Author Topic: Panel Vmp required for a 24 volt battery pack  (Read 117 times)

harryn

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Panel Vmp required for a 24 volt battery pack
« on: August 08, 2019, 03:36:06 PM »
Hi, we routinely build mobile power systems using RV size solar panels, AGM batteries, and a solar charge controller.  Sometimes we have control of the panel selection, but often not.

For various reasons we have been using another brand but would like to consider switching to the kid.

One of the challenges of the other brand controller is that it requires a Vmp at the controller to be at least 34 volts and more ideally 35.

Since the panels we typically use are nominal Vmp = 18 volts in good weather, and then there can be voltage drops due to wire lengths (sometimes 30 - 50 ft each side), sometimes the voltage is just not high enough to get the controller to run.

Of course in cold weather, AGM battery charge  voltages can also rise, which can complicate life even more.

Question - does anyone know what the required voltage delta ( Vmp - Vbattery) is for the kid controller?

Alternatively, for a 24 volt system,  the kid panel input voltage minimum ?

I read through the manual but missed seeing this information.

Thanks
« Last Edit: August 08, 2019, 03:43:27 PM by harryn »

boB

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Re: Panel Vmp required for a 24 volt battery pack
« Reply #1 on: August 08, 2019, 04:38:33 PM »
As I remember, the wakeup voltage for the Kid is the same as the Classic (?)

If so, that would be battery voltage PLUS 1/3 Vbatt and then minus 1.0 volt.

So, if the battery voltage was presently 24.0 V, the Voc of the panel would have to be,

24 + 24/3 - = 24 + 8 - 1 = 31 volts.

So, if your Voc is 2X18V = 36 volts, that should work fine in that case.

Typically the MPP voltage will be around 80% of Voc so for the 31Voc case, that might be 24.8 volts which is really on the edge but JUST enough to charge and as the battery gets charged, it will require less current.   This is probably why they want Vmp to be 34 volts at least which is a better spot to be at if possible.

You MAY be able to use a PWM style controller like the Brat just fine in a case like this too.



K7IQ

harryn

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Re: Panel Vmp required for a 24 volt battery pack
« Reply #2 on: August 08, 2019, 08:04:49 PM »
Thank you, that is very helpful.

The challenge we face is that our products are used by people in a wide variety of situations, and we have limited influence on it, just suggestions.

It is common to be 2, 3, 4, .....6 panels, 1 string, 2, etc.

Some of our products are 24 volt, some are 48 volt.

I am attempting to simplify our production by using fewer variations to stock and deal with changes with software settings.

I attempted this once before, but had some challenges with mounting that I hope to overcome this time.
« Last Edit: August 08, 2019, 08:08:42 PM by harryn »

harryn

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Re: Panel Vmp required for a 24 volt battery pack
« Reply #3 on: August 09, 2019, 11:48:02 AM »
Just checking something

On a cold day, an AGM battery charge voltage can be 30 volts.

(30 volts) + ( 30/3) - 1 volt = 39 volts Vmp?

boB

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Re: Panel Vmp required for a 24 volt battery pack
« Reply #4 on: August 09, 2019, 06:48:26 PM »
Just checking something

On a cold day, an AGM battery charge voltage can be 30 volts.

(30 volts) + ( 30/3) - 1 volt = 39 volts Vmp?

No, that 39V would be the PV open circuit voltage, Voc necessary for the CC to turn on.

The rough-ish estimate for Vmp would be about 80% of that so, about  31.2 volts

I used to see around 75% of Voc for Vmp but recent years I have seen the ~80% Voc typically.

But as you can see, 31.2V Vmp is kind of close for 30V BUT on the good side, the battery is most likely going to be almost fully charged at that 30V so the current shouldn't be all that high I wouldn't think.  You might have some other current draws but the battery should be good (educated guess)

This is why a higher PV voltage than the battery is better...  Not stupidly high of voltage, but higher like one nominal voltage or two.  This can also help if there is partial shading on the PV array where the Vmp is dropped even further.

K7IQ

UpNorthMan

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Re: Panel Vmp required for a 24 volt battery pack
« Reply #5 on: August 12, 2019, 10:20:19 AM »
If I have my information correct, the colder your panels are. The higher voltage they will produce also. Someone please correct me if I am wrong. That is the reason to double check your configuration with the sizing tool. To prevent over voltage at your lowest temperature.
Ed

harryn

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Re: Panel Vmp required for a 24 volt battery pack
« Reply #6 on: August 16, 2019, 12:46:35 AM »
Just checking something

On a cold day, an AGM battery charge voltage can be 30 volts.

(30 volts) + ( 30/3) - 1 volt = 39 volts Vmp?

No, that 39V would be the PV open circuit voltage, Voc necessary for the CC to turn on.

The rough-ish estimate for Vmp would be about 80% of that so, about  31.2 volts

I used to see around 75% of Voc for Vmp but recent years I have seen the ~80% Voc typically.

But as you can see, 31.2V Vmp is kind of close for 30V BUT on the good side, the battery is most likely going to be almost fully charged at that 30V so the current shouldn't be all that high I wouldn't think.  You might have some other current draws but the battery should be good (educated guess)

This is why a higher PV voltage than the battery is better...  Not stupidly high of voltage, but higher like one nominal voltage or two.  This can also help if there is partial shading on the PV array where the Vmp is dropped even further.

Thanks for the info.

Yes, the challenge I get into is that on conversion vans, the panels are mounted facing "up".  On cloudy winter days the van owners need to capture every possible electron so it is critical that the controller activates as early in the day as possible and stays on as late as possible.   This is regardless of the battery state of charge. 

Since at least  30 watts is always being consumed essentially 24x7 , the max charging capability possible is needed, even during AGM finishing stages.