Hi everyone.

I have two Axpert Inverters connected in parallel. 

Here are pics of each ones specs.

I do not see any limitation specs on the input side of the solar charge controller but on other inverters, I happened to notice that they show a 18A PV array input limit.

Would this be standard for all of them? 

And what is the maximum number of panels that I can use on both seeing their PV specs differ slightly?

Thanks.

21 hours ago, Anton Roothman said:

I have two Axpert Inverters connected in parallel.

I'm amazed that you can safely connect those two in parallel.

21 hours ago, Anton Roothman said:

I do not see any limitation specs on the input side of the solar charge controller but on other inverters, I happened to notice that they show a 18A PV array input limit. Would this be standard for all of them?

No, that 18 A input limit refers to the high PV voltage models, which have a completely different solar charger.

One of yours (the one with the 105 V max PV voltage) is probably a PWM model. That means that during bulk charging, the panels are connected directly across the battery; the total panel voltage is the battery voltage. Also, the total panel current is the solar charger output. Since the rated output current is 50 A, that means that the rated PC input current is also 50 A. Besides, 18 A at say 54 V is less than 1000 W. Usually you would have many strings of two "24 V" panels (with Voc around 40 V) in series. Any more panels in series would just be a waste, since the battery voltage will pull them far from their Vmp (voltage where maximum generated power is achieved). Here you might have 54 V at 50 A at the input, and 54 V and 50 A at the output. It's a very simple arrangement, at least when in the bulk stage. During the absorb and float stages, pulses of say 50 A are delivered to the battery, averaging just enough to maintain the absorb or float voltages.

Your other inverter with the 145 V maximum PV voltage is an MPPT model. That one actually varies the load on the panels to achieve Maximum Power Point Tracking. It is a buck converter, meaning that the output voltage is lower than the panel voltage, but also the output current (a maximum of 60 A in this case) is higher than the panel current. Here you might have 100 V at 30 A on the input producing 50 V at 60 A at the output. The output is smooth at all times, never pulsed.

As a final example, a high PV voltage model might have 330 V at 15 A at the input,  producing perhaps 380 V at 13 A at the output, which powers the DC bus. Part of that power might be chopped up to power the load via the DC-AC converter (the "inverter proper"), and part of it might wind its way through the buck converter and DC-DC converter to charge the battery. These are the ones that are limited to around 18 A of PV input current. The reason for this is that the high voltage MOSFETs or IGBT(s) used have limited current ratings. The MOSFETs in your models are rated at much lower voltage, and have higher current ratings.

Both of your solar chargers are pretty hardy, so there is no need to worry about exceeding the maximum PV input current with those.

As a point of interest, if it's no trouble to find out, what are your main (U1) firmware versions? It should be possible to leave the details of MPPT versus PWM to the two solar charge controllers (these have their own small 8-bit microcontroller with associated firmware), but I didn't think that they achieved that. I would have expected you to run into fault codes due to mismatched "paralleling constants", resulting in fault code 71. Good on Voltronic for allowing these models to be paralleled, as most users would  expect to be possible.

Hi Coulomb.

Thanks for the very interesting and detailed response.....following the forum, I was hoping to get a reply from you. I actually followed you to get clarity on the firmware to make sure I don't buy inverters that are clones etc. and that I can upload firmware to parallel connect these units.

I have uploaded the same firmware onto both inverters that I found a link to in one of your posts. The firmware version on bot currently is U1_72_40.

I haven't had any issues with the parallel connection this far. Just had to figure out how the charge controllers work with the solar panels. 

Currently I have two 555w Canadian solar panels connected in series on each inverter with two 100ah 48V lithium ion batteries connected in parallel to the the inverters. I did notice that one inverter, the one with the 105V maximum PV voltage that is also usually slave between the two inverters as it tends to switch on a bit slower the the 145V one, tends to be the one supplying the loads connected rather than the master from the PV's connected to it....bit strange....only when you pull more power than what is available from those PV's does the master one (145V) also supplies power from it's solar panels....but the batteries are staying fully charged.

Wanting or opting to go with the same size PV's....the 555W ones....can you pls advise as to how many I can connect to each of the Inverters without over doing it?

For example...in my mind...would I be able to connect four strings of 555W X 2 connected to the 105V inverter....giving me I guess at 96V x 4440W and 46.25A array for that specific inverter and with the 50A limit (the 105V one) and there strings of 555W x 4 connected to the 144V inverter...giving me once again I guess at 144V x 8880W and approximately 62A array for the inverter with the 60A limit(the 145V one)....

Once again....I really appreciate your inputs on the forum and look forward to your valued reply.

Just a pic of my firmware...

 

 

Edited December 11, 20232 yr by Anton Roothman

45 minutes ago, Anton Roothman said:

Hi Coulomb.

Thanks for the very interesting and detailed response.....following the forum, I was hoping to get a reply from you. I actually followed you to get clarity on the firmware to make sure I don't buy inverters that are clones etc. and that I can upload firmware to parallel connect these units.

I have uploaded the same firmware onto both inverters that I found a link to in one of your posts. The firmware version on bot currently is U1_72_40.

I haven't had any issues with the parallel connection this far. Just had to figure out how the charge controllers work with the solar panels. 

Currently I have two 555w Canadian solar panels connected in series on each inverter with two 100ah 48V lithium ion batteries connected in parallel to the the inverters. I did notice that one inverter, the one with the 105V maximum PV voltage that is also usually slave between the two inverters as it tends to switch on a bit slower the the 145V one, tends to be the one supplying the loads connected rather than the master from the PV's connected to it....bit strange....only when you pull more power than what is available from those PV's does the master one (145V) also supplies power from it's solar panels....but the batteries are staying fully charged.

Wanting or opting to go with the same size PV's....the 555W ones....can you pls advise as to how many I can connect to each of the Inverters without over doing it?

Once again....I really appreciate your inputs on the forum and look forward to your valued reply.

Just a pic of my firmware...

 

 

Just a point while waiting for @Coulomb to answer. Those 2x550W panels on the inverter with Voc of 105V can reach over 105V and damage your MPPT if temp ever reaches 0 deg C. This depends on your area. 

Thanks Scorp007.

I thought about it as the temperature in my area does sometimes goes to below 0 and was concerning. Would definitely try and see if I can get another inverter before winter comes. Did read about the effect of the cold weather when it comes to the voltage of the panels and was quite surprised that the colder weather it actually increases the voltage. 

Thanks for the reply and is definitely appeciate it.

Great forum and appreciate everyone's feedback...makes life much easier and I hope lots of people joins this very informative forum... that's why I joined. Thanks Scorp007

Just now, Anton Roothman said:

Thanks Scorp007.

I thought about it as the temperature in my area does sometimes goes to below 0 and was concerning. Would definitely try and see if I can get another inverter before winter comes. Did read about the effect of the cold weather when it comes to the voltage of the panels and was quite surprised that the colder weather it actually increases the voltage. 

Thanks for the reply and is definitely appeciate it.

Great forum and appreciate everyone's feedback...makes life much easier and I hope lots of people joins this very informative forum... that's why I joined. Thanks Scorp007

You can connect those panels and even more than 2 in parallel. This will keep the volts below 60V.

Not essential to change a working inverter just for this. 

Knowing the inverter obviously better that I do, what would you suggest be my PV array...taking into account my wish to stay with the current panels I have...the 555W ones, should be a good array to stay within good limits and maximise the inverter's charge controller capability...would really appreciate the advise 

Hi Scopr007.

Just to clarify on the 105V inverter....I think what you say I should RATER do is to connect single 555W panels in parallel to reduce the voltage and that will protect the inverter. Would I then be correct in saying that I should rather connect 4 x 555w panels on that particular inverter all in parallel to be safe?

2 minutes ago, Anton Roothman said:

Hi Scopr007.

Just to clarify on the 105V inverter....I think what you say I should RATER do is to connect single 555W panels in parallel to reduce the voltage and that will protect the inverter. Would I then be correct in saying that I should rather connect 4 x 555w panels on that particular inverter all in parallel to be safe?

You can leave it as 2 in parallel on the 105V inverter. But if needs be you can also run 4 in parallel on it. For now run 2 in parallel on the 105V inverter and the 2 in series on the 145V inverter. 

Get what you are saying...makes sense...thanks.

But up to what number of panels on each can I go to reach maximum without damaging my two inverters..? Your opinion...it counts...believe me...just want to know so I do not cause damage and spend unnecessary money on solar panels for what I have inverter wise.

.....also...just as a point mentioned by @Coulomb....the 105V inverter is most probably a PWM charge controller and not a MPPT, would that make any difference to your argument...just out of interest as you mentioned that it could damage my 105V inverter's MPPT controller as opposed to it probably being a PWM controller....difference?

 

1 hour ago, Anton Roothman said:

.....also...just as a point mentioned by @Coulomb....the 105V inverter is most probably a PWM charge controller and not a MPPT, would that make any difference to your argument...just out of interest as you mentioned that it could damage my 105V inverter's MPPT controller as opposed to it probably being a PWM controller....difference?

 

I did not see the comment about the PWM. Let's see if he adds anything but panels in parallel would suit a PWM better. I just thought a 105V solar controller might not be a PWM that's why I took it as a MPPT. 

11 hours ago, Anton Roothman said:

I have uploaded the same firmware onto both inverters that I found a link to in one of your posts.

Ah. Then I think that you're lucky that the MKS 1 74.xx firmware works in your MPPT inverter. I'm pretty sure that it would not have been running similar firmware to the other one.

11 hours ago, Anton Roothman said:

Currently I have two 555w Canadian solar panels connected in series on each inverter

Those seem to be 84 cell (14 x 6) panels; the inverters were designed back in the days of 72-cell panels (12 x 6). 84 cells is an unfortunate size for the 105 V max PV model. Two panels in series has too high a Voc in winter, and one panel (or any number all in parallel) doesn't have enough voltage to charge the battery.

2S of those panels is fine for the 145 V max inverter. But the PV on those (like mine) is rated at 60 A max, or 60 x 50 = 3000 W max. The general rule is not to "overclock" my more than a factor of 1.2, so that's 3000 x 1.2 = 3600 W, or about 6.5 555 W panels. So really you should limit the 145 V max PV inverter to 3 strings of 2S panels, or 3330 W nominal.

That leaves you with a lot of panels left over. My suggestion is to leave the 105 V max PM inverter without any panels, and connect an external MPPT for the other panels. Configure those panels, and size that MPPT, based on what you can get. Connect the external MPPT directly to the 48 V battery that powers the inverters. I would set the external MPPT bulk and float voltages to 0.1 V less than what you set in the inverter with panels; that way, it takes control of the last part of charging the battery, and won't get confused.

If you want, you could add other panels (say two 72-cell panels in series, or several such strings in parallel) to the 105 V max inverter, to a maximum of about 2500 Wp of panels. I don't think that you can "overclock" the PWM solar charger at all; it has no real way of "using only what it needs". I guess it could PWM even in the bulk stage, but I doubt that it's programmed to do that. In fact, I think that you should ensure that Imp for the array is under 50 A, even if that means less than 2500 Wp of panels. I have no practical experience with PWM solar chargers.