looking at 12 vs 24 volts

ikk

Adventurer
I am currently debating on two different solar panels both are Semi-flexible monocrystalline panels. I plan on an MPPT controller. I have a 225 ah battery setup. I plan on using the 225ah battery for emergency home use, and a smaller system for camping. I was looking at attaching whichever panels I get to a light aluminum frame for transport and setup. for the 24 volt I was thinking of getting 2 panels running 300 watts at 24 volts and a 15amp MPPT. For the 12 volt I was thinking 2 or 3 panels running 240 or 360 watts at 12volts now with only 2 I can get by with a 15amp MPPT but 3 would push me up to 20amp. The 15amp mppt would cost around $50.00 which I like due to its rugged no screen and waterproof construction. the 20amp would be around $100.00. Looking at the numbers below I don't see that big of a difference in IPMAX 6A for 12volt vs 6.2A for the 24volt system. from what I have read the 24 volt system would give me the ability to run longer wires with thinner gauge. any other advantage for going 24 volt.(this is where I am leaning right now)

Specs of 12 volt panels are:
Cost $135 X2=$270 X3=$405
PMAX 120 watt
IPmax 6.0A
VPmax 20V
ISC 6.65A
VOC 22.8V

Specs of 24 Volt Panel
Cost $189 x2=$378
PMAX 150
IPMAX 6.2
VPMAX 24V
ISC 6.7 A
VOC 30V
 

dwh

Tail-End Charlie
Those are both "12v nominal" panels. They have a high enough voltage to charge a "12v nominal" battery, but not high enough to charge a "24v nominal" battery.



The amperage rating of a charge controller is the max amps it can flow.

240w / 20v = 12a INPUT
240w / 13v = 18.4a OUTPUT

300w / 24v = 12.5a INPUT
300w / 13v = 23a OUTPUT

360w / 20v = 18a INPUT
360w / 13v = 27.6a OUTPUT


A 15a charge controller isn't going to cut it.
 

jonyjoe101

Adventurer
which mppt controller are you looking at for 50 dollars? that is kind of low for mppt, all the cheap ones are in the 90/100 dollar range. Some say mppt but are not true mppt.

Most panels I seen are either 21 volts or 36 volts. The 36 volt ones work better with mppt, the 21 volt panels you won't see any improvement with mppt. If you get the lower voltage panels, connect them in series to get them to a higher voltage for mppt to work with them. With mppt it needs the higher voltage to actually increase your amps, even a 24 volt panel would be low voltage for it.
 

dwh

Tail-End Charlie
With mppt it needs the higher voltage to actually increase your amps, even a 24 volt panel would be low voltage for it.

Sorry, but that's a common myth. It is absolutely not true.


An MPPT controller will find the maximum power point of the panel and maximize the watts harvesyed regardless of whether it's a 20v panel or a 36v panel.

The voltage conversion section of the MPPT controller will down-convert the voltage from panel Vmp to battery voltage, regardless of it's a 20v panel or a 36v panel.


Down-converting from a higher voltage can make it appear that there is a bigger gain, but that's just an illusion.

240w / 36v = 6.6a in
240w / 13v = 18.4a out

240w / 20v = 12a in
240w / 13v = 18.4a out

It appears that the higher voltage "boosts" 12a - from 6a to 18a, but the lower voltage only gets a 6a "boost" - from 12a to 18a.

But it's 240w in, 240w out either way. It's 18a out to the battery either way.

The input voltage is basically irrelevant. The MPPT gets the same power to the battery either way.
 

jonyjoe101

Adventurer
the GreeSonic Authentic MPPT, looks too small to be a true mppt controller, its only 1.5 inches thick is what worry me that its not mppt. There is a review on youtube "Cheap MPPT(?) Solar Charge Controller - 12v Solar Shed" that looks very similar to the greensonic but smaller 10 amp, it proved not to be mppt.
All mppt controller on the inside have what is called a toroid which is a large donut shape inducter with copper wires around it. That takes up considerable space.
On the reviews they used them on 100 watt panels and said it was real mppt. I have my doubts about the reviews. If they tested it on a 200 watt panel, then it would be more realistic. I would research it more before acquiring it.

This is what I get from using mppt and pwm on different panels, real everyday results with lead acid battery. Why I don't thing mppt works on low voltage panels. They do work on low voltage panels but will produce same results as a pwm controller.

240 watt 36 volt panel (sharp brand mono)
ecoworthy 20 amp mppt = 12 amps output
pwm 20 amp = 6 amps output

120 watt 21 volt panel (kycora brand mono)
ecoworthy 20 amp mppt = 6 amps output
pwm 20 amp = 6 amps output

toroid on mppt
toroid.jpg
 

dwh

Tail-End Charlie
240 watt 36 volt panel (sharp brand mono)
ecoworthy 20 amp mppt = 12 amps output

PV operating at Vmp

pwm 20 amp = 6 amps output

PV operating at battery voltage (WAY WAY below Vmp)


120 watt 21 volt panel (kycora brand mono)
ecoworthy 20 amp mppt = 6 amps output

PV operating at Vmp

pwm 20 amp = 6 amps output

PV operating at battery voltage (way below Vmp)


Okay, the first thing to notice here, is that the 240W PV puts out twice as much as the 120w PV when both are connected through the MPPT. Even though one is 36v and one is 21v.

IF the MPPT were more efficient with a higher voltage, then the 240W 36v PV should be putting out MORE than double the 120w 21v PV. Or, conversely, the 21v 120w PV should be putting out LESS than half the 36v 240w PV.

The fact that the 240w puts out double what the 120w puts out, PROVES that the MPPT is doing the same job regardless of the input voltage.


The next thing to notice, is that the 240w 36v panel, when connected through the PWM only produces (assuming a 12v battery voltage just to make the math easy)...

6a x 12v = 72w

And the 120w panel, through the PWM, also produces only 72w.

This is because with PWM, the panels are operating at battery voltage, NOT at their respective Vmps.

So the 240w 36v panel, operating at 30% of its Vmp only produces 30% of its rated power.

Whereas the 120w 21v panel, operating at 60% of its Vmp produces 60% of its rated power.

Well, yea...the greater the differential between Vmp and actual operating voltage, the greater the inefficiency and the greater the loss.





The 120v panel producing only 6a output through the MPPT is what makes it look like there is no difference between the PWM and the MPPT at the lower 21v Vmp.

But that's a red herring. Something else is wrong...

240w ÷ 12v = 20a
120w ÷ 12v = 10a

But through the MPPT, the panels are only producing 12a and 6a.

Which could mean the battery voltage is higher than 12v.. so...

240w ÷ 12a = 20v
120w ÷ 6a = 20v

And there is no way the battery voltage is 20v... so...

12v x 12a = 144w
12v x 6a = 72w

Which means, that even through the MPPT, these panels are not operating at full capacity.

There could be several reasons for that. The most likely is that the battery is already at a pretty high voltage. Second most likely is that they aren't facing dead-on to the sun. Third is that they are operating at high temps. Or all three.

Regardless, the 120v panel putting out only 6a whether through PWM or MPPT is a coincidence.

It does not prove that MPPT doesn't work at a 21v Vmp.

That is a misintetpretation of the data.
 
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