Measuring amps

[SJHall]

How do I measure amps produced by my solar panel?

I have a new Helios 6t-245 watt panel attached to a TriStar-45 controller energizing 2-12 volt deep cycle batteries, a 24 volt system. My TriStar-45 says, on a beautiful sunny summer day in north Idaho, that I'm getting ~30 volts but only .5, that is 1/2 an amp.

So, I took the controller out of the system and hooked the panel directly to the battery bank, inserted an ammeter between the negative lead and the negative battery terminal and it still only shows 1/2 an amp flow. Even if the batteries are fully charged, shouldn't the panel still put out it's best effort? I'm stumped!

Thank you for your help,
SJ

[billvon]

So, I took the controller out of the system and hooked the panel directly to the battery bank, inserted an ammeter between the negative lead and the negative battery terminal and it still only shows 1/2 an amp flow. Even if the batteries are fully charged, shouldn't the panel still put out it's best effort?

Not if the operating voltage point is not at its peak power point. Solar PV only outputs max power at one voltage.

For a quick test, cover the panel, connect a 10 amp DC ammeter across the two leads, then uncover the panel. You should see Isc (short circuit current) which should be a lot higher.

[SJHall]

Thanks Billvon,

The short circuit current shows 8.4 amps @ ~30-32 volts at 12 noon on this clear, sunny summer day.

Can you think of a reason the panel won't push more than .5 amps when it's wired directly to the battery bank, that is, with no controller in the mix?

SJ

[Sunking]

The short circuit current shows 8.4 amps @ ~30-32 volts at 12 noon on this clear, sunny summer day.

Wrong! There is no voltage at Isc, that is impossible.

Voltage = Current x Resistance. Short Circuit = 0 resistance. ZERO x X = ZERO.

Example: 8.4 amps x 0 Ohms = 0 volts.

8.4 amps x 31 volts = 260 watts

[SJHall]

Sunking,

You are right of course. The voltage reading was taken right after the amp reading, not simultaneously. I included a voltage reading because it sounded like the voltage was also important since the panel only puts out maximum power at a certain voltage. I am often confused; is there another way to do this so you have all the data you need?

Thank you,
SJ

[Sunking]

Sunking, You are right of course. The voltage reading was taken right after the amp reading, not simultaneously.

Then that was Voc or voltage open circuit where resistance is infinite. The only reading you as a DIY can take is Isc which is invalid, and Voc which is valid.

Isc can only be measure accurately in a lab with a know light source and test conditions. However you can get an idea on a very clear cool day around noon with the panel facing directly into the sun around solstice and equinox for your area. You might see 90% under those idea conditions.

You can take Voc with a flashlight in a dark room at night. Solar panels are current sources, and the amount of current they can produce is dependent on the amount of available light striking their surface. In the vacuum and cold of space you gets lots of current. In the heat, atmosphere, and low sun angle of the North or South Pole on earth suks .

[inetdog]

Thanks Billvon,

The short circuit current shows 8.4 amps @ ~30-32 volts at 12 noon on this clear, sunny summer day.

Can you think of a reason the panel won't push more than .5 amps when it's wired directly to the battery bank, that is, with no controller in the mix?

SJ

Unless something really strange is going on (like a partially shaded or defective panel which cannot deliver full current at full voltage) the explanation would have to be that the voltage difference between the internal panel terminals and the battery at .5 amps (.5A times lead resistance and internal battery resistance) is equal to the difference between V(.5A) of the panel and V(battery).

One example of weirdness: Suppose the panel has three sections of cells wired to deliver ~10 volts each, with internal bypass diodes.
Then also assume that one of the three sections is shaded or defective and cannot produce more than .5A at full exposure.
The result will be that Voc will be measured at 30V, and Isc will be measured at 8.4 amp because the current from the two good sections is passing through the bypass diode of the defective section. That would give the exact observations that your report and whenever the current draw is greater than .5A, the panel voltage will drop sharply from 30 volts to 20 volts and no more charging current can be delivered to a 24 volt battery.

The best way to test this is to use a variable load resistor bank (high power!) to draw increasing amounts of current from the panel and observe the output voltage as a function of current. For a good 32 volt panel, the voltage should remain above Vmp all the way out to at least 5 amps (or whatever Imp is specified as. ) If the voltage drops sharply at a lower current, the panel is defective.

[billvon]

The voltage reading was taken right after the amp reading, not simultaneously. I included a voltage reading because it sounded like the voltage was also important since the panel only puts out maximum power at a certain voltage. I am often confused; is there another way to do this so you have all the data you need?

You can do a load test with a programmable load but that's generally not needed. Isc and Voc (the two numbers you got) are pretty good diagnostic numbers. Your Impp will be about 90% of your Isc; Vmpp will be about 80% of Voc.

[SJHall]

Great! Thanks guys, that is the thinking I need! Now I feel comfortable talking to the panel mfgr.

SJ