PWM Confused, please help!

[Jchance]

About a year and a half ago I installed 200 watts of solar panels on my RV roof, connected them to the pre-wire input on the roof, then terminated them to a PV 16amp breaker in a compartment. I Never got around in finishing the project. We rarely boondock and don’t have an inverter and I never purchased a charge controller, until today! I’m about to install a 12V fridge directly to my lead acid battery with a cutoff switch and an inline 20 amp fuse. So I figured I should finish connecting the solar system and take advantage of having them charging my battery. I know 200w is not much, but it’s a start. After spending over $1k on the fridge I’m Trying to keep my budget down, so I purchased a PWM charge controller. My system specs are as follows.

The panels I installed are 4- 50 W sun power flexible panels, 2 pairs in series, (50W x 2=100W), then in parallel, (100W x 2=200W), to the breaker. The specs on the panels are as follows;
Voltage (VMP) 17.6 V
Current (Imp). 2.80 A
Open circuit voltage (VOC). 21.5 V
Short circuit current (Isc). 2.95 A
Max fuse. 15. A

The charge controller I just purchased is an HQST 20 amp 12/24V controller. The sped are;
Rated Working Voltage: 12V/24V
Solar Input Voltage: <55V
Rated Working Current: 20A
Float Charging Voltage (adjustable): 13.8V; ×2/24v
Over Voltage Protection: 17V; ×2/24v
Over-discharge Recovery Voltage: 12.6V; ×2/24v
Charging Recovery Voltage: 13.2V; ×2/24v
Max. Voltage At The Battery End: <35V
No Load Loss: ＜10mA /12V; <12mA/24V
Temperature Compensation: -3mV/°C /2V

My question is; is it ok to leave the 2 sets of in series panels in parallel to the controller?
I realize this type of setup is best for a MPPT controller but again, I’m trying to keep the budget down. I read wills book on installing RV solar systems about 2 years ago, but I misplaced it! So I only have been internet searching info.

I have a basic understanding of adding up and not adding up the different voltage’s & amperage’s as it relates to series or parallel, I’m just not sure if I’m going to do any harm or if I’m going to waste the amount of power I’m harvesting from the sun to my system by the way I’m installing everything with a PWM controller?

Any and all advice greatly appreciated by this learning new-B!

 

[sunshine_eggo]

My question is; is it ok to leave the 2 sets of in series panels in parallel to the controller?

No. All 4 panels must be in parallel. PWM controllers work by shorting the panels to the battery thus forcing the panels to operate at battery voltage. If you have Vmp = 35V, that gets shorted to ~14V, so rather than getting 35V * 2.80A = 98W per 2S string, you get 14V * 2.80A = 39W per 2S string.

Thus a 2S2P array will only get you 78W.

A 4P array will get you 156W. An MPPT would work either way 2S2P, 4P or possibly even 4S to get the full 200W potential output.

The 55V solar input rating of the PWM is simply what it can handle, not what you should shoot for.

You will need a 5-15A MC4 fuse on the (+) lead of each panel before you put them in parallel.

 

[Jchance]

Thank you very much for your help!

Understood. So if I left my panels connected as they are, and install a 20 amp MPPT controller, I’ll be ok?

Also are the fuses necessary at the end of each panel, even though I have a PV breaker at the end of the run, (before the controller)?

I did find a 20 amp EPEVER MPPT, but it’s double the price of the PWM. There are cheaper MPPT’s, but I remember reviews saying their not really MPPT’s, their junk !

Again, your response and guidance is greatly appreciated.

 

[Jchance]

5-15A MC4 fuse

Also if the fuses are necessary on my PWM setup, (after changing panels to all parallel connections), you said 5-15A MC4, one on each end of the + of the panel lead, that’s 4 panels/fuses, what’s the 5th for? Spare?

And finally, if I leave my current wiring configuration, and I need the fuses, where and how do they get connected?

Thanks again!

 

[sunshine_eggo]

Thank you very much for your help!

Understood. So if I left my panels connected as they are, and install a 20 amp MPPT controller, I’ll be ok?

Yes. 2S2P would be fine, and you would have the potential to harvest the full 200W.

Also are the fuses necessary at the end of each panel, even though I have a PV breaker at the end of the run, (before the controller)?

On a 3P or more array, 1 fuse per string. On a single string or two strings in parallel, no fuses/breaker is needed.

I did find a 20 amp EPEVER MPPT, but it’s double the price of the PWM. There are cheaper MPPT’s, but I remember reviews saying their not really MPPT’s, their junk !

Almost always the case.

Also if the fuses are necessary on my PWM setup, (after changing panels to all parallel connections), you said 5-15A MC4, one on each end of the + of the panel lead, that’s 4 panels/fuses, what’s the 5th for? Spare?

Spares never hurt, but even the 5A is over spec. Unlikely to ever blow unless SHTF.

And finally, if I leave my current wiring configuration, and I need the fuses, where and how do they get connected?

You technically don't need fuses/breakers on a 2P array, but if you used them a single fuse per string anywhere in the (+) side.

 

[Jchance]

(On a 3P or more array, 1 fuse per string. On a single string or two strings in parallel, no fuses/breaker is needed)

Ok, I’m a little confused. Let me talk myself through this, and please correct me if I’m wrong. If I purchase a 20amp MPPT, keep my current wiring setup, (2 pair of 2/50Wp in series, that is then connect in parallel, I would only need 1-15amp fuse on the + of “each pair” before they go into the parallel combiner, which goes into my PV 15amp breaker?

 

[sunshine_eggo]

(On a 3P or more array, 1 fuse per string. On a single string or two strings in parallel, no fuses/breaker is needed)

Ok, I’m a little confused. Let me talk myself through this, and please correct me if I’m wrong. If I purchase a 20amp MPPT, keep my current wiring setup, (2 pair of 2/50Wp in series, that is then connect in parallel, I would only need 1-15amp fuse on the + of “each pair” before they go into the parallel combiner, which goes into my PV 15amp breaker?

On a 2P2P array, you don't NEED any fuse or breaker at all anywhere between the panels and the MPPT.

Fuses/breakers exist to protect the wires. Single strings or two strings in parallel can't generate enough current to overload the wires. 3P or more can produce enough current to exceed the wires.

If you already have a 15A breaker between the parallel adapters and the MPPT, it's not needed, but there's no harm having it. You do not need to add any additional fuses.

 

[Jchance]

On a 2P2P array, you don't NEED any fuse or breaker at all anywhere between the panels and the MPPT.

Fuses/breakers exist to protect the wires. Single strings or two strings in parallel can't generate enough current to overload the wires. 3P or more can produce enough current to exceed the wires.

If you already have a 15A breaker between the parallel adapters and the MPPT, it's not needed, but there's no harm having it. You do not need to add any additional fuses.

Great info! Now I fully understand, you explained it perfectly, Thanks. What about a fuse/breaker between the 20amp solar controller and the battery? I found through a little research it’s recommending 25 A. But I’m not sure if that’s correct?

 

[sunshine_eggo]

Great info! Now I fully understand, you explained it perfectly, Thanks. What about a fuse/breaker between the 20amp solar controller and the battery? I found through a little research it’s recommending 25 A. But I’m not sure if that’s correct?

Wire should be sized for 1.25X the maximum current. Very long wires may require more to address losses from wiring resistance.

Fuses/breakers should be sized for 1.25X the wire rating.

Folks often take shortcuts. 10 awg is good for 30A. MPPT might produce 20A. To avoid nuisance blow/trip, you go 1.25X 20A even with 30A wire, so 25A breaker is fine. In both cases you have a wire margin and a fuse/breaker margin.

 

[Goboatingnow]

I never fuse small to medium solar arrays. I always pull wire capable of handling Isc.

 

[sunshine_eggo]

I never fuse small to medium solar arrays. I always pull wire capable of handling Isc.

So, if you have 50A going to the MPPT from 5-6 strings, you use 6awg in EACH of the 5-6 strings? What do you do about the Panel wires themselves (usually 10-12awg)? Do you open the junction box and replace?

Or is the answer to the above "no," and you are just being boastful about doing it wrong thinking you're doing it right?

 

[Jchance]

Wire should be sized for 1.25X the maximum current. Very long wires may require more to address losses from wiring resistance.

Fuses/breakers should be sized for 1.25X the wire rating.

Folks often take shortcuts. 10 awg is good for 30A. MPPT might produce 20A. To avoid nuisance blow/trip, you go 1.25X 20A even with 30A wire, so 25A breaker is fine. In both cases you have a wire margin and a fuse/breaker margin.

Thanks, yes I am using 10 AWG and the run is under 5’, more like 4’

Thanks again for all your advice and information and help. I truly appreciate it and appreciate forms like this very much. I’m on other types of forms And try to help people with my knowledge and experience as well. I always believe in giving and not just taking. Whenever I can help someone I go out of my way to make sure I do so, and you sir, seem to do the same. Thank you again.

I’ll follow up if I need to or if something else Pops up in my spinning mind!

 

[Goboatingnow]

So, if you have 50A going to the MPPT from 5-6 strings, you use 6awg in EACH of the 5-6 strings? What do you do about the Panel wires themselves (usually 10-12awg)? Do you open the junction box and replace?

Or is the answer to the above "no," and you are just being boastful about doing it wrong thinking you're doing it right?

Yes I pull big wires.

 

[sunshine_eggo]

Yes I pull big wires.

And you replace panel wires with bigger ones?

 

[Goboatingnow]

And you replace panel wires with bigger ones?

The panel wires to the intermediate junction boxes are capable of handling ISC of the panels they come from.

 

[sunshine_eggo]

The panel wires to the intermediate junction boxes are capable of handling ISC of the panels they come from.

And here we have the key piece of information indicating you have no idea what we're talking about or why, and your input is valueless.

Thanks for stopping by.

 

[Goboatingnow]

And here we have the key piece of information indicating you have no idea what we're talking about or why, and your input is valueless.

Thanks for stopping by.

Abuse like that should get you banned.

As a professional EE I know where all my currents run and the fault currents issues with PV.

 

[sunshine_eggo]

Abuse like that should get you banned.

Aw.

As a professional EE I know where all my currents run and the fault currents issues with PV.

But you clearly don't know the rules on over-current protection for arrays and need to make irrelevant comments.

 

[Goboatingnow]

Aw.



But you clearly don't know the rules on over-current protection for arrays and need to make irrelevant comments.

I’m aware of the NEC code regarding panels. It didn’t apply in my case anyway.

 

[sunshine_eggo]

I’m aware of the NEC code regarding panels. It didn’t apply in my case anyway.

So just posting to be boastful and irrelevant. Gotcha.