Testing solar amperage

86Scotty · 01-03-2015, 02:24 PM

Probably a dumb question for anyone handy with a multimeter, but I don't trust what I've found so far.
I just finished installing a 100 watt Renogy panel and their cheap PWM controller, which has only dummy lights, no amperage reading. I will someday upgrade to an MPPT controller but this is my first go at solar. Anyway, I would like to occasionally test the amperage my panel is putting out, using a multimeter if I can. I have an easy place to probe, on the wire screw-downs in or out of the controller. I'm thinking I would just set my multimeter to amps (10A) and should get a reading, but when I did this I got a spark so I stopped. I don't want to damage anything. One Youtube vid I saw hinted that they will always spark since they are always live if in daylight, so is the momentary spark OK?

Thanks

**WVvan** · 01-03-2015, 03:08 PM

On your average multimeter don't touch the probes to the two input screws on the controller to get an amp reading. The probes are used to complete the circuit. All the current must pass through the meter. You got a spark when you measured between the two screw downs because you just shorted them out.

An ideal meter (in theory, not in practice) will have infinite resistance when being used to measure ohms and zero resistance (aka a short) when measuring amps.

This is why meters have a amp fuse but not a ohm fuse. If you use the meter to complete a circuit in the amp setting you only lose the fuse if the currents is too high as opposed to burning out the meter

The way to measure would be to disconnect one of the two wires from the controller and attach that to one of the probes then attach the other probe to the screw terminal where you just removed the wire. Now the meter completes the circuit.

86Scotty · 01-03-2015, 04:11 PM

Ok. Thanks! Not a good solution for quick readings though. Is there a cheap easy way to add a digital amp readout? I think this is called a shunt but not sure.

**WVvan** · 01-03-2015, 05:05 PM

This should do for you with the understanding that accuracy isn't the greatest but good enough for what we're using it for. Be sure you get one that comes with it's own shunt.

http://www.ebay.com/itm/New-100V-100...item43c5b7c269

Warning: If you go this route be sure find a way to protect the shunt from contacting anything it not supposed to. It will be after all "live". I build a plexiglass box that fits over mine.

Another consideration with these types of low price meters from across the sea. Hook up the polarity wrong just once, and it's toast. Don't ask me how I know.

jony101 · 01-03-2015, 11:41 PM

get a dc 90 volt 30 amp combo meter from ebay, they cost about 23 dollars. I have that on my solar system. your controller out wires connect to the meter in, then the meter out go to the battery. No shunt is required. Its pretty accurate my controller has an lcd screen and the voltage and amps usually match.

Just by looking at the voltage you can tell if your controller is in bulk or float mode plus you know how much amps your panel/controller is putting out. At night time you can see at a glance what your battery voltage is, I leave mine connected 24/7, the power the meter uses is insignificant.

Another option is to get a dc wattmeter, these also are available in ebay, they cost about 13 dollars, they also connect between the controller and battery (no shunt required) these are also very accurate and measure volts/amps, peak amps and you an leave it connected 24/7.

I use the xt60 connector on all my connections, they can handle up to 60 amps.

boywonder · 01-04-2015, 01:00 AM

Just disconnect one of the wires going from the panel to the controller and use your meter leads as a jumper wire to reconnect the removed wire, ie place the meter in-line with the wire (in amp/current mode).

As already mentioned, your meter has (almost) infinite resistance in voltage mode and (almost) zero resistance in amp mode. If you think about this for a minute it means that you can probe anything in voltage mode without shorting anything (since the meter has infinite resistance no current will flow), but when in current mode it's a dead short, same as connecting a wire between two points. That's also why you physically have to move the meter leads when using it as an ammeter.

If you get no amps with the sun shining on your panel when using your meter, check the fuse in the meter, your prior mistake may have popped it.

JWA · 01-04-2015, 04:45 AM

It should be said most hand-held DMM's are internally fused at max 10 amps---exceed that and that fuse definitely blows.

The eBay link to a meter and included shunt is great if only because it shows how to connect the parts, shows the shunt has to be in line with power being measured.

For ease of operation and installation the watt meter type device is best---the shunt is internal, connections are extremely simple.

Scalf77 · 01-04-2015, 07:29 AM

Scotty,
If you put a fuse in-line with panels coming from the roof, as shown in WVvan's build.

Then all you would need to measure current of the panel would be to pop the fuse and use your hand held amp meter. Still not convenient, but relatively safe as apposed to removing connections, etc. Obviously this is just getting you the current available from the panels.

If I was going to chose one of those meters, the second would be my choice, one of the negatives being that it would have to be in series with your controller to battery connection, how much extra wire that creates would be the concern.

The first meter has a external shunt, witch theoretically should enable you to keep the interruption to your circuit at a minimal, problem is that I don't see that there is anything that they do to compensate for noise, so the shunt although external still has to be close to the meter.

In the long run it may better to save the money and put it towards a good RV battery monitor that could give you data on all your charging devices. One of the problems with so many readouts is the comparison between them, it get's challenging when you have multiple sources giving you slightly different information. Then again, the inner techno geek comes out and we can never have enough data Glad to see you are going solar.

-greg

86Scotty · 01-04-2015, 10:20 AM

Thanks everyone. Awesome info. I'll post some finished pics of what I come up with in the next few days. I chose to hide my controller since it doesn't have a digital readout, but it's pretty easy to get to. I think I can add one of these where I can see it without adding much wire.