Solar panel volts but no amps

[Ho jo]

I ve been seeing extremely low watts lately. Testing my panels 6 amps on 3 but .8 on one. Voltage is still 22.5 on all. Getting it replaced under warranty 8 months old. Always thought I'd be replacing batteries first

[Sunking]

So what makes you think the panels are bad?

If the batteries are charged, you will not have any current. No place for it to go. My guess is everything is working as designed. If anything wired poorly or incorrectly. I assume you have 36 cell battery panels based on Voc of 22 volts. No current flows in a panel when the voltage is Voc.

Voc = Voltage Open Circuit. No current can flow in an Open Circuit. If the batteries are charged, essentially you have an open circuit. Wai tuntil you have bright sunshine at noon. Turn everything on you can can and see what happens.

[Ho jo]

Sun king yes 36 cell 18.9 22.5 volts on clear days my readings at 12 had been 308 watt peaks at at over 21 amps. Then all of a sudden 16.5 224 peaks. I was thinking low sun but decided to check it out. Unplugged panels tested each around 22 volts. 3 tested 5.99 5.98 5.95 and 1 put out .8 called the panel company. They sent me a replacement. They couldn't tell me what happened. Now today I'm back to over 308 watt peaks. I need help on what voltage to put in calculator for pv run 75 ft on 8 awg is what I have

[Sunking]

3 tested 5.99 5.98 5.95 and 1 put out .8 called the panel company.

OK I missed that part. In the meantime short circuit that panel out so the other 3 can work.

As for the calculator, you are doing it wrong. You do not use a calc to see what a 8 AWG can do. Like saying I have $10,000 to buy a Rolls Royce, you will get laughed out of the showroom. What you are looking for is WHAT GAUGE OF WIRE YOU NEED to meet the specification of 2 to 3% voltage drop. To do that you need to know is the Vmp at maximum power, maximum current, and 1-way distance.

Example lets say you have all 4 panels in series. Your Vmp will be 72 volts with 5.5 amps. 2% of 72 volts is 1.44 volts. With 1.4 volts / 5.5 amps = .25 Ohms max resistance. Fortunately there are all kinds of calculators on the web to use. Input Vmp, Imp, and 1-way wire distance. Using my numbers will give you 10 AWG.

Now if you have a PWM controller you are screwed because you must wire all 4 panels in parallel. Vmp = 18 volts and Imp = 22 amps. That will require a bank loan to buy 150 feet of 1/0 AWG. No tonly do you loose a lot of cash but you turn your 400 watt panels into 250 watts.

[Ho jo]

Ok 18.9 vmp x 21.16 imp 2 awg c 2.65% 75 foot on 8 awg 10.53 % .7.88 % loss from small cable if my math is correct I'm getting 308 watts now with 10.53% loss so I'm losing 308x.0788=30.57 watts

[Ho jo]

Wow $315 for 1/0

[karrak]

You do not need to limit the power losses to 2%-3%, you have to firstly calculate the smallest cable size that can safely carry the current without overheating, then you have to make sure that the voltage loss does not drop the voltage so much that equipment you are running or batteries you are charging have enough voltage to work or charge properly, the third and last criteria is the economic cost of the losses.

In your case what is the cost of loosing 10% of your panel output and is it cheaper reducing the power loss with a larger cable or buying more panels. At a panel cost of $1.00 per watt you would have to spend an extra $30 on panels or spend $315 on cable.

Simon

[Sunking]

You do not need to limit the power losses to 2%-3%, you have to firstly calculate the smallest cable size that can safely carry the current without overheating, then you have to make sure that the voltage loss does not drop the voltage so much that equipment you are running or batteries you are charging have enough voltage to work or charge properly, the third and last criteria is the economic cost of the losses.

Ignore that Jackwagon. He is FUBAR

Power losses add add up real quick with low voltage and you have 3 points of power loss.

1. Between Panels and Controller.
2 Between Controller and Battery
3. Between Battery and Load.

If you took Karrak's advice and only used wire large enough to be safe, you can easily loose 30% or more of your power between the panels and controller. Totally unacceptable and just a poor design practices. If you keep losses tight to 2% to 3# on each of the three legs, you are already looking at loosing 6% max. Run these numbers in this CALCULATOR. Perfectly safe for 22 amps.

Wire = Copper
Wire Size = 12 AWG
Voltage = Vmp or 18 volts
Phase = DC
# of Conductors = Single Set
Distance = 75 feet
Current = 22 amps.

See what you get and play around with it. Change voltage to 72 volts and current to 5.5 and see what happens.

[Sunking]

Ok 18.9 vmp x 21.16 imp 2 awg c 2.65% 75 foot on 8 awg 10.53 % .7.88 % loss from small cable if my math is correct I'm getting 308 watts now with 10.53% loss so I'm losing 308x.0788=30.57 watts

OK why are you wiring your panels in parallel? Let me guess, you have a PWM controller and have no choice.

You could have saved a ton of money if you bought a MPPT controller. That gets your voltage up to 72 volts and current down to 5.5 amps. The money you saved on wiring more than pays for the controller. With 10 AWG = 1.2%. 12 AWG = 1.8%, and 14 AWG = 2.9%. Code prevents you from using anything smaller than 14 AWG but at 5.5 amps using Karraks method you could use 20 AWG safely but loose 11%.

Dude 14 AWG cost 9-cents per foot. 150 feet x 9-cents = $13.50 just call it $15 dollars. A 20 amp MPT controler cost just under $200. You really made a mistake using a PWM Controller, and compounded it using 12 volt battery panels. You could have used much higher power GT panels that cost half as much as you paid for battery panels. Panels saving alone would pay for the MPPT Controller You are paying double what you should be paying for everything.

Say $300 to $400 for a single 300 watt panel, $200 for a 30 amp MPPT Controller, and $20 for panel wiring. Do not forget with 3 parallel panels you now need an expensive Combiner and Fuses for each panel leg. Not to mention 3 racks for the panels.

[Ho jo]

Kinda what I was thinking but I'm a solar idiot.does 308 watts sound about right for 400 watts on pwm with a 75 foot 8awg. So with the right size wire I'd pull 19% more. I already have a 80 ft 200 amp house service wire for when I build my cabin. With all 4 panels working and a bigger wire I may have enough to get me through the winter with out having to buy any thing else. one can only hope. Thanks All

[Ho jo]

The reason I did 12 volt panels 300 watt panels cost more to ship than the panels. had I known I'd be needing 6 to 800 watts I would have. in my mind 12 volt rv and batterys 12 volt panel. Hind sight is 20 20. 1 started out with 200 watts. Without knowing my needs like a lot newbies on this forum Backwards.

[Sunking]

Kinda what I was thinking but I'm a solar idiot.does 308 watts sound about right for 400 watts on pwm with a 75 foot 8awg.

That would be about right, but to really know would take 3 voltage measurements, and the current

V1 = Panel Voltage at the junction where the 4 panels are connected together
V2 = Controller, Input terminal from panels
V3 = Battery Term Post

From there simple math. Example the loss on the wire between panels and controller = 1 - [V2/V1]. Or say 1 - [16/18] = 11.1%. The real interesting ratio and the most important is 1 - [V3/V1] Say 1 - [13/18] = 27.7% the absolute best you can get with PWM running full power.

If you want to know power at any point is easy with PWM because Input Current = Output Current. The current on the 3 legs to the battery are the same at any point along the length. So power at any Point is:

V1 x C
V2 x C
V3 x C

Power losses will equal Voltage losses. So using the examples above with 22 amps of current the panels are producing 396 watts, Controller input is 352 watts, and the battery is charging with 286 watts.

[Sunking]

The reason I did 12 volt panels 300 watt panels cost more to ship than the panels. had I known I'd be needing 6 to 800 watts I would have. in my mind 12 volt rv and batterys 12 volt panel. Hind sight is 20 20. 1 started out with 200 watts. Without knowing my needs like a lot newbies on this forum Backwards.

Understood maybe. Is this in an RV? How would you have a 75 foot run in an RV? Otherwise I understand.

You can make the panels work and I have a good idea. Sell the PWM controller, and buy a MPPT controller. That will allow you to wire all the panels in series so you can run 72 volts @ 5.5 amps. You can use cheap 14 AWG wire instead of wire the size of you thumb, Additionally save another ton of money because you will not need a combiner and 4 breakers for the panels. That is a $150 bill saved. The cost savings of smaller wire and less hardware will pay for the MPPT Controller and might even put an extra coin back in your pocket.

[karrak]

That would be about right, but to really know would take 3 voltage measurements, and the current

V1 = Panel Voltage at the junction where the 4 panels are connected together
V2 = Controller, Input terminal from panels
V3 = Battery Term Post

From there simple math. Example the loss on the wire between panels and controller = 1 - [V2/V1]. Or say 1 - [16/18] = 11.1%. The real interesting ratio and the most important is 1 - [V3/V1] Say 1 - [13/18] = 27.7% the absolute best you can get with PWM running full power.

If you want to know power at any point is easy with PWM because Input Current = Output Current. The current on the 3 legs to the battery are the same at any point along the length. So power at any Point is:

V1 x C
V2 x C
V3 x C

Power losses will equal Voltage losses. So using the examples above with 22 amps of current the panels are producing 396 watts, Controller input is 352 watts, and the battery is charging with 286 watts.

So the PWM controller is dissipating 84W (352-286), i don't think so.

No Sinking, the PWM controller acts as a switch between the solar panels and the battery. When it is switched on the voltage drop across the controller is the voltage across the switching transistor inside the controller which should be less than 0.5 volts. At 22A this equates to a power loss less than 11W. That is why the heat sinks on PWM controllers are so small.

So what happens to the extra power? The answer is that it is never generated by the solar panels. If you draw too much current from a solar panel the voltage drops but the current stays the same (it actually goes up a little but we can ignore this). Now power=volts x amps so if we drop the volts but keep the amps the same the power goes down.

So in this case the panel output voltage reduces to the battery voltage + ~0.5V + the voltage drop in the cable.

Simon

[sensij]

So the PWM controller is dissipating 84W (352-286), i don't think so.

No Sinking, the PWM controller acts as a switch between the solar panels and the battery. When it is switched on the voltage drop across the controller is the voltage across the switching transistor inside the controller which should be less than 0.5 volts. At 22A this equates to a power loss less than 11W. That is why the heat sinks on PWM controllers are so small.

So what happens to the extra power? The answer is that it is never generated by the solar panels. If you draw too much current from a solar panel the voltage drops but the current stays the same (it actually goes up a little but we can ignore this). Now power=volts x amps so if we drop the volts but keep the amps the same the power goes down.

So in this case the panel output voltage reduces to the battery voltage + ~0.5V + the voltage drop in the cable.

Simon

Sunking's math was right for the example he created, but yeah, if the OP measures the V1, V2, and V3 suggested, he is unlikely to find 3 V drop across the controller.

This thread relating to the OP's bad panel is now closed, there are other threads in which the design details of this system is more on topic.