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What happens to charge controller when maximum voltage exceeded?

fafrd

Solar Wizard
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Aug 11, 2020
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I’ve got an MPPT charge controller rated for 55V maximum input voltage.

The panels I’m considering have a 50V Voc @ 25C and an 0.27V TC factor.

The average coldest annual temperature where I live is 39F (3.9C).

So on a rare morning that drops all the way down to 39F, Voc will be higher by 21.1 x 0.27V = 5.7V or 55.7V total.

So my question is, if input voltage to an MPPT controller very occasionally exceeds it’s rating by a little bit (0.7V in this case), will it permanently damage the controller?
 
We try to absolutely obey the maximum spec. It could potentially cause damage above that. Either blow a capacitor or a transistor.
0.7V, about 1%, is so little it seems unlikely to be a problem.

"average coldest annual temperature"
What matters is actual temperate it experiences. Safer to use coldest temperature on record, not average.
Your location may vary from where the published value was recorded.

0.27V/degree for a 50V panel is slightly above 0.5%. I'm used to seeing 0.4% or 0.3%

Some "24V" panels would be about 44Voc or 45Voc. Yours at 50Voc would be a few more cells. You can find some that are low enough to be no concern.
 
The only way to know how much headroom is available is to disassemble and examine the circuit. It is very unlikely that a few volts will have any impact at all.
 
We try to absolutely obey the maximum spec. It could potentially cause damage above that. Either blow a capacitor or a transistor.
0.7V, about 1%, is so little it seems unlikely to be a problem.

"average coldest annual temperature"
What matters is actual temperate it experiences. Safer to use coldest temperature on record, not average.
Your location may vary from where the published value was recorded.

Safer, sure, but not really realistic. I can count on one hand the number of days it has dropped into the 30s over 25 years where I live, let alone the into the 20s (20F is the coldest on record).

I suppose as a fall-back, I could always just disconnect the array on any days expected to drop below 44F...

0.27V/degree for a 50V panel is slightly above 0.5%. I'm used to seeing 0.4% or 0.3%

Some "24V" panels would be about 44Voc or 45Voc. Yours at 50Voc would be a few more cells. You can find some that are low enough to be no concern.

Oh, I know I can fit other panels to avoid this concern. I’m looking at 450W or 500W 144 Half-cut cell panels (equivalent in size to 72-cell) which push voltage levels to the upper-limit of my already-purchased cheapo 30A MPPT charge controller.

I’ll eventually get a much more capable 80A charge controller which will have 150V voltage tolerance, but I’m trying to understand whether I can get something working more quickly using this existing controller.

it only cost me $30 and it’s not the end of the world if it gets damaged, so between being ready to risk it and being ready to shut down the array ahead of (unusually) cold snaps, sounds like I shouldn’t be terribly concerned.

And if there is typically any margin at all built into these voltage limit specs, 55.7V may not cause any damage.
 
The only way to know how much headroom is available is to disassemble and examine the circuit. It is very unlikely that a few volts will have any impact at all.
Not worth the trouble of disassembly. If 1 or two volts over the maximum once or twice a year is unlikely to cause failure, I should be OK, thanks.
 
Not worth the trouble of disassembly. If 1 or two volts over the maximum once or twice a year is unlikely to cause failure, I should be OK, thanks.
If your unnamed SCC only cost $30 are you sure it is really MPPT or only PWM?
 
If your unnamed SCC only cost $30 are you sure it is really MPPT or only PWM?
You tell me: https://hqsolarpower.com/content/HCC30MPPT-G1-Manual.pdf

I’m pretty sure that design has been discontinued so I got it for cheap on eBay as they sluffed off the old inventory getting ready for the new.

I jumped on that after having a look at those little blue 10A ‘MPPT’ charge controllers from Amazon (most of which turn out to actually be PWM).

Havn’t actually tested it yet but it is waaay better built than those pack-of-playing-cards sized controllers and reviews indicate that it is a true MPPT...

Was originally pulling together bits and pieces to have some backup power available during extended fire-related grid outages but now planning a larger, more permanent off-grid install (hence why I’m interested to see if I can start to play around with this little 30A charge controller while deciding what 80A controller I eventually want to get.

Or if this little 30A model proves to work well, I could keep it as part of the final system and add a second 50A MPPT for the second string (since I will be dealing with a bit of shade on several panels).
 
What's likely to be the hottest temperature where you live?

HEAT is your enemy when you're VOC is too close to your chargers voltage input rating.
 
it only cost me $30 and it’s not the end of the world if it gets damaged, so between being ready to risk it and being ready to shut down the array ahead of (unusually) cold snaps, sounds like I shouldn’t be terribly concerned.

Wonder what would fail ...
Capacitor shorts? (PV voltage goes to zero)
Transistor shorts? (battery voltage goes to PV voltage)

I would order a couple spares. You'll likely be needing them?. ;)

Charge controllers?

Or batteries?

I try to think of the worst thing that could happen.
It's my job.
:devilish:

Better have a fire extinguisher nearby!
? ?
 
There's most likely some voltage derating. A static 56V voltage will not damage the mosfet. But switching mosfet OFF creates inductive spikes which causes avalanche current that can now destroy the mosfet.
 
What's likely to be the hottest temperature where you live?

HEAT is your enemy when you're VOC is too close to your chargers voltage input rating.
Can you explain?
I think you may have this backwards.

Or possibly what you are saying is that heat is the enemy when your Vmp is too close to your chargers minimum voltage input rating?
 
I have thought about this. You can buy zener diodes rated to 10 amps.
If you had say a 50v zener diode across the scc input current would flow at 50v.
That could eliminate burning out the scc.
Maybe someone has the energy to work it out.
 
Can you explain?
I think you may have this backwards.

Or possibly what you are saying is that heat is the enemy when your Vmp is too close to your chargers minimum voltage input rating?

Usually for MPPT the maximum voc rating is given for minimum environmental operating temp.

At 25c the rating usually is considerably less, say 90%.

The hotter the environmental temp the less you have to go to stay safe.

If I've got this backwards, then someone has written my product manuals wrong and HEAT is my friend :)
 
I have thought about this. You can buy zener diodes rated to 10 amps.
If you had say a 50v zener diode across the scc input current would flow at 50v.
That could eliminate burning out the scc.
Maybe someone has the energy to work it out.

I've thought about that too, possibly using zener and transistor to pull down the voltage. Zener by itself has limited power handling, also series resistance giving an I/V response that would be a poor clamp for variable current.
A considerable amount of power has to be dissipated to lower the voltage slightly. It would take full PV voltage at some moderate current so considerable heatsinking required.

A lower power circuit could be implemented that carries full current, is held in saturation normally (low voltage drop) but either adds a few volts drop (burns 10's of watts) or goes open circuit when voltage goes too high.

I think the SCC with "Hyper Voc" feature that tolerate higher voltage use this approach, have a transistor or relay to disconnect the PV input.

Another implementation would be crowbar - if voltage exceeds threshold it clamps PV output to ground. That carries Isc but doesn't dissipate much power if MOSFET, Vds is low. However, it needs a source of voltage to keep Vgs about 10V. An SCR would remain latched until current stopped flowing (sun goes down or PV disconnected by switch) and would dissipate power based on its voltage drop.


I contemplated one of these when I found panels I had didn't fit my SCC specs, but decided I'm better off getting other panels with suitable voltage range. (also dimensions better fitting my mobile application.)
 
Usually for MPPT the maximum voc rating is given for minimum environmental operating temp.

At 25c the rating usually is considerably less, say 90%.

The hotter the environmental temp the less you have to go to stay safe.

If I've got this backwards, then someone has written my product manuals wrong and HEAT is my friend :)
My MPPT will be in the basement at 65F even when my panels on the roof are well over 100F.

My only concern is whether the early-morning Voc of my panels can increase to levels that could damage my MPPT on rate exceptionally-cold days.

Additional protective circuitry is a good idea but not worth the trouble in this case (I’d rather just switch the system off on historically cold days or purchase another MPPT controller rated for higher voltage).
 
My only concern is whether the early-morning Voc of my panels can increase to levels that could damage my MPPT on rate exceptionally-cold days.

I'm not picking up what you're putting down.

Your VOC will be at it's highest in full sun. Your MPPT will be happy so long as your stay within the manufacturers specifications.
 
Okay thanks

Today I am the fool. Thanks for setting me on the right path, It never occurred to me that a panel makes higher than it's rated voltage when the temp is less than 25c
 
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