another newbie not wanting to get burnt

[Lndmvr]

I have 2 sunpower spr-e20-435-com panels, 75 volt, 435 watts. My quest is to run a 35 ft RV with a 24 volt deep well pump for water. I am thinking Of using a Flexmax 80. I expect to start with 2-12 volt 220 ah batteries in series with a 24v to 120 vac tripp light 2424 hardwire inverter into a breaker panel. New to this and I’m thinking 24 volt will be more efficient as I add battery strings. Might step down 24 to 12 volt for some lighting. Also thinking 12v-propane- ac refrigerator. I look around to figure the math but so many variables confuse me. I am just trying to determine if it will work. I did go to electrical vo-tech in the 70's but can't remember at this age. Again, I'm just starting out and looking for help. Got the panels free and as the saying goes, free gets expensive. Thanks

[Sunking]

Operate at the highest panel and battery voltage possible, and avoid parallel batteries.

The Flexmax 80 is a MPPT controller and has roughly the following maximum solar input power vs battery voltage.

1000 watts at 12 volts
2000 watts @ 24 volts
4000 watts @ 48 volts

Regardless of battery voltage you can run input voltage on the FM 80 to roughly 120 volts Vmp or 150 Voc. However if you hav enot bought the charge controller I highly suggest Midnight Solar 150 or 200. Much better product and more versatile.

[Lndmvr]

Operate at the highest panel and battery voltage possible, and avoid parallel batteries.

The Flexmax 80 is a MPPT controller and has roughly the following maximum solar input power vs battery voltage.

1000 watts at 12 volts
2000 watts @ 24 volts
4000 watts @ 48 volts

Regardless of battery voltage you can run input voltage on the FM 80 to roughly 120 volts Vmp or 150 Voc. However if you hav enot bought the charge controller I highly suggest Midnight Solar 150 or 200. Much better product and more versatile.

Thanks for the quick reply. After looking through the site I should add this,
I am in 33612, Florida at this time and not going more than 200 miles north. I have looked at midnite controllers and it's a tossup but I will buy the best. I was advised to put the panels in parallel and should have 11.94 amps to the controller. In series it would be 150 volts which looks to me to be upper limits. I expect day usage to be ok but am concerned about running 120 volt ac at night from the inverter. I expect to run water to a lifted tank and run a 12 volt gravity fed for water. I will read all I can about wire sizes, fuses etc before putting it all together. Nothing will be on continuously except for internet. I think I'm looking at 6 batteries tops when the time comes. I am wondering if I can make buss bars out of copper pipe crimped and soldered at termination points.

[Lndmvr]

Knife switch and buss fuse or breakers

I would like some input as to using buss type fuses compared to dc breakers. Trying to compare panels layouts and costs between premade and being able to layout my board as my needs change and grow. Thanks

[OceanArcher]

re: Fuses vs Breakers

Just a personal opinion here. I spent several years installing and operating an AC to DC system (480 input -- 200 vdc output at 3400amps). I'm partial to breakers (dc rated) for use in any solar system. Yes, they cost a little bit more initially, but over the lifetime of the system, they are much more cost effective than fuses ...

[Lndmvr]

before I pull the trigger

Merry Christmas. I went to midnites calculator but cannot input my configuration. Panels are Sunpower spr-e20-435-com times 2.
Pmax = 435
Vmp = 72.9
Imp = 5.97
VOC 85.6
ISC = 6.43

So 2 in parallel is around 870 watts. voltage should be 73, amps near 12 ( rounding here). The calculator won't accept 0 series string. I have the opportunity to get 2 more so I think double the former values but voltage the same. Will the classic 150 accept this into a 24 volt bank ( 2-12 volt, 220 ah marine) as a starting point? Future expectations are 2nd bank same as 1st. The 2 panel setup is for off grid camping in rv and the 4 panel setup is for when it's parked at my land. The setup will be mounted on the rear carrier when camping with panels on a hinged butterfly arrangement. I have weight concerns when towing. Thanks for any input. I believe this stays in the 2000 watt - 24 volt consideration. Output from batteries will go to 2400 watt inverter and 24dc to 12 volt dc converter for minor lighting and electronic items. Thanks ( so many more calculations to do!)

[Mike90250]

I'm not a midnight person yet (another 4 weeks) but I think that the 2 parallel, 72V panels will be just fine to charge a 24V battery bank. You should have plenty of headroom in both the Voc and watts departments.

[Lndmvr]

After looking at Sunkings battery calculator

So in my project the 24 volt x 2 panels looks correct. But to go to 4 panel setup it looks like I would have either go to 48 volt bank stepped down to 24 volt or separate the bank to 2 24 volt strings and add another controller. Is my thinking correct?

[Sunking]

With those panels you should operate them in parallel regardelss of battery voltage. At 72 Vmp you can operate upt to 48 volt battery with no issues. Depending on exactly which Midnite Solar controller you use will determine maximum panel wattage vs battery voltage. For your RV application the Classic 150 is your best option. For a Classic 150 operating @ 70 volt Vmp input at battery voltages maximan power input is:

1380 watts @ 12 Volt Battery
2700 Watts @ 24 Volt Battery
5022 Watts @ 48 Volt Battery

As for the question about breakers or fuses is economics and convenience. What is important is that whatever the over current protection device you use is rated for DC and voltage you will be operating at.

[Lndmvr]

Fusing parrellel panels

With those panels you should operate them in parallel regardelss of battery voltage. At 72 Vmp you can operate upt to 48 volt battery with no issues. Depending on exactly which Midnite Solar controller you use will determine maximum panel wattage vs battery voltage. For your RV application the Classic 150 is your best option. For a Classic 150 operating @ 70 volt Vmp input at battery voltages maximan power input is:

1380 watts @ 12 Volt Battery
2700 Watts @ 24 Volt Battery
5022 Watts @ 48 Volt Battery

As for the question about breakers or fuses is economics and convenience. What is important is that whatever the over current protection device you use is rated for DC and voltage you will be operating at.

I read after 2 panels in parallel each panels should then be fused individually. So i should just install fuses at each panel at the start?

[Sunking]

I read after 2 panels in parallel each panels should then be fused individually. So i should just install fuses at each panel at the start?

Incorrect there are no requirements to have any kind of over current protection for your application. Anyone who tells you differently does not know what they are talking about and should be ignored.

From a design POV you have two options.

1. Is to size the wire from the panels to safely conduct full fault current x 1.25 based on the cable insulation type. In your application say using MTW or SF1 cable insulation all that is needed is 12 AWG stranded.

2. Or using the same 12 AWG use a over current protection devices if you wish.

[Naptown]

Incorrect there are no requirements to have any kind of over current protection for your application. Anyone who tells you differently does not know what they are talking about and should be ignored.

From a design POV you have two options.

1. Is to size the wire from the panels to safely conduct full fault current x 1.25 based on the cable insulation type. In your application say using MTW or SF1 cable insulation all that is needed is 12 AWG stranded.

2. Or using the same 12 AWG use a over current protection devices if you wish.

HUH?
If he goes to three panels (read strings) in parallel he will need to fuse each panel.
With the 2 strings (2 panels in parallel) he needs to size the conductors (cables) at 156% of the short circuit current. These would be source or output conductors.

[SunEagle]

HUH?
If he goes to three panels (read strings) in parallel he will need to fuse each panel.
With the 2 strings (2 panels in parallel) he needs to size the conductors (cables) at 156% of the short circuit current. These would be source or output conductors.

I think what Sunking was refering to was that if he stays with just the 2 panels he does not need to add fuses now. Only if he expands to more than 2 panels.

[Naptown]

I think what Sunking was refering to was that if he stays with just the 2 panels he does not need to add fuses now. Only if he expands to more than 2 panels.

That would be correct however the OP was asking about adding another panel.
Even in this application he would need to size conductors to 156%

[Lndmvr]

Trying to clear this up

I think what Sunking was refering to was that if he stays with just the 2 panels he does not need to add fuses now. Only if he expands to more than 2 panels.

2 panels in parallel ---- furthest panel - 6 amp @ 75 volt, closest panel 12 amp @ 75 volt

4 panels in parallel------ furthest panel - 6 amp @ 75 volt, 3rd closest panel 12 amp @ 75 volt, 2nd closest panel 18 amp @ 75 volt, closest panel 24 amp @ 75 volt.
Lets consider 10 gauge wire so maximum fuse value is 30 amp. ( all figures rounded here for sake of simply figuring out the theory). And I believe I am protecting the WIRE not anything else.
So fusing at each panel figuring for the 4 panels inline at some time would be, furthest 6amp, 3rd closest 12 amp, 2nd closest 18 amp and nearest panel 24 amp. The 24 amp fuse would be as close to the Classic 150 ( which shipped today) as possible. When in 2 string then it should not matter what the fuses are because I am still fusing the WIRE at below its rated maximum. edit: but I would be using the closest 2 panels in the 2 string. ( 24 amp fuse)

Things that make you go hmmmmm.