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Old 06-21-2016, 08:10 PM   #1
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Help me understand current relating to solar

I am feeling uneducated at the moment.

I've been playing with my solar panels, hooking them up and trying to look at energy harvested. I've got two 100 watt, 12 V (nominal) panels and a Renogy Wanderer 30amp PWM charge controller.

Take this evening for instance: getting late, sun low, I use my cheap multimeter to measure amps between the + and - lines coming from the solar panels. I get 0.98 amps. I didn't check voltage tonight, but the other day, it was at 18 volts. So then, I touch my probes to the + and - on the wires exiting the converter to the battery, I get 15 amps. My meter only handles 10 amps and the wires get warm quickly so I pull away. While the 15 may not be accurate because it's beyond the meters range, I'm pretty sure it's over 10 amps.

So here is my question. How does solar output of 1 amp at 18 volts, come out of the meter reading 15 amps (at 14.8 volts, the controllers boost charge voltage)? I must be misunderstanding the measurement of amps or something. This can't be right unless the amps I'm measuring are coming from elsewhere? Come to think of it, I still had the camper plugged into show power. Could that be 15 amps from the converter/charger plus the panels?

What am I missing or misunderstanding?
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Old 06-21-2016, 08:36 PM   #2
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Simply put, Watts = volts x amps.

You have 200w of panel, which likely output at at least 24v. Late in the day, you may be getting 20% efficiency, which means 40w. 40w @ 24v will push at 1.67amps.

The controller converts this to 14.5v. You still have 40w, but at 14.5v, this is now 2.76amps. The "extra" amperage that you are reading is likely from your shore power charger, but you should only be drawing that heavy a load if your battery is low, or your current usage is high.

This is where Jayco "doesn't get it" when it comes to prewiring for solar. They wire the roof connection with 8awg wire from the roof to the controller mounting area. This is good for up to 300w, at 34.5v (8.69amps). The controller converts this to 14.5v, at 20.69 amps. Unfortunately, Jayco uses the same 8awg ire from the controller area to the battery. I pulled all of mine out, and replaced it with 2/0 wire to carry the higher amperage with limited line-loss.

All of these numbers are notional, as they don't factor in line-loss, or loss in the controller function. But they are close enough for a simple (I hope) explanation.

Hope that all of this helps you, somehow......
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Old 06-21-2016, 09:07 PM   #3
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Quote:
Originally Posted by Fire_Instructor View Post
Simply put, Watts = volts x amps.

You have 200w of panel, which likely output at at least 24v. Late in the day, you may be getting 20% efficiency, which means 40w. 40w @ 24v will push at 1.67amps.

The controller converts this to 14.5v. You still have 40w, but at 14.5v, this is now 2.76amps. The "extra" amperage that you are reading is likely from your shore power charger, but you should only be drawing that heavy a load if your battery is low, or your current usage is high.
That's what I was remembering, watts = volts x amps, so no way 0.98 amps out of the panel can turn into over 10amps at the voltages I can have.

To satisfy my curiosity, I unplugged shore power and measured between the same contacts, without the panels hooked up and still get 15 amps. I'm not believing it and there has to be another answer. It cannot create that much current and if there was a problem, I would think I'd see heavy draw on my cheap battery monitor.
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Old 06-21-2016, 09:19 PM   #4
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I'm wondering if you're measuring current properly. You have to unhook a lead and put the meter between the open you just created in order to read current.
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Old 06-21-2016, 09:26 PM   #5
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Never mind.....

I'm a freaking idiot......

Hopefully someone else will learn from my mistake. I was NOT using the meter right and putting the probes on the right things. I was measuring across + and -, not inline on the plus like I should have been. At least it was only a $10 harbor freight meter that I probably fried.

I feel like such a moron. But at least now I know better and I didn't burn the camper down!
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Old 06-21-2016, 09:34 PM   #6
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LOL! I've got 4 or 5 Harbor Freight meters, and haven't paid for any of them. I limit my trips there to whenever they have the meter or e double LED flashlight pack as the free gift with a purchase....
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Old 06-21-2016, 09:34 PM   #7
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Quote:
Originally Posted by Cdash View Post
I am feeling uneducated at the moment.

I've been playing with my solar panels, hooking them up and trying to look at energy harvested. I've got two 100 watt, 12 V (nominal) panels and a Renogy Wanderer 30amp PWM charge controller.

Take this evening for instance: getting late, sun low, I use my cheap multimeter to measure amps between the + and - lines coming from the solar panels. I get 0.98 amps. I didn't check voltage tonight, but the other day, it was at 18 volts. So then, I touch my probes to the + and - on the wires exiting the converter to the battery, I get 15 amps. My meter only handles 10 amps and the wires get warm quickly so I pull away. While the 15 may not be accurate because it's beyond the meters range, I'm pretty sure it's over 10 amps.

So here is my question. How does solar output of 1 amp at 18 volts, come out of the meter reading 15 amps (at 14.8 volts, the controllers boost charge voltage)? I must be misunderstanding the measurement of amps or something. This can't be right unless the amps I'm measuring are coming from elsewhere? Come to think of it, I still had the camper plugged into show power. Could that be 15 amps from the converter/charger plus the panels?

What am I missing or misunderstanding?
CDash,
The only way to measure current correctly is to place the meter "inline" with either the positive or the negative wire. You cannot measure current "across" the +/-. If you do not have a DC clamp on meter, you would have to open your connection to one side and put the meter in series.
To answer your question, you could not get 15amps out when you only have 1 amp in.
If you are going to do this type of thing regularly, I highly recommend you get a clamp on DC meter.
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Old 06-21-2016, 09:40 PM   #8
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Hey, don't feel bad! We all do it and that's what this forum is for. Now you know.
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Old 06-22-2016, 01:42 PM   #9
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One additional point regarding the Harbor Freight (FREE) meters, of which I also have 3, you need to make sure they have fresh batteries inside them or you will be scratching your head trying to figure out why the voltages you are measuring are not correct. Don't ask how I found this out x*$%#^!

Don
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