how to monitoring voltage and wattage of array while in use

Urbandialect · August 2011

Hey I have a question, the charge controller I have doesn’t have an LCD screen on it that shows the amount of Voltage and amps the array is producing.

My question is, is it possible to add one to it? I tried hooking up my amp panel meter but when I do, it drops the voltage of the array from 18 volts to 2 volts, (charging stops) They say the amp meter has a built in shunt, I don’t know what a shunt is, but I’m guessing that’s what is causing the voltage to drop to a safe level to measure amps?

Is there a way to hook the amp meter up to the array with out causing a voltage drop? Also I saw something called an RC power analyzer. http://www.amazon.com/Watts-Meter-Analyzer-WU100-Version/dp/B001B6N2WK/ref=sr_1_1?s=lamps-light&ie=UTF8&qid=1313418076&sr=1-1 <- This device shows the DC Voltage and Amps coming in and out, is it possible to hook this up to the charge controller? And if so is it as easy has hooking up the + and – leads from the array to the + and – leads to the power analyzer, then running it into the charge controller, then hook another one up the same way between the charge controller and battery, to see what i'm losing?

BB. · August 2011

Re: how to monitoring voltage and wattage of array while in use

Yes, there is the Doc Watson line of DC AH/WH meters. As far as I know, they only measure current in one direction--so they are not great to measure AH in/out of a battery bank. I believe these can be "hacked" to add external shunts--but of course, they would need a "conversion" factor because the screen will not be calibrated to the new shunt.

And there are external shunts (which are precision power resistors with two sets of electrical connections. One set for the big power connections, a second small set for connecting a precision milli-volt meter):

Deltec 100 amp, 100 millivolt current shunt
Price: $24.00

These shunts can be connected to a panel meter of some sort, or to a Battery Monitor. Battery Monitors are designed to measure current in both directions so you can keep track of how full the battery bank is (in percent, or actual numbers). Also, battery monitors have "fudge factors" that allow them to more accurately estimate battery state of charge (losses, temperature, etc.).

While you can use the Doc Watson as a replacement for the current display... I really do like the battery monitors instead. They show what current is going into/out-of the battery bank itself--which is really more important.

Also, if you have AGM/Sealed batteries, this is the only way to accurately estimate the present state of charge as you cannot measure specific gravity of the electrolyte in sealed batteries.

-Bill

Urbandialect · August 2011

Re: how to monitoring voltage and wattage of array while in use

What is a shunt? I keep seeing people with them in their wiring, but i don't know what they are and what they do, all i know is my amp meter has a built in shunt and when I go to measure the amps from the panel the voltage drops to 2 volts.. So i can't measure amps while the system is working at the same time.

Why can't i just hook the amp meter up to the panel like the voltage meter w/out any problems? what causes the voltage drop? What can i use where I can measure the watt/amps with out the voltage dropping when I hook it up?

BB. · August 2011

Re: how to monitoring voltage and wattage of array while in use

In general, most meters do not measure current (at least large currents) directly, they instead use a precision power resistor and measure the voltage drop across it.

From the picture I supplied, the two large bolts are where you connect the heavy battery cables (typically between battery bank - and the negative bus bar, so you measure all current going into/out of the battery bank).

In this case a 100 amp shunt with 100 mV (0.1 volt) drop will have:

V=I*R
R=V/I= 0.1 volts / 100 amps = 0.001 ohms

So, if there is a 1 mV drop (0.001 volt), there is 1 amp flowing through the shunt. 0.001 ohms is very low resistance.

Standard DMM (digital multi meters) do have internal shunts to measure current (and voltage drop from current flow)--however, typically the largest shunt rating is for 10 amps. When you switch to lower scales, they use a higher value resistor to measure the voltage drop (for more accuracy).

Because we are talking about such small voltages (0.001 volt), they use two screws in the side of the above shunt so it can accurately measure the voltage drop back to the mVolt meter/Battery Monitor.

Depending on the maximum current flow and the maximum voltage drop a circuit can support (0.1 volt and 100 amps in this case), there are different models/sizes of shunts to use with different meters (many battery monitors can be reprogrammed for different shunt values).

-Bill

Cariboocoot · August 2011

Re: how to monitoring voltage and wattage of array while in use

A shunt is a fixed, stable resistance. You pass current through it, and by measuring the Voltage differential on the two sides you can determine the amount of that current.

You can attach a Voltmeter and an Ammeter directly to the panel leads and measure what they're producing at any one time, providing it's the right meters used. I don't know the specs on the meter you were trying, but the Amps function should not drop the Voltage at all. Did you by any chance connect the leads in parallel to the panels when trying to check Amps? If so, that would do it. For Voltage the leads go (+) to (+) and (-) to (-). For Amps the leads go PV (+) to Ammeter (+), Ammeter (-) to Charge Controller; the current has to flow through the meter. That's why it is vital to have a meter that can handle the full output potential of the panels - and then some.

Ordinary meters don't log data, of course, which is why Bill suggested the Doc Watson. It will have a shunt (connect in-line on the negative side) and with it track the current flow.

BB. · August 2011

Re: how to monitoring voltage and wattage of array while in use

The Doc Wattson has an internal shunt, that, in theory, is good for ~60 amps maximum... In reality, the wires are not really rated for more than 15-20 amps continuous load.

The RC-Electronics link I gave you has some application notes/hacking instructions for the Doc Wattson family of meters.

Like all things electrical, you have to know the voltage/current to pick the correct test equipment.

-Bill

PS: I should add "I think it has an internal shunt"... I have never played with a real unit before.

Urbandialect · August 2011

Re: how to monitoring voltage and wattage of array while in use

Cariboocoot wrote: »

Did you by any chance connect the leads in parallel to the panels when trying to check Amps? If so, that would do it..

Yes, i hooked it up just like i would a volt meter, I have 3 analog volt meters hooked up, one is on the array itself outside to see what the panels are doing, then I have one inside going from the charge controller to the battery to see what the charge controller voltage is chargering at, then I have one hooked up to the battery to see what the charge of the battery is at....

Cariboocoot wrote: »

For Amps the leads go PV (+) to Ammeter (+), Ammeter (-) to Charge Controller; the current has to flow through the meter. That's why it is vital to have a meter that can handle the full output potential of the panels - and then some.

What?, so i hook the + of the amp meter to the + of the solar panels and the - to the - of the charge controller? which negative, the one going to the battery or the one coming from the array?

Cariboocoot · August 2011

Re: how to monitoring voltage and wattage of array while in use

Urbandialect wrote: »

What?, so i hook the + of the amp meter to the + of the solar panels and the - to the - of the charge controller? which negative, the one going to the battery or the one coming from the array?

Nope. Not the (-) of the charge controller; the (+).
Negative from PV goes to the negative of the charge controller.
Ammeter goes in line with the positive wiring. Current flows from PV through the Ammeter to the charge controller.

Putting it in parallel the way you did connected that shunt right across the (+) and (-) thereby creating a short that would definitely drop the Voltage. You do it that way when you're measuring Isc: short-circuit current. That's PV (+) to Ammeter (+), Ammeter (-) to PV (-) and no charge controller involved. The Voltage will drop drastically, but if there's enough light on the panel you'll get something approximating the Isc value.

bmet · August 2011

Re: how to monitoring voltage and wattage of array while in use

DVM has to be inline to measure Current

mike95490 · August 2011

Re: how to monitoring voltage and wattage of array while in use

BEWARE -
Never, EVER, connect an ammeter to see how many amps your battery has. Instant Arc Welder.

This usually happens when you forget to reset your multi-meter to the volts mode, after measuring amps. BOOM !

how to monitoring voltage and wattage of array while in use

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