Watt Meter on Small Solar System Problem

bte52 · April 2014

I did a search of this very helpful forum and found a couple of threads that address my question however I am still a bit confused. Perhaps a new outlook will help me understand better. I very new to solar and wanted to start with an inexpensive, portable setup for emergency power. I purchased a set of two (2) 50W mono solar panels that fold up like a suitcase. They came wired together and included a cheap 15 amp 12V PWM charge controller. I hooked everything up with to a 100+ amp hour deep cell marine battery and a 1100 watt inverter. The panels seem to charging the battery just fine. Here’s the problem: I also wired in a GT Power RC Watt Meter (same as Turnigy or WattsUp). I have the source leads on the watt meter to the battery and the load leads wired to the battery inputs on the solar charge controller (per a YouTube video I saw). The watt meter only displays data relating to volts and the battery. NO data regarding amps (load) comes up. In the scrolling display I am supposed to be able to see Watts, Wh, Wp, Amps, Ah, Ap . The only value that comes up in the scrolling data is Vm. I would like to ask if it is possible that I have incorrectly wired this meter to my system. I know that the GT Power Watt meter is good because I tested it using a battery charger on the source side and a battery to the load side. The meter scrolled perfectly through all metrics. NOTE: There is nothing wired to the load terminals on the charge controller. I thought that I might add a small, switched LED light to that later. Any assistance would be appreciated.

As an aside, I also wanted to test the output of the solar panels to see how they compared to the sticker on the back. The VOC is listed at 22.4V and the ISC at 6.2 amps. I did the test on a clear bright day at straight up noon here in the Mid Atlantic. I disconnected the wires leading to the charge controller to test each panel independently. I tested at the junction box of each. The panels each read around 22V buts seemed to drop off to around 20 rather quickly. The amps for each were 2.8 to 3 amps. If each panel ISC is 6.2A I don't get why they are half of the rated value. Thank you very much.

Cariboocoot · April 2014

Re: Watt Meter on Small Solar System Problem

Welcome to the forum.

It's backwards.
You have the Source connected to the battery and the Load connected to the charge controller. That would read any Watts the battery fed to the controller, but the power flow is in the other direction: the charge controller is the Source as it supplies energy to recharge the battery which is the Load.

Your panels are 50 Watt, probably Vmp 17.5 and Imp 2.85 - there is no way the Isc of such a panel would be >6. It would be more like 3, and I bet the Isc rating given is for the two panels in parallel as that is the way they were sold, right?

BB. · April 2014

Re: Watt Meter on Small Solar System Problem

bte52 wrote: »

The only value that comes up in the scrolling data is Vm. I would like to ask if it is possible that I have incorrectly wired this meter to my system. I know that the GT Power Watt meter is good because I tested it using a battery charger on the source side and a battery to the load side. The meter scrolled perfectly through all metrics. NOTE: There is nothing wired to the load terminals on the charge controller. I thought that I might add a small, switched LED light to that later. Any assistance would be appreciated.

Sorry, I cannot help you with the meter. Beware, that if this is a copy of the Doc Wattson type--They do not keep track of the direction of current flow (AH in vs AH out simply add to the total).

As an aside, I also wanted to test the output of the solar panels to see how they compared to the sticker on the back. The VOC is listed at 22.4V and the ISC at 6.2 amps. I did the test on a clear bright day at straight up noon here in the Mid Atlantic. I disconnected the wires leading to the charge controller to test each panel independently. I tested at the junction box of each. The panels each read around 22V buts seemed to drop off to around 20 rather quickly. The amps for each were 2.8 to 3 amps. If each panel ISC is 6.2A I don't get why they are half of the rated value. Thank you very much.

Isc is current "short circuit"--So it is not useful for predicting output. You need Vmp and Imp ratings to understand the capabilities of the panel.

Assuming this is Vmp~18 volts, then the Imp would be around:

50 watts / 18 volts Vmp = 2.78 amps Imp (estimate).

So, your readings of ~2.8 to 3.0 amps is close to the best we would expect from a single 50 Watt panel charging a 12 volt battery.

Solar panels are roughly "constant current" output devices. I.e., a set amount of sun will give X.X amps from zero to ~Vmp voltage output on the solar array. The power into the battery (with a PWM type charge controller):

2.78 amps * 12.0 volts "near dead" battery charging = 33.3 Watts actual charging "power" into near dead battery
2.78 amps * 14.2 volts battery charging = 39.5 watts actual charging "power" into near full battery

So, you can see the power into the battery with a constant current device is also dependent on battery voltage.

-Bill

bte52 · April 2014

Re: Watt Meter on Small Solar System Problem

Thank you very much. Right, they were wired together. That being said, I tested each panel individually. They were not tied together for my test.

zoneblue · April 2014

Re: Watt Meter on Small Solar System Problem

That meter is ok to test a system like that, but i wouldnt really leave it in place, certainly dont try to run the inverter through it! The wires arent thick enough and the insides while technically rated to something like 100A, wouldnt handle anything like that on a continuous basis. And as others said they dont work backwards so you need a proper battery meter. Learning exercise fine, but if you are serious id be starting to collect quality gear. The best money these days in on midnite kid plus Wbjr.

bte52 · April 2014

Re: Watt Meter on Small Solar System Problem

Thanks to all for your help. I guess the YouTube video I watched to install this meter was wrong. All fixed now.

It's almost noon here and I have the system up and running. The inverter is off and there is nothing running. Perhaps you can tell me if these numbers are acceptable. The battery was pretty much fully charged. When I first hooked the cables up to the battery it was reading 5.78 amp but settled back to 4.12A now. The volts read 13.6V and 54.6W. The scrolling readings are 13.62Ah, 17.9Wh, 5.88Ap and 12.81Vm. Thank you again.

BB. · April 2014

Re: Watt Meter on Small Solar System Problem

13.6 volts is "float" voltage for a lead acid battery (long term storage/charging). If your battery is still drawing >4 amps and the battery is not very large, I would suspect that it still needs a whole lot more charging.

What is the AH rating and type of battery/bank you have (i.e., 100 AH and flooded cell or AGM, etc.)?

In general, a battery that is properly in float with no other loads should draw about 1-2% maximum float current unless the battery is undercharged and/or has a shorted cell.

If you have a flooded cell battery, get a decent quality glass hydrometer and measure/log the temperature corrected specific gravity of each cell.

If these are sealed batteries, you need to measure the "resting voltage" (i.e., no charging/discharging for 3+ hours, then measure voltage--I.e., >12.7 volts, battery is charged. Below ~12.2 volts, battery is 1/2 charged. Below 11.5 volts, battery is near dead).

-Bill

bte52 · April 2014

Re: Watt Meter on Small Solar System Problem

BB. wrote: »

13.6 volts is "float" voltage for a lead acid battery (long term storage/charging). If your battery is still drawing >4 amps and the battery is not very large, I would suspect that it still needs a whole lot more charging.

What is the AH rating and type of battery/bank you have (i.e., 100 AH and flooded cell or AGM, etc.)?

In general, a battery that is properly in float with no other loads should draw about 1-2% maximum float current unless the battery is undercharged and/or has a shorted cell.

If you have a flooded cell battery, get a decent quality glass hydrometer and measure/log the temperature corrected specific gravity of each cell.

If these are sealed batteries, you need to measure the "resting voltage" (i.e., no charging/discharging for 3+ hours, then measure voltage--I.e., >12.7 volts, battery is charged. Below ~12.2 volts, battery is 1/2 charged. Below 11.5 volts, battery is near dead).

-Bill

Here are the battery specs. Thank you:

Cold Cranking Amps at 0°F : 575amps
Marine Cranking Amps at 32°F : 715amps
Minutes at 25 Amp: 175min.

Cariboocoot · April 2014

Re: Watt Meter on Small Solar System Problem

bte52 wrote: »

Here are the battery specs. Thank you:

Cold Cranking Amps at 0°F : 575amps
Marine Cranking Amps at 32°F : 715amps
Minutes at 25 Amp: 175min.

Not a battery suitable for renewable energy systems. Notice the complete lack of Amp hour rating. The specs given are for an automotive battery or Marine/RV battery.

BB. · April 2014

Re: Watt Meter on Small Solar System Problem

Looks like a Deka DC27 ~80 AH marine/rv/deep cycle battery... The AH rating is missing from the website (as Marc says, usually a sign that these are not really deep cycle batteries).

Anyway, using our various rules of thumb, a 5%-13% rate of charge would be around 4 amps minimum. And a 1% rate of charge would be around 0.8 amps (assuming full charge and float voltage).

Probably a flooded cell battery which will allow you to measure the specific gravity. This will tell you a whole bunch more about your battery bank state of charge/health.

If you only have one battery, more or less, I would suggest a maximum inverter of:

80 AH * 12 volts * 1/5 discharge rate * 0.85 AC inverter eff = 163 Watt nominal AC inverter

Or, at most, about 2x 163 Watts (~320 Watt) maximum AC inverter... Any more than that, and the battery will probably "collapse" before it can output that amount of power/current.

-Bill

bte52 · April 2014

Re: Watt Meter on Small Solar System Problem

Thank you all. Yes, it is a Deka DC27. I did a lot of research on batteries but it looks like I should have done more. There is a battery store in town called Pasco and I went in with very specific requirements and allowed the counter person to change my mind. Oh well, I have a boat with an older battery and will install this one in it and buy another.

Given that I already have my Power Bright 1100 inverter, are there any recommendations on exactly what 12V battery I need for my small backup solar system (2 50 watt panels). Thank you guys so very much for your patience and help. I am trying to learn.

Ed

BB. · April 2014

Re: Watt Meter on Small Solar System Problem

It is all about building a "balanced" system...

Too big of AC inverter--You may waste some power (larger inverters have more "tare" losses).

Make the battery bank larger, then you need larger solar panels/charging source (batteries should have around 5% to 13% rare of charge). Too small of solar array (for battery bank size), the harder it is on the battery bank.

Or, if you have a solar array limitation (your 100 watts of panels), then that limits the size of battery. Which then limits the size of AC inverter/loads you can support.

So--We can design a system around any one component. But, it usually works out better if you design the system to support the loads you need. Using a Kill-a-Watt type meter, or DC AH/WH meter to measure your loads is very helpful.

-Bill

Watt Meter on Small Solar System Problem

Comments

Categories