I don't have one to test with, so hopefully someone is willing to try.

@Justin @ray_pridham@bigpond.com @Guy Stewart (Victron Energy Staff)

My 0.5 ohm NTC's have arrived and first tests suggest 1 x NTC works very well. Voltage drop at approx 3 amps is around 1 volt and the fridge starts without error with the fridges low battery value set at 10.1v.

The best news is the 100/20 doesn't cut the load off but starts the fridge in all modes including float. The NTC temp settles at about 75 deg C .

More testing on a road trip over the next few days, but for now a big thank you to Warwick and all who chipped in.

BTW something I read along the way said 2 NTC's in parallel probably won't work, one would heat first and carry all the load.

@WKirby

I finally got my NTC's , I will be trying it on the weekend , I will keep you updated on the results.

Holding thumbs it works ,

Hey there.

Just wanted to chime in and say I was having what sounds like a similar problem with my 75/15 when trying to run a fridge on a recent camping trip.

Of course, the fridge runs fine off the battery directly but I kind of wanted to utilise the battery saving tech of the load output, not to mention get an accurate meaure of how much power i used relative to how much I had been generating with the panels.

Sounds like it's a limitation of the hardware, which is disappointing but if that's the case, so be it.

Cheers

I am not technical but wondered if the config was 24v would this reduce the amps demand on the load and reduce the likelihood of it cutting out?

I suspect the cause. I have the same problem. My roof fan MaxxFan automatically switched off when it is a sunny hot day. Exactly if you want the fan to work. On such a warm sunny day, I completely monitored it once and saw that my Votronic info panel switched on again when my fridge started. The power was interrupted for a short moment. The roof fan connected directly to the battery and that solved the problem of the roof fan. The peculiar thing is that power interruptions only occur on sunny days. In my opinion, this means that the MPT 75/15 adds up all the amperes, both the incoming and the outgoing. On a sunny day with my 2x 110 watt solar panels I am already over 10 ampere and then a starting fridge of 4.5 ampere can no longer reach it.
The mppt reduces the number of amps depending on how the battery is charged. You probably have to measure the full capacity of the solar panels of that moment (without the reduction of the MPP). The specified 15 ampere consumption current is, I think, not separate from the charging current. If these are counted together, you will soon be over 15 ampere.
I find this information from Victron misleading because I assumed that the capacity is 15 ampere charging current as well as 15 ampere consumption current.
Very disappointing because the more solar energy there is, the fewer devices I can switch on. It sometimes happens that the refrigerator no longer works.