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Must 3KVA PV1800 Solar Charging


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A colleague has one of those red 3KVA hybrid inverters (Must iteration I suppose) similar to the image but without the Must branding. I do not know if anyone here has had experience with them. The setup of the 24V system:

6x265W Suntech modules

2x 2x 200Ah Vision batteries

3KVA 2400W Inverter

He is currently off-grid and typical load 200-300W. During peak he runs a 1HP submersible pump for about 30min max.

The batteries are not holding charge and I suspect they are not hitting absorption which is strange because we are in a clear sky good solar hours period. The panels are in 3 strings of 2x series. I’ve heard of these Musts not actually giving their rated charging amps and that backup they work ok but not so ok for offgrid. Suggested fix include paralleling all the 6 panels to boost amps (+48A open). Is there any wisdom in this?

The inverter doesn’t display PV yield like the Axpert. Whats the best to calc this? Amps x system voltage or actual displayed panel voltage?

7DD8BADE-234B-4FFA-9B5E-93BD2879E4C7.jpeg

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40 minutes ago, Kalito said:

Must iteration

Axpert clone if I recall. "Illegal" in as much as it can be illegal in a country where such things are generally not enforced.

First thing to do is to find out if the solar charger is a PWM or a MPPT model. An easy way to do it if you cannot find the documentation, is to measure the voltage on the PV side while the unit is bulk charging. If that voltage is higher than the battery side, then it is usually MPPT.

Assuming it's an MPPT, another non-invasive test is to look at the Vmp (voltage at max power) on the back of the panel, work out the Vmp for the whole array, and then measure how far away the PV voltage is from the Vmp. If the MPPT is working properly the voltage will be around the calculated Vmp, in bright sunshine.

Finally, what remains is to put an amp meter in the charge line, ideally a Victron BMV, and record some actual values for the thing. If you could rig it up to a Raspberry Pi running Venus, it can log the details measured by the BMV to VRM (Victron's telemetry site), and then you have all the details you need for the whole day. Of course now we're talking a couple k's of expenditure :-) Way less than the cost of batteries though.

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1 hour ago, Kalito said:

The setup of the 24V system:

 

1 hour ago, Kalito said:

Suggested fix include paralleling all the 6 panels to boost amps (+48A open). Is there any wisdom in this?

Hi Kalito, if this is the correct MODEL: then there is wisdom in your suggested fix. I see this inverter has a PWM charge controller, so no reason to series two panels to charge a 24 volt system. but we have to make sure that the voltage the panel charge at is higher than the required charging volts for the batteries. 

Manual As info: https://d.mustpower.com/manual/420-00268-00-PV1800-PK.pdf

Second option is to install a Victron MPPT to get all the power from the panels to the batteries..

EDIT: I just checked a Suntec 265watt panel's rating and it is rated at 31V optimum operation voltage. I would connect all in parralel as suggested and see how big the improvement is. If it doesn't work , ill connect a separate MPPT to the system. 

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8 hours ago, Jaco de Jongh said:

The volts when all panels in parallel might be to low to charge the batteries sufficiently. Two in series will result in a lot of wattage wasted

Yeah but last week I was assured that MPPT is a waste of money... :-P

A 265W panel is going to be a 60 cell panel or larger. A 60 cell panel has a Vmp of around 30V. So it should be good enough for charging a 24V bank. It's definitely the better option compared to two in series (which will effectively kill half the power).

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1 hour ago, plonkster said:

Yeah but last week I was assured that MPPT is a waste of money... :-P

A 265W panel is going to be a 60 cell panel or larger. A 60 cell panel has a Vmp of around 30V. So it should be good enough for charging a 24V bank. It's definitely the better option compared to two in series (which will effectively kill half the power).

Thanks guys very insightful Sunday education much appreciated. Please shed more light on the wattage wasted for 24V  in this configuration of two in series but I assume its ok for 48V?  Yes they are 60 cells. To ask is wisdom -lol

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45 minutes ago, Kalito said:

Please shed more light on the wattage wasted for 24V

With your current setup you have 60Volts x 25.68 Amps (8.56 amps per string x 3) = 1540 Watts available. 

PWM: 

The PWM will charge the batteries at Bulk charge (assume) 27.5 volts x 25.68 amps = 706.2 Watts. 32.5 volts x 26.68 amps can not be used as the PWM can not do anything with it so you will loose 876 Watts. Less than 50% reach your battery's. 

If all panels were in parallel,  the PVM will have 30 volts x 51.36 Amps  (8.56 amps per string x 6) = 1540 Watts available. The PWM will be able to pass 27.5 volts x 51.3 amps = 1410 watts -  internal losses,  to the batterys . In this setup you will loose only 2.5 volts x 51.3 Amps = 128.25 Watts that the PWM can not use. 

The last option is in line with the max charge current of the Unit. 

A.PNG.d8508399dcb1486c45437978501610f0.PNG

MPPT:

The MPPT will convert the access voltage into amps and sent 1540 Watts (available power) / 27.5 volts @ 56 Amps - small internal losses to the battery.   

 

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15 hours ago, plonkster said:

First thing to do is to find out if the solar charger is a PWM or a MPPT model. An easy way to do it if you cannot find the documentation, is to measure the voltage on the PV side while the unit is bulk charging. If that voltage is higher than the battery side, then it is usually MPPT.

I've long wondered about PWM charge controllers. I know I never want one, but I wanted to know what they leave out in a PWM controller. I finally Googled it, and it seems to me that the essential difference is that the PWM model doesn't have a big inductor, so there is no "DC transformer" effect. It's like a variable resistor between the panels and the battery, but way lower loss. Once you have the inductor there, you can vary the "turns ratio" of the "DC transformer" to find the point where maximum power is achieved. Since this is "just software", it's nearly always done, so the choices are PWM or MPPT, even though theoretically you could have a converter with an inductor and a fixed "turns ratio". Actually, part of my solar charging is via an old Blue Sky Energy solar controller, and it has a trimpot inside to set the PWM ratio ("turns ratio"), so it's one of these rare "in between" devices.

https://www.hurricanewindpower.com/blue-sky-energy-solar-boost-3048l-charge-controller-without-display/

Anyway, my point is, surely with a PWM controller, it would be the current that is the same at the panels and battery, not the voltage?

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22 hours ago, Coulomb said:

what they leave out in a PWM controller

The inductor and the lower switch/flyback diode is also left out (not needed, the job of that switch is to harness the back-EMF from the inductor).

22 hours ago, Coulomb said:

current that is the same at the panels and battery, not the voltage? 

The PWM only turns on when the battery reaches absorption voltage, during bulk the PV is essentially latched to the battery and the voltage will be the same.

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2 minutes ago, plonkster said:

The PWM only turns on when the battery reaches absorption voltage, during bulk the PV is essentially latched to the battery and the voltage will be the same.

Even if that would result in much less current, due to being so far from Imp? Ah, but Imp and Isc are nearly the same. And if the voltage is constant, then Imp isn't the maximum current anyway. So these PWM charge controllers are always operating near to Isc. I finally get it, thanks @plonkster

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15 minutes ago, plonkster said:

One of the cleverest tricks I saw was with a MPPT for an electrical borehole pump. It had no inductor in the controller... what they do is use the windings inside the motor to get the same effect.

Electric Vehicle motor controllers, DC or AC, are the same. So there is a minimum inductance that the motor can have for any particular motor controller. If the inductance is too low, the current ramps too fast, control is poor, and the current will likely exceed specifications, with predictable results. 

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On 2018/09/30 at 4:48 PM, Kalito said:

The inverter doesn’t display PV yield like the Axpert. Whats the best to calc this? Amps x system voltage or actual displayed panel voltage?

I finally understand that question, it took a while to percolate through. It depends on which current you are using. Axperts report battery side current, not PV side current, so you need to multiply by battery voltage. If your inverter has a PWM charge controller, then the two currents would be the same. I would use battery voltage in that case, since PV voltage will be a square wave with battery voltage at the bottom and panel Voc at the top. Hopefully they are giving you the average solar charge current, not the peak charge current. These will be different during the absorb stage. 

If you had PV side current and an MPPT charge controller, then you'd needed to use PV voltage. In this last case, you'd get PV power, slightly higher than the solar charge power into the battery due to losses. 

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UPDATE:

I wrongly stated the inverter as PV1800 its PH1800 and the model is PH18-3K as per spec plate:

574662874_PH18-3KPK.jpg.0828cad09c4c63a86e099044943e6e64.jpg

In the original configuration (3 strings of 2x panels in series) the following were the displays at about mid-day:

BeforeAmps.jpg.a3fd99b684e152dbda0a60415c360827.jpgBeforeV.jpg.d1e444e9345b7d4f8e3b611dda06cd04.jpg

After the panels were all paralleled the current was 11amps more at 38A and the voltage started creeping upwards. I believe the problem is resolved.

1653679112_IncreasedAmps.jpg.5337081be7cc94eb1cf6f50c5f75af9b.jpg1672004204_IncreasingVolt.jpg.18552938f0d02933774e08d69df7f9e1.jpg

 

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On 2018/10/02 at 9:30 PM, Kalito said:

After the panels were all paralleled the current was 11amps more at 38A...

Err, 38 - 17 = 21, so a bit more than double the current. The "bit more" was possibly because it was later in the morning, closer to solar noon. [ Edit: so it looks like this is a clone of an Axpert KS, a PWM model. ]

The sticker on the side is quite different to the Axperts, and they've gone to some effort to make the LC Display somewhat different in detail as well. I wonder if that's an attempt to escape prosecution as a blatant clone, or whether they merely used the same general specifications and designed their own from scratch?

My feeling is that it's a thinly veiled attempt to hide the fact that it's a total clone. [ Edit: actually, this one seems to be a hybrid model, it seems to be a more or less total clone with the ability to grid-tie somehow added. ]

Edited by Coulomb
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  • 4 weeks later...
On 2018/09/30 at 8:48 AM, Kalito said:

A colleague has one of those red 3KVA hybrid inverters (Must iteration I suppose) similar to the image but without the Must branding. I do not know if anyone here has had experience with them. The setup of the 24V system:

6x265W Suntech modules

2x 2x 200Ah Vision batteries

3KVA 2400W Inverter

He is currently off-grid and typical load 200-300W. During peak he runs a 1HP submersible pump for about 30min max.

The batteries are not holding charge and I suspect they are not hitting absorption which is strange because we are in a clear sky good solar hours period. The panels are in 3 strings of 2x series. I’ve heard of these Musts not actually giving their rated charging amps and that backup they work ok but not so ok for offgrid. Suggested fix include paralleling all the 6 panels to boost amps (+48A open). Is there any wisdom in this?

The inverter doesn’t display PV yield like the Axpert. Whats the best to calc this? Amps x system voltage or actual displayed panel voltage?

7DD8BADE-234B-4FFA-9B5E-93BD2879E4C7.jpeg

You were lucky. Max PV is 60 V and you put series of two pannels, when Vo is more than 70 V. If It had risen absort stage with those series It would have blown...

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On 2018/10/02 at 12:14 AM, plonkster said:

One of the cleverest tricks I saw was with a MPPT for an electrical borehole pump. It had no inductor in the controller... what they do is use the windings inside the motor to get the same effect.

Yes, even a solar VFD with MPPT algorithm hasn't got an inductor. I would say:

If we have solar pannels in a side and a load, not a battery, in the other side, we only need a capacitor to track MPPT.

If we have a resistive load, current and Voltage signals are square.

If we have inductive load, Voltage Signal is square and Current Signal could become sinusoidal.

Edited by Javi Martínez
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3 hours ago, Javi Martínez said:

If we have solar pannels in a side and a load, not a battery, in the other side, we only need a capacitor to track MPPT.

Mmmmh, I had to to think about that for a while. But I think you are right. If you're not doing any voltage conversion, you're merely passing as much current as possible to the motor (up to some defined limit perhaps), then you can essentially just run the motor at Vmp, and then you need no inductor. 

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On 2018/10/27 at 11:03 AM, plonkster said:

Mmmmh, I had to to think about that for a while. But I think you are right. If you're not doing any voltage conversion, you're merely passing as much current as possible to the motor (up to some defined limit perhaps), then you can essentially just run the motor at Vmp, and then you need no inductor. 

I'm sure, solar VFD are VFD with direct access to DC bus.

Even, if you feed a VFD with PV power enough, tipically R-S terminals, or LN terminals, It works.

Edited by Javi Martínez
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