Determining State of Charge on Solar Systems

[bvansnell]

I have been monitoring the indicated State of Charge (SoC) on our solar system according to our GoPower GP-PWM-30 solar regulator. I have noticed that during the day the SoC can get to 100%. This indication seems close to the SoC that would be found by interpreting the voltage on our Equus Battery Monitor.

According to the reference below SoC can't accurately be determined when a battery is being charged or discharged -- only when has been at rest for some time.
Measuring State-of-charge - Battery University

Does anyone know whether solar regulators such as the one we have take into account all the complexities in determining a true SoC?

How do you determine SoC on your system?

[cpaharley2008]

Brian,
You have way too much free time on your hands, all I know it works and that keeps me happy.

[bvansnell]

Jim:

I thought it was a reasonable question. Most people expect to have a working gas gauge in a car. An accurate indicator for the amount of energy remaining in a battery is the equivalent for a solar system. Judging by the discussions there have already been on this forum people are concerned about how to manage their solar systems and need help in doing this.

For now I will assume that the State of Charge taken while the battery is being charged or discharged is not likely going to be accurate, and it would be better to pay attention to the readings early in the morning or after the sun has set.

I'm also trying to decide whether it is worthwhile to replace my three year old single group 27 battery with a new group 29 to increase the reserve capacity.

Going solar is a new adventure and there is a lot to learn.

[Vermilye]

It isn't a bad question, but the problem is the only accurate way to measure SOC is to remove all charging devices, apply a small load to the batteries for a short time, remove the load, then measure. Because most solar systems don't "unhook" all that easily, it is difficult to do. If you have the typical Escape Go Power installation, I suspect you will get a pretty accurate measurement if you wait until after dark (and the converter & tow vehicle are not charging) and check the meter. It will be close.

Another way to check is to measure the specific gravity of the battery cells.

I do know the single 95 watt panel combined with the Go Power PWM controller has no problem keeping my pair of 6V, 225 amp/hr batteries topped off in all but the most shaded conditions.

[cpaharley2008]

No offense meant Brian, I was trying to make some levity out of a serious question.

[btroxell]

I wanted to track my power usage so I bought a Clipper Marine Instruments BM-1. I find it fun and partially useful, to watch as the panels charged the batteries. As a gauge to determine how 'full' the batteries are, it's an OK guide. If there is any device pulling current, it will read real low so if the heater fan is blowing, I have to wait until it turns off to check it. It also drops to a 80% reading fast and then hangs there for a long time. I was worried the first few times I saw it drop to 80% because I was concerned if I was going to make it through the night. But that didn't make sense since we have dual 6v batteries and we have dry camped for 3 days prior to getting the gauge with no problems. I ended up getting a hydrometer and took measurements of my batteries and compared the hydrometer's readings with that of the BM-1's readings. This confirmed that the BM-1 read low in the top part of the scale but was close in the mid-range, where it is more useful.
It did come in handy last summer when camping under lots of trees. It told me that I did not get a full charge a few days; not critical but useful in that I needed to place the panels in different locations when were out and about during the day.

Ben

[bvansnell]

Jon, Jim and Ben:

Thank you for your replies.

Here are some figures from my solar regulator recorded today: SR=solar regulator, BM=battery monitor. The only load on the battery was the "phantom load" from smoke, propane and carbon monoxide monitors and any others I don't know about.

Time...........State of Charge.......Voltage (SR)....Voltage (BM)......Comment
4:00 PM...........100.......................12.8....... .......12.89...............Bright cloud
7:00 PM.............91.......................12.6...... ........12.68...............Dusk
8:00 PM.............86.......................12.5...... ........12.60...............Dark

For my 100 Amp-hr group 27 battery if useable power is between a given State of Charge and 50%, the reading taken at 4:00 PM would indicate 50 Amp-hr available. The reading at 8:00 PM would indicate 36 Amp-hr available, 28% less.

[gbaglo]

This thread just reminds me of other solar threads I've read and affirms my decision to buy a generator.
In my next life, my next retirement, I may have enough time to think about a solar installation.

I can't even figure out an appropriate Smilie.

[barry]

Originally Posted by bvansnell

Jon, Jim and Ben:

Thank you for your replies.

Here are some figures from my solar regulator recorded today: SR=solar regulator, BM=battery monitor. The only load on the battery was the "phantom load" from smoke, propane and carbon monoxide monitors and any others I don't know about.

Time...........State of Charge.......Voltage (SR)....Voltage (BM)......Comment
4:00 PM...........100.......................12.8....... .......12.89...............Bright cloud
7:00 PM.............91.......................12.6...... ........12.68...............Dusk
8:00 PM.............86.......................12.5...... ........12.60...............Dark

For my 100 Amp-hr group 27 battery if useable power is between a given State of Charge and 50%, the reading taken at 4:00 PM would indicate 50 Amp-hr available. The reading at 8:00 PM would indicate 36 Amp-hr available, 28% less.

Hi Brian

It seems to me that if you are using that much juice with just phantom loads there is something wrong. My batteries basically stay at 95% and higher all the time with the trailer sitting in the driveway (not plugged in to shore power). Bright sun or cloud they will be at 100% but if it rains for a couple of days then they might go down to 95%.

Maybe your battery is on the way out?

Lucky you having "bright cloud" today; we had torrential rain and hail.

Barry

[bvansnell]

Barry:

While it is true our battery may be on its way out (it is three years old), another explanation might be that the "true" State of Charge of our battery in the middle of the day is quite a bit less than 100% -- the solar regulator is being influenced by the charging voltage. Our trailer is parked at the back of our lot and it is mostly shady even on a sunny day. There is rarely any measurable charging amperage. You will notice that the voltage is not particularly high while charging. When on shore power the voltage is usually over 13 volts.

I don't think the change in voltage later in the day is due to the phantom load. When no charging is going on the solar regulator can better determine the actual State of Charge.

I suppose a way of checking this would be to move the trailer to our front driveway where it will be in full sun and measure the difference.

Another way of checking the condition of the battery would be to connect to shore power and fully charge the battery, then disconnect it and see whether the battery voltage stays up. Since very little sun is forecast, maybe I'll try this next.

[bvansnell]

baglo

I think you are just seeing confirmation of the truth of Kermit's observation that it is not easy being green, and on the eve of St. Patrick's Day too!

Brian

Originally Posted by gbaglo

This thread just reminds me of other solar threads I've read and affirms my decision to buy a generator.
In my next life, my next retirement, I may have enough time to think about a solar installation.

I can't even figure out an appropriate Smilie.

[barry]

Originally Posted by bvansnell

While it is true our battery may be on its way out (it is three years old), another explanation might be that the "true" State of Charge of our battery in the middle of the day is quite a bit less than 100% -- the solar regulator is being influenced by the charging voltage. Our trailer is parked at the back of our lot and it is mostly shady even on a sunny day. There is rarely any measurable charging amperage. You will notice that the voltage is not particularly high while charging. When on shore power the voltage is usually over 13 volts.

Brian

You are probably right. Mine goes up to over 14 volts sometimes and usually is over 13 volts when sitting at 100% but my trailer is sitting in a driveway that faces south west so it gets a fair bit of sun during the day ....... when we see it. If I go into the trailer and turn on a couple of lights the charging amps will go up to .3 or .4 until I shut them off. If I turn on the inverter then it will shoot up to 2 or 3 amps if the sun is out.

I'd plug the trailer in for a couple of days and then do the same test if I was you.

Barry

[cpaharley2008]

I feel once you establish your 12v needs and a newer battery to meet those needs your numbers will increase. A 3 year old liquid factory stock battery maybe your issue. I always heard that AGM and solar are a good match, perhaps a group 29 may fit. That is what I'm going to install once my factory unit starts to drop. Have you check your fluid levels in your battery and the sg of each cell?

[ddevin]

Originally Posted by gbaglo

This thread just reminds me of other solar threads I've read and affirms my decision to buy a generator.
In my next life, my next retirement, I may have enough time to think about a solar installation.

I can't even figure out an appropriate Smilie.

Baglo, you don't need to understand any of this to enjoy solar power. I would suggest that a generator is more complicated because you have to DO something! With solar, all you need to do is...well, enjoy the benefits! We have enjoyed our solar for 3 seasons, and have not done a single calculation or measured a single amp, volt or joule! Go green!

[bvansnell]

I plugged the trailer into shore power for 18 hours and let the converter charge it, then disconnected from shore power. The solar panel did what it could to maintain the charge. There were no loads apart from phantom loads. Here are the results:

Time......State of Charge....Voltage(BM)......Comment
8:00AM.........100%..................13.43........ ..Just disconnected from shore power
9:00AM.........100%..................13.10........ ..Bright diffused light (trailer in shade)
8:00PM............93%..................12.73...... ....Dark

The initial high voltage was due to the battery just coming off charge. In my opinion, the state of charge at 8:00 PM is likely a reasonably accurate estimate of the real state of charge. The solar panel was not able to do much to charge the battery during the day since the trailer was in shade. (You need sun to run a solar system!) Temperature was cool (10 deg C = 50 deg F.) which may effect results. This test does not show the reserve capacity of the battery because no load was applied. I used a clamp-on DC ammeter to check the size of the phantom load. It was about 300 milliamps, or about 4 amp-hrs for 12 hours, which explains most of the loss of State of Charge. (The single group 27 battery has a theoretical maximum capacity of about 100 amp-hours when new, i.e. 100% State of Charge = 100 amp-hrs.)

[bvansnell]

I just came across an excellent source of information on managing power on trailers and RVs. The author recommends a Bogart Engineering Trimetric 2020 or 2025 for determining State of Charge. This device calculates State of Charge from measuring power into the battery and power out and other factors rather than estimating it from battery voltage.
Here is a link: https://handybobsolar.wordpress.com/...ging-puzzle-2/

Here is a link to Bogart Engineering: Bogart Engineering Products | Bogart Engineering

[ChuckS]

Brian when I do my solar charging system I will have the Bogart on the trailer. It shows how much goes into the batteries and how much is being used. I have room for two 120V panels on the roof. PantherRVProducts sells lots of the Bogart system.
Chuck

[Brian B-P]

A very commonly mentioned device for tracking state of charge is the Xantrex LinkLITE Battery Monitor (or its predecessor). It works on an in/out balance basis, like the Bogart product. The measurement basis is actually current, rather than power.

[ChuckS]

Originally Posted by ChuckS

Brian when I do my solar charging system I will have the Bogart on the trailer. It shows how much goes into the batteries and how much is being used. I have room for two 120 watt panels on the roof. PantherRVProducts sells lots of the Bogart system.
Chuck

The panels are 120 Watt.

[Carol R]

I will have solar on my new 17B and have purchased an EMS from Progressive which Reace is installing for me. I will show my ignorance here - will the EMS do what the LinkLite battery monitor does?