Disconnect a series panel in cold weather

[Mof]

I have a Midnite Classic 150, currently being fed by 2 panels with the following specs:
Pmax: 579W
Vmp: 40.9V
Imp: 14.16A
Voc: 48.9V
Isc: 15A

All good.

I would like to add a third panel, but will run into Voc issues when it is cold.

Attached is a PDF of the sizing tool output from Midnite Solar. It indicates the Classic150 will go into HyperVOC at <=17 Celsius. I will experience plenty of days warmer than this, and many more significantly colder than this.
Other than switching to a Classic 200(too expensive), my idea is to install a DPDT switch in the string so i can manually disconnect one of the 3 panels when the temperature puts the Classic 150 into HyperVOC.
Anyone tried this? Any pitfalls, advice, or wisdom?

Assuming this idea is workable, another question:
Lets say I have 2 panels connected on a cold morning. Later in the day the panels warm up in the sun to a point where the output of 3 panels would no longer exceed the 150V charge controller limit. Any issues just flipping the switch and reconnecting the 3rd panel? IE will the sudden jump in voltage harm the Classic150 controller?

 

[Crowz]

Parallel is the way to go when worried about over volting.

 

[n2aws]

Parallel is the way to go when worried about over volting.

I would have to agree with this. It's just not worth the risk of damaging your midnite classic because you got busy, dozed off, went to dinner, or otherwise just forgot to flip a switch.

 

[timselectric]

No
Stop
Please don't do it

Oh well, I tried.

 

[SamDeleted]

17 Celsius

Maybe do this if you only over to switch over below freezing (0c)

But almost anywhere in the world temperatures could drop below 17c without much warning , blowing up your MPPT

 

[DIYrich]

The HyperVOC will protect the inverter if you leave 3 connected accidentally. If Temps will warm up to drop the voc to 150v, then you can leave it connected.

If you are going to use a switch, consider a rapid shutdown device, and only switch when panels are off.

 

[Mof]

I would have to agree with this. It's just not worth the risk of damaging your midnite classic because you got busy, dozed off, went to dinner, or otherwise just forgot to flip a switch.

No
Stop
Please don't do it

Oh well, I tried.

Why?
A little more background to explain why I push back...

The Midnite classic has a feature called hyperVOC where it will go into protection mode when the 150V voltage limit is exceeded. When the Classic is in hyperVOC, it will not charge. VOC can get as high as 174V (=150V CC rating + 24V nominal battery voltage) before the controller is at risk of being damaged. See description here:

https://www.midnitesolar.com/pdfs/whyHyperVOC.pdf

With 3 panels as I propose, the temperature would have to drop to -43 Celsius before VOC reaches 174V (calculations below). So frying my controller is not a big concern for me. The reason I want to put the switch in is so I can take the controller out of protective hyperVOC on cold mornings and still get 2 panels charging. The switch is easier than unplugging/re-plugging MC4 connectors.

For anyone interested, math is as follows with 3 panels:

VOC at 25 Celsius: 146.7V
Panel Temperature Coefficient (Voc): -0.27 % / °C
HyperVOC max allowed(as per Midnite Solar): 174V

146.7 x 0.27% = 0.39609V/ºC = how much VOC increases per ºC drop
174 - 146.7 = 27.3V delta = how many volts VOC can increase from 146.7V before CC is at risk
27.3 / 0.39609 = 68.92 = how many ºC below 25ºC the temperature can drop before CC is at risk
25ºC - 68.92ºC = -43.92ºC = temperature, below which, the CC is at risk of damage.

 

[DIYrich]

You could have a temperature controlled switch. At 5am, it reads the temp. if it is below a set point, it flips the switch disconnecting the 3rd panel. If it is above the setpoint, it flips the switch to connect the 3rd panel. The you run that way all day, even if the temps increase into the operating range. You could set it for something like 10 degrees below 150v VOC, and hope it warms up during the day to allow the system to turn on.

I need to think why a zener diode with a high wattage resistor doesn't work.

 

[efficientPV]

Why not add a couple lower voltage panels.

You made the fatal flaw of getting the biggest panel you could find.

My panels never see open voltage, I have a diversion shunt regulator.

 

[Hedges]

HyperVoc should protect the SCC.
Why Midnight uses nominal battery voltage not actual, I'm not sure. To keep it simple, and relying on SCC actually handling slightly higher than 150V spec in case battery is drained below 24V in the morning?

Can you get by with letting SCC produce nothing until it warms up?

Instead of open-circuiting PV string and reconnecting minus the 3rd panel, consider using Y cables to branch before & after one panel, and use a string to short out the extra panel.

Open the junction box and see if you can move wire connection to skip one of the (3?) diode-bypassed sections of PV panel. Or short out one diode. Or add a switch to short the diode.

A switch making/breaking no more than 15A at 20V is more doable than open-circuiting 15A at 174VDC.

Why not add two more panels, wire 2s2p?
Still within Midnight 150's max output current into 24V.
Orient one 2s differently from other 2s and enjoy more hours production and reduced current.
Or if only 3 panels, 3p as was suggested. Add a fuse per string.

 

[Mof]

Why not add a couple lower voltage panels.

I have these ones, so want to make them work. It's the CS3W-445MB-AG
https://www.canadiansolar.com/wp-co...dian_Solar-Datasheet-BiHiKu_CS3W-MB-AG_EN.pdf

You made the fatal flaw of getting the biggest panel you could find.

They were inexpensive, So I *had* to buy them. The seller had a few pallets of them and abandoned his project.

My panels never see open voltage, I have a diversion shunt regulator.

I will research a diversion shunt regulator. Thank you.

 

[Mof]

HyperVoc should protect the SCC.
Why Midnight uses nominal battery voltage not actual, I'm not sure. To keep it simple, and relying on SCC actually handling slightly higher than 150V spec in case battery is drained below 24V in the morning?

Can you get by with letting SCC produce nothing until it warms up?

Yes, assuming it warms up enough to drop out of HyperVOC.

Maybe I just save myself some headaches and run 3S in the warm months, and 2S in the winter months. In which case the MC4 plug/unplug method will suffice.

Instead of open-circuiting PV string and reconnecting minus the 3rd panel, consider using Y cables to branch before & after one panel, and use a string to short out the extra panel.

Open the junction box and see if you can move wire connection to skip one of the (3?) diode-bypassed sections of PV panel. Or short out one diode. Or add a switch to short the diode.

Good idea. Thanks for that.

A switch making/breaking no more than 15A at 20V is more doable than open-circuiting 15A at 174VDC.

Why not add two more panels, wire 2s2p?
Still within Midnight 150's max output current into 24V.
Orient one 2s differently from other 2s and enjoy more hours production and reduced current.

Yes, this is the correct solution, but will require me building a new ground mount. I have room on my current ground mount for 1 more panel, that's why I was exploring this 3S switched idea. I have 6 panels total, but the other 3 were earmarked for something else. Perhaps I re-evaluate.

Or if only 3 panels, 3p as was suggested. Add a fuse per string.

 

[Hedges]

"J-BoxIP68, 3 diodes"

Shorting out one diode is simple.
If you use a switch, ought to have rubber seal over it. maybe plastic handle not metal handle, to reduce shock risk when rubber boot eventually fails. Better to KISS and just hardwire a short.

External Y cables to suitably rated and waterproof switch is simple but more expensive. Just Y cables plugged to short, done at night or with a cover over the panel to stop current. Maybe make mating MC4 connectors for weather seal when not shorting.

3P would appear to meet voltage requirements.

Can you get creative on ground mount to fit four panels, like mounting rails across existing rails?

 

[Hedges]

Be very careful relying on HyperVoc.
Midnight's engineers have done a great job on everything, from the early days of Trace to now. But operating modes with lithium batteries could defeat the protection.

I think the way HyperVoc works is by disabling switching of buck converter when over-voltage, so FET only sees Voc - Vbat. Once switching starts, the inductor kicks voltage down to battery (-), and FET sees entire Voc. That should work with typical lead-acid system, if SCC and inverter are separately fused to battery. If battery could become disconnected (or is over-discharged below normal voltage), then FET could see excessive voltage. Maybe SCC disconnected by itself, even without inverter to load it down, can't provide HyperVoc protection. It would have capacitors with 0V on the battery terminals.

With Lithium, BMS can disconnect battery. It would also do so to prevent charging below 0C, which could be a problem for you.

 

[Rocketman]

instead of adding one panel to go 3s and run into more costs and issues with the diversions…

Add TWO panels into a 2s2p arraignment. You will never have overvoltage issues, and you get even more power! If this causes you to over panel your array - you get the advantage of more power earlier.

The extra solar panel will be much cheaper than the SCC if the protection stuff ever fails.

 

[Mof]

instead of adding one panel to go 3s and run into more costs and issues with the diversions…

Add TWO panels into a 2s2p arraignment. You will never have overvoltage issues, and you get even more power! If this causes you to over panel your array - you get the advantage of more power earlier.

The extra solar panel will be much cheaper than the SCC if the protection stuff ever fails.

Yes. That is the correct solution and what I will do once I get around to building a new ground mount for the other 2 panels. 2S2P is nice since no combiner or fuses required. Realistically I will probably get to it next spring. My current ground mount is oriented for morning sun, so the new one will be oriented for afternoon sun.

Until that gets built and for the next few warm months (I'm in Ontario) I will run 3S. Come mid-October when the temperature really starts to drop, I will bypass one panel and run 2S for the winter.

Thanks everyone for their input and advice!

 

[DIYrich]

My panels never see open voltage, I have a diversion shunt regulator.

Shunt the power to a heater under the panels ....

 

[Hedges]

Yes. That is the correct solution and what I will do once I get around to building a new ground mount for the other 2 panels. 2S2P is nice since no combiner or fuses required. Realistically I will probably get to it next spring. My current ground mount is oriented for morning sun, so the new one will be oriented for afternoon sun.

Until that gets built and for the next few warm months (I'm in Ontario) I will run 3S.

Consider 3s || 3s, and maybe short out one diode in one panel per string.
With morning orientation and afternoon orientation, it could probably sustain near full 82A up to 28.7V (3.6V/cell), 2350W. While 6x 579W = 3474W, panels normally only produce 75% to 85% of STC ratings. And if strings are at 90 degree angle to each other I expect 0.7 times as high a peak. So probably never clips.

How much charge current do you want? I think 82A is within charge specs for most LiFePO4 cells, but only at nominal temperatures. There are some tables for various brands showing reduced current, like 0.15C rather than 0.5C, closer to freezing. With Classic, you should be able to add Whiz Bang Jr. battery shunt and it can regulate max charge current, while delivering additional power when used by loads. Of course there are also battery heaters.

 

[Mof]

I got the third panel mounted ~2 weeks ago. It is the horizontally mounted panel on the bottom. Where I am, current morning temperatures are anywhere between 8°C and 18°C. There does not seem to be a pattern when the Midnite Classic enters HyperVOC. For example, yesterday with temperature at 12°C it did not... highest pv voltage observed was 132V. The previous day, at around 16°C, it did enter hyperVOC... highest pv voltage observed was 153V around 8:15AM. In both cases there were clear blue skies, but panel were shaded due to surrounding trees.

 

[Jet]

I have these ones, so want to make them work. It's the CS3W-445MB-AG
https://www.canadiansolar.com/wp-co...dian_Solar-Datasheet-BiHiKu_CS3W-MB-AG_EN.pdf



They were inexpensive, So I *had* to buy them. The seller had a few pallets of them and abandoned his project.




I will research a diversion shunt regulator. Thank you.

Does the seller have any left. Could you get a 4th panel and series parallel (2s2p).