Mixing Kyocera KD205GX-LPU & Suntech STP170s-24/ab-1 with Flexmax 80

[yaug2]

Hello all,

As an avid RV'yer, I have developed an interest in solar panels so that I can go boondocking. Last year, I had the opportunity to buy 6 siemens sp75, which have a Vmp of 17.1 volts. They are connected in parallel to a "commercially customized" charge controller, and are charging a bank of 12v batteries of 460 amp/h.

Because I was looking at upgrading my charger to an MPPT one, I had the opportunity to buy a package which had 4 solar panels and the Flexmax 80. The package is 2 X Kyoceras KD205GX-LPU, and two Suntech STP170s-24/ab-1.

As I am trying the best way to hook them together, I realized that their Vmp was much different, even though their nominal voltage is 24v. Indeed, the Kyoceras have a 26.6 Vmp while the Suntech have a 35.2 Vmp.

Should I hook all 4 panels in parallel thus losing on the Suntech (which would be brought down to 26.6v) or should I wire the Kyoceras in Series, the Suntech in series, and then parallel them? The only reason I'd want to do that would be to minimize the voltage drop I am getting on my 60 ft 10awg wire run I have between the panels and the Flexmax, as it would cut it in half.

Also, what might be the most optimal way of using them along with the Flexmax?

I am also thinking of using the Aux output on the Flexmax to diverse a load to a small space heater when my batteries are full. How hard is that? I already have a solid state relay that is good for 25 amps AC...

Thanks in advance,

[Amy@altE]

The voltage of those panels is very different from each other. The Kyoceras are actually nominal 18V panels, the Suntechs are 24V. I would never run them together on an MPPT charge controller.
Kyocera 205
Maximum Power (Pmax): 205W (+5W/-0W),
Maximum Power Voltage (Vmpp): 26.6V
Open Circuit Voltage (Voc): 33.2V
Maximum Power Current (Impp): 7.71A
Short Circuit Current (Isc): 8.36A

SunTech 170
Maximum Power at STC (Pmax) 170Wp
Optimum Operating Voltage (Vmp): 35.2V
Open - Circuit Voltage (Voc): 43.8V
Optimum Operating Current (Imp): 4.83A
Short Circuit Current (Isc): 5.14A

[Sunking]

You have a bunch of panels that are not compatible. Choose the two highest voltage panels, wire them in series to the MPPT controller. Parallel th eother 4 with 18 volt VMP and use a PWM controller.

[yaug2]

The voltage of those panels is very different from each other. The Kyoceras are actually nominal 18V panels, the Suntechs are 24V. I would never run them together on an MPPT charge controller.
Kyocera 205
Maximum Power (Pmax): 205W (+5W/-0W),
Maximum Power Voltage (Vmpp): 26.6V
Open Circuit Voltage (Voc): 33.2V
Maximum Power Current (Impp): 7.71A
Short Circuit Current (Isc): 8.36A

SunTech 170
Maximum Power at STC (Pmax) 170Wp
Optimum Operating Voltage (Vmp): 35.2V
Open - Circuit Voltage (Voc): 43.8V
Optimum Operating Current (Imp): 4.83A
Short Circuit Current (Isc): 5.14A

Well that serves me well. I should have done my homework prior to buying them. The guy told me they were 24 volts, and that he had them wired together with the Flexmax. He also told me (stupid of me to believe him) that he was installing photovoltaic systems.

Anyways, I had look the specs up online, but I guess I got confused in between the Voc of the Kyoceras (33.2v) and the Vmp of the Suntech (35.2). My bad. Ah well.

[yaug2]

You have a bunch of panels that are not compatible. Choose the two highest voltage panels, wire them in series to the MPPT controller. Parallel th eother 4 with 18 volt VMP and use a PWM controller.

Ok, but if I do this, it means that the Siemens (17.1v) are going to bring down the Kyoceras (26.6v) to 17.1v, meaning I am loosing the difference (26.6v - 17.1v = 9.5v * 7.71amp = 73watts) so 73 watts per panels, I'd lose close to 146 watts out of the 410w they are supposed to produce at the peak.

On the other hand, if I hook the Suntech in parallel with the Kyoceras, the bottleneck becomes the Kyoceras at 26.6v and therefore it would bring down the Suntech from 35.2 down to 26.6v, a difference of 8.6v * 4.83amps = 41.5w or 83 watts lost for the two panels together. Now the amps may not be totally linear when the voltage is not optimal, but I suspect I may not lose as much this way.

In both cases, some panels have to adjust to the lower voltage, resulting in a loss of power.

Am I wrong? What am I missing?

Thanks in advance...

[Amy@altE]

The Siemens are actually better suited for this than the Kyoceras. The 6 Siemens and 2 SunTechs would max out the charge controller for a 12V battery bank. If you want to use the Kyoceras, get a second charge controller like the Midnite Kid.

3 parallel strings of 2 of the Siemens 12V panels in series, turning them into 24V 175W panels.
2 parallel strings of SunTech 170 24V panels.

Siemens 75W, 12 V nominal
17.0Vmp x 2 = 34Vmp in series
21.7Voc x 2 = 43.4Voc in series
4.4Imp
4.8Isc

Suntechs
Maximum Power at STC (Pmax) 170Wp
Optimum Operating Voltage (Vmp): 35.2V
Open - Circuit Voltage (Voc): 43.8V
Optimum Operating Current (Imp): 4.83A
Short Circuit Current (Isc): 5.14A

[SunEagle]

The Siemens are actually better suited for this than the Kyoceras. The 6 Siemens and 2 SunTechs would max out the charge controller for a 12V battery bank. If you want to use the Kyoceras, get a second charge controller like the Midnite Kid.

3 parallel strings of 2 of the Siemens 12V panels in series, turning them into 24V 175W panels.
2 parallel strings of SunTech 170 24V panels.

Siemens 75W, 12 V nominal
17.0Vmp x 2 = 34Vmp in series
21.7Voc x 2 = 43.4Voc in series
4.4Imp
4.8Isc

Suntechs
Maximum Power at STC (Pmax) 170Wp
Optimum Operating Voltage (Vmp): 35.2V
Open - Circuit Voltage (Voc): 43.8V
Optimum Operating Current (Imp): 4.83A
Short Circuit Current (Isc): 5.14A

Looks like a good wiring system for those 8 panels. He will need to get a combiner box with 5 circuit breakers.

[yaug2]

The Siemens are actually better suited for this than the Kyoceras. The 6 Siemens and 2 SunTechs would max out the charge controller for a 12V battery bank. If you want to use the Kyoceras, get a second charge controller like the Midnite Kid.

3 parallel strings of 2 of the Siemens 12V panels in series, turning them into 24V 175W panels.
2 parallel strings of SunTech 170 24V panels.

Siemens 75W, 12 V nominal
17.0Vmp x 2 = 34Vmp in series
21.7Voc x 2 = 43.4Voc in series
4.4Imp
4.8Isc

Suntechs
Maximum Power at STC (Pmax) 170Wp
Optimum Operating Voltage (Vmp): 35.2V
Open - Circuit Voltage (Voc): 43.8V
Optimum Operating Current (Imp): 4.83A
Short Circuit Current (Isc): 5.14A

Thanks! That's a good explanation for it. Curiously enough, I have looked at the past history of the system on the Flexmax 80, up to 130 days back actually, and I have seen peak wattage of 750 and even 800 watts, supposedly with these 4 panels. I find it surprising that given the mismatch between them, the Flexmax still reported these peak wattages. Even 800w escapes me, at the panels together would be rated for a total of 750 watts, which is usually under ideal conditions.

If anything, the mismatch would have brought down the expected 750 to say 600 watts, so the reported peak watts make no sense, unless he also had other panels along with the four I bought from him.

What do you guys think?

[yaug2]

Looks like a good wiring system for those 8 panels. He will need to get a combiner box with 5 circuit breakers.

How does that work? A breaker for each string within a combiner box? Where would it need to be located? If close to the battery, then it means I need 5 runs of cable (pairs) from the strings to the converter... that seems complicated and costly. Otherwise, you put the combiner on top on the RV, close to the panels?

I am really new at this, so any help is greatly appreciated.

Thanks,

[SunEagle]

How does that work? A breaker for each string within a combiner box? Where would it need to be located? If close to the battery, then it means I need 5 runs of cable (pairs) from the strings to the converter... that seems complicated and costly. Otherwise, you put the combiner on top on the RV, close to the panels?

I am really new at this, so any help is greatly appreciated.

Thanks,

Anytime you wire more than 2 sets of panels in parallel you are required to include an over-current protection (circuit breaker or fuse) for each string. With your 8 panels you will now have 5 strings wired in parallel (3 strings of 2 x 75watt panels in series and those 2 170watt Suntech panels).

The combiner box can be located anywhere between the panels and charge controller. The location is dependent on accessible (for maintenance or trouble shooting) and the wire size needed to meet Code for amp rating and <3% voltage drop.

[Amy@altE]

Use a Midnite PV6 with five 8A breakers in it. You'll need 2-hole strain reliefs for each pair of wires to get into the box.

[yaug2]

Thank you all, will try and do this when the weather allows me to work on my Siemens Sp-75 ,as they are still covered up in snow, and it still -25C here!!

In the meantime, will play around with the two Kyoceras and the 2 Suntech, and the Flexmax. I have tried to set up AUX - St. Relay, but I am not sure of the reference voltage. Is it in aborbing mode or in float mode that it seeks the difference the user puts in?

Did an EQ of my batteries yesterday, and it seemed to have helped them. Ran 130 watt load via an inverter for 3hours, and my 460 amp/h battery bank was still showing 12.56 when the load was gone. Not to bad for old Interstates U2200, at least 10Y olds!

Thanks in advance.