How to measure amp OUTPUT not DRAW.


Dirtman

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Dumb question.... I hooked up a series of solar panels to my shed. In series not parallel so volts are the same but amps is increased. Using it for charging 12 volt batteries and gizmos.

I understand how to measure the amp draw of a device on a power source (aka measuring headlight draw on a battery). But how do I measure the maximum amp output of my solar panel array? I need to know so I know what size controller to buy. Each panel says it has a maximum of 3amps at 17vdc. But im putting out about 21vdc on a cloudy day. Do i just trust the specs on the panels or is there a true way to measure the amp output?
 


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Have you tried an ammeter?
 

Dirtman

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i have a dmm. Like i said i know how to measure amps on a dmm for stuff like parasitic draw. But how do you do that with nothing actually drawing current? Just open circuit.
 

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i have a dmm. Like i said i know how to measure amps on a dmm for stuff like parasitic draw. But how do you do that with nothing actually drawing current? Just open circuit.
Hook up a starter, or a stove heating element.
 

PetroleumJunkie412

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Direct the load to an ideal ground. This is dangerous as f*ck to do.

Do not do the following:
Dig a narrow post hole, dump a bag of Epson salts or five in it. Pound a copper rod through the center of the salt mound, backfill and irrigate with lots of water. Instant perfect earthing ground. Measure discharge from panel to ground.

There are ways to measure.total current output from a given source, but the ones I know all involve discharging or charging an "ideal" load and measuring draw.

There's probably a German built capacitor based meter that accomplishes this that's €50,000.
 

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So go by the sticker... got it
 

PetroleumJunkie412

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So go by the sticker... got it
Pretty much. Current is tough to meter. Adsm08's idea is pretty accurate. You have to get the thing to dump all it has and measure that.
 

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Do not do the following:
Dig a narrow post hole, dump a bag of Epson salts or five in it. Pound a copper rod through the center of the salt mound, backfill and irrigate with lots of water. Instant perfect earthing ground. Measure discharge from panel to ground.
Why not?


Pretty much. Current is tough to meter. Adsm08's idea is pretty accurate. You have to get the thing to dump all it has and measure that.
Basically the safest way I can come up with is to hook it to a load that will try to pull more amps than the system can produce, do that you can see the maximum output.
 

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Ammeter is series on the panel output to a battery... and put a load on the battery.
 

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If you hooked the panels in series, the voltage adds up and current stays the same. Ie; two 17 volt/3 amp panels in series gives 34 volts at 3 amps. Same with batteries. Two 12 volts batteries in series gives 24 volts.

Put your panels in parallel and the voltage stays the same but the current adds together. Two panels would give 17 volts at 6 amps.

Easiest thing is to go by the nameplate ratings. It should be close. I haven't played with solar panels too much. But my assumption would be more current available on sunny day as opposed to cloudy day. Voltage should remain at rated voltage unless you are drawing too much current. Then it would probably droop.
 

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In series not parallel so volts are the same but amps is increased.
This wouldn't be correct...

Two panels in series would double the voltage, while maintaining the same current (it's exactly the same as with batteries... two 12v 90 ah batts in series = 24v @ 90ah). Two panels in parallel would do as you state.

What voltage is your battery system? (12V or 24V?)

Edit-
too slow :LOL:

Here's the info that is important for sizing what controller you need:

Vmp = volts @ maximum power output.
Voc = open-circuit (no-load) voltage (and will always be higher than Vmp)
Imp = amps @ max output.
Isc = amps @ short-circuit (generally not real important, but is usually listed anyway).

A panel designed for use with a 12V lead-acid battery system typically has ratings around 17-18 Vmp, and 21-22 Voc. Imp & Isc correlate to the physical size of the panel.

For similar panels in series, the Vmp figures add together, as also do the Voc figures, while Imp and Isc remain the same (this would allow use of two 12V-rated panels to charge a 24V system). For panels in parallel, the Imp and Isc add, while Vmp and Voc remain the same.
 
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PetroleumJunkie412

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Why not?




Basically the safest way I can come up with is to hook it to a load that will try to pull more amps than the system can produce, do that you can see the maximum output.

Big enough source of power, and there's a good chance if things go just wrong enough, no more Dirtman. Not sure if this is a solar cell battery charger, or an entire roof.


Also. You're absolutely correct.


We do the earthing rods with electrolytes at work. At times, a frac fleets just won't ground back to Earth (14-18 locomotive diesels running dual triplex pumps at once). Works like a charm, but there's always a slim chance of discharging into someone/something unintentionally.
 

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I have a 500 amp battery tester. Could that be used as load?
In fact, I think it has both a volt and amp meter integrated. I have to go look....
 

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Dumb question.... I hooked up a series of solar panels to my shed. In series not parallel so volts are the same but amps is increased. Using it for charging 12 volt batteries and gizmos.

I understand how to measure the amp draw of a device on a power source (aka measuring headlight draw on a battery). But how do I measure the maximum amp output of my solar panel array? I need to know so I know what size controller to buy. Each panel says it has a maximum of 3amps at 17vdc. But im putting out about 21vdc on a cloudy day. Do i just trust the specs on the panels or is there a true way to measure the amp output?
You don't "put out" volts. You "put out" amps. I have a solar panel on my bus and the controller can show 20v, but absolutely no amps. You have to actually draw current so you can measure it. Amps is current. Volts is just potential difference. If it says 20v, it's like it's saying it is 20 feet in the air. But it might be a feather, or it might be a grand piano. You have to drop it (hook up the wires with a meter) to know what it is.

You have to trust the specs. Don't worry about volts measured straight off the panel. Whatever the volts you read off the panel are, the controller will sort that part out. You need to worry about amps. The controller will be designed to charge a 12v battery, which you will be drawing power off of. Whatever amps they are using will be at the nominal 12v.

Which brings us to watts. The panel is actually rated in watts--which is electrical pressure--volts--(potential difference) combined with the current (amps). Watts is the work done by the system. I have a single 150w panel on my bus. You need to look up all the watt requirements for your panel and use that for the baseline.
 

Dirtman

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The panels are rated at 30 watts each and I ran 5 of them (in parallel not in series, it is still 12vdc output). I always get that mixed up. According to the specs they are 1.74 amps each so I just ordered a 10amp controller. I would have just bought the biggest controller possible but from what I've read the controller needs to be near the actual output of the panels to work properly (i.e slapping a 200 amp controller on a 2 amp panel won't work). This is why I figured I'd "attempt" to measure the current instead of just going by the spec sheet.

Now I'm figuring out how to wire in the isolator so I can have two batteries hooked up at once. One to a permanent battery that powers the shed lights and such and the other line for trickle charging my boat/truck battery.

Im inching ever closer to my goal of going 100% off the grid and never interacting with society again!
 


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