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Old 03-16-2013, 03:13 PM   #1
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Determining State of Charge on Solar Systems

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?
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Old 03-16-2013, 03:18 PM   #2
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Brian,
You have way too much free time on your hands, all I know it works and that keeps me happy.
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Old 03-16-2013, 06:20 PM   #3
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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.
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Old 03-16-2013, 06:50 PM   #4
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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.
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Old 03-16-2013, 07:49 PM   #5
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No offense meant Brian, I was trying to make some levity out of a serious question.
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Old 03-16-2013, 08:46 PM   #6
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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.

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Old 03-16-2013, 11:40 PM   #7
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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.
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Old 03-17-2013, 12:26 AM   #8
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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.
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Old 03-17-2013, 12:50 AM   #9
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Quote:
Originally Posted by bvansnell View Post
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
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Old 03-17-2013, 01:19 AM   #10
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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.
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