Please advise my 3S2P2S over-panel plan

coolbz

New Member
My current system components and setup:

MPPT voltage range 120v - 250v
Santan 250watt solar panel voltage range 30v - 36v
7S setup running around 150-240v and producing between 100watt to 1200watt during day hour depending on shade level. When there is partial shade on 2-3 panels, max watt is only around 200-300watt.

I want to remove watt bottleneck as much as possible by adding 5 more panels and re-arrange to a 3S2P2S topology (see illustration below, each +p- represent a panel).

Code:
         +p- +p-          +p- +p-          +p- +p-

-----|              |--|             |--|             |------

         +p-  +p-         +p-  +p-         +p-  +p-

By doing this, I hope when shade happen on one of those 2S branch, the other paralleled 2S will help to carry watt forward. Please let me know if you see any issues or have better ideas. I have to use my limited backyard space efficiently so shade is not avoidable.
 
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Bud Martin

Photon Sorcerer
What is VOC of the panel?
What is the SCC Max PV Input Voltage?
BTW, your graphic and topology for ' 3S2P2S topology' does not make sense.
 

coolbz

New Member
Panel VOC is 36v. SCC Max PV input volt is 250v. So in theory, doing 7S will exceed 250v, but in reality, I checked console and monitoring data, it never go above 240v. In fact, when the load is heavy, MPPT volt drops to 140v+.
 

Bud Martin

Photon Sorcerer
Once the panels are loaded (current being pulled) the Voltage will drop, but in the early morning with little Sun and with little current draw the PV Voltage will be high close to VOC, VOC also goes up in cold temperature so you have to factor in 10 ~ 15% headroom to not exceed max PV input, so 36V x 7 = 252V, add 10% to that = 277V, so you do not want to take a chance of damaging the SCC.
7S and it drops to 140V with heavy load? That is big drops. How much power do you see at that point?
Right now having odd number of panels does not fit well with the wiring, I have to think what you can do with 12 panels.
 

740GLE

Solar Addict
By keeping multiple panels in series you’ll still have issues with partial shading, just run 3p4s try and keep the panels that get shad in one series run.
 
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coolbz

New Member
By keeping multiple panels in series you’ll still have issues with partial shading, just run 3p4s try and keep the panels that get shad in one series run.Havong

4S will not reach the minimum 120v requirement. The panel's nominal 30 volt never realize under real load. I even tried with 6S, under load, it can easily drop below 120V and switch to grid all together.

I'm hoping by carefully mix S and P connection, it can deal with shade much better than one string of panels.
 

coolbz

New Member
Once the panels are loaded (current being pulled) the Voltage will drop, but in the early morning with little Sun and with little current draw the PV Voltage will be high close to VOC, VOC also goes up in cold temperature so you have to factor in 10 ~ 15% headroom to not exceed max PV input, so 36V x 7 = 252V, add 10% to that = 277V, so you do not want to take a chance of damaging the SCC.
7S and it drops to 140V with heavy load? That is big drops. How much power do you see at that point?
Right now having odd number of panels does not fit well with the wiring, I have to think what you can do with 12 panels.
Yes, by adding a 1000+ watt dishwasher, 7S can easily drop below 140 volt. The maximum voltage of my 7S so far is 241 volt. With any load, even just idle solar charger/inverter, it drops, which I think is how MPPT system works to find the max watt output.
 
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coolbz

New Member
I even thought of 6S2P connection, which also satisfy MPPT votage requirement , but it requires too many splits and wires between panels.
 

Bud Martin

Photon Sorcerer
Yes, by adding a 1000+ watt dishwasher, 7S can easily drop below 140 volt. The maximum voltage of my 7S so far is 241 volt. With any load, even just idle solar charger/inverter, it drops, which I think is how MPPT system works to find the max watt output.
Panel Vmp of 30V, so 30V x 7 = 210V, the SCC brings that down to 140V (20V per panel) with heavy load. I am not sure that is what to expect or not.
 

coolbz

New Member
Panel Vmp of 30V, so 30V x 7 = 210V, the SCC brings that down to 140V (20V per panel) with heavy load. I am not sure that is what to expect or not.
I think the way MPPT work is to try drawing different amp and see at what point solar system can generate max watt (watt = amp * volt). When amp draw is higher, voltage drops, when you short a solar panel, you reach that max nominal amp (8 amp for my panel), but that also bring down volt to zero. There has to be an optimal point that maximize watt, which is the goal of MPPT charger.

PWM charger does not have this algorithm so can not achieve MPPT's efficiency level.
 

Bud Martin

Photon Sorcerer
I think the way MPPT work is to try drawing different amp and see at what point solar system can generate max watt (watt = amp * volt). When amp draw is higher, voltage drops, when you short a solar panel, you reach that max nominal amp (8 amp for my panel), but that also bring down volt to zero. There has to be an optimal point that maximize watt, which is the goal of MPPT charger.

PWM charger does not have this algorithm so can not achieve MPPT's efficiency level.
I know that the MPPT will try to find the maximum power point, but the panel also has its own spec of Vmp and Imp; I never study to find out how they interact with each other (Panel and MPPT).
 

coolbz

New Member
I think some MPPT charger has software glitches including growatt. In the case of 6S panels, it can sometimes draw too much amp so voltage drop below it's lower limit which caused system to switch to grid. Too much to such switches do not seem good.
 

crossy

Solar Enthusiast
Once the panels are loaded (current being pulled) the Voltage will drop, but in the early morning with little Sun and with little current draw the PV Voltage will be high close to VOC, VOC also goes up in cold temperature so you have to factor in 10 ~ 15% headroom to not exceed max PV input, so 36V x 7 = 252V, add 10% to that = 277V, so you do not want to take a chance of damaging the SCC.

Take notice of this!!! ^^^

Never, ever, ever exceed the maximum input voltage of your MPPT, down this road lies the exit of the Magic Smoke and the entry of heartache.

The panels come up very rapidly to VOC as soon as it gets light even if no appreciable power is available. This is the danger time for your controller.
 

740GLE

Solar Addict
Why would the system need to have Voltage of the array above 120v? You should only need they array voltage above the battery voltage to initiate charging.

Either way stick to 2p6s to keep the strings simple, if two panels of one string are shaded your down to one string being useful.
 

rcrracer

Solar Enthusiast
Does it make sense to have one panel wiring configuration for summer/shoulder months and another for winter? Ex. 3s2p or 2s3p winter. Do people buy specific panels and quantities based on their preferred charge controller and/or latitude?
 

coolbz

New Member
Take notice of this!!! ^^^

Never, ever, ever exceed the maximum input voltage of your MPPT, down this road lies the exit of the Magic Smoke and the entry of heartache.

The panels come up very rapidly to VOC as soon as it gets light even if no appreciable power is available. This is the danger time for your controller.
I wonder what level of over voltage will cause permanent damage. From another thread, one person reported his growatt system was able to to give over-volt warning and cut off instead of burnt. It sounds reasonable.

Sales people may tell everyone it's prohibited and will cause absolute damage because they don't want to take any chances for nothing.

We are talking about a system capable of handling 3kw constant load and 6kw instant load at 120-250 volt range. I cann't imagine the electronic components are designed so that it fries at 251v. It's not that the wire had a spike of 1000v.
 

coolbz

New Member
Why would the system need to have Voltage of the array above 120v? You should only need they array voltage above the battery voltage to initiate charging.

Either way stick to 2p6s to keep the strings simple, if two panels of one string are shaded your down to one string being useful.

I guess 120v min volt is by-design. It is a 48v system so that old rule of SCC only need be a few volts above battery is no longer hold. Maybe true for PWM but not true for newer MPPT.

I'm debating between 2p6s and the more complicated 3s2p2s. There is just too many shade through the day at various points. There is so many pro and cons between lower volt vs higher volt solar systems.
 

740GLE

Solar Addict
I think the assumption is that full sun could last solid 2-3 hours at max charging current, if the batteries topped up and no longer charging, MPPT will throttle current draw back and V PV input will creep up, thus you need to handle high Voc for hours on end. Also factor Voc of a system is temp dependent, at -20f you’ll be wayyy higher.

Also simpler, Current heats up but voltage flashes over, sparky sparky!!
 

coolbz

New Member
Does it make sense to have one panel wiring configuration for summer/shoulder months and another for winter? Ex. 3s2p or 2s3p winter. Do people buy specific panels and quantities based on their preferred charge controller and/or latitude?

I bought these panels mostly because it is a super great deal at $50 per 250watt. Then I picked 48v solar charger system because of the wiring simplicity and energy transmission efficiency due to higher MPPT voltage. But it apparently suffers from shade more than lower voltage parallel system.

No I can not use anything lower than 5S because MPPT won't get triggered to charge at lower volt.
 

coolbz

New Member
I think the assumption is that full sun could last solid 2-3 hours at max charging current, if the batteries topped up and no longer charging, MPPT will throttle current draw back and V PV input will creep up, thus you need to handle high Voc for hours on end. Also factor Voc of a system is temp dependent, at -20f you’ll be wayyy higher.

Also simpler, Current heats up but voltage flashes over, sparky sparky!!
I do plan to drop from 7s to 6s going into fall/winter. And maybe focus on a few hours per day, when shade is minimum, then over-panel to remove bottleneck.

My system has no battery yet. So far it has cut my grid power usage signficantly.
 
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