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Post by cybrmarc on Oct 26, 2019 11:39:41 GMT
I've had this problem with all of the charge controllers I've owned, where they show the voltage of batteries as much lower (by .2 - .6 or more volts) than a direct read from the battery terminals with the voltmeter. I installed my new Rover yesterday and same thing. Using two flooded 12V lead acid batteries in parallel.
The only cause I can think of would be cable length to the charge controller, which is maybe 2' or so. But that's about the same length as the wire leads on my voltmeter, so I don't get it. Will try decreasing cable length to controller to ~6" today and see what that does.
Any ideas?
I wouldn't care that much except for that this seems like it would make my low voltage cutoff wildly inaccurate, as well as possibly the other charging stages (boost, equalization, etc.) When controller read 14.4 yesterday for boost, battery was actually 14.56
Thanks, Marcus
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Post by tattoo on Oct 26, 2019 12:27:53 GMT
I've had this problem with all of the charge controllers I've owned, where they show the voltage of batteries as much lower (by .2 - .6 or more volts) than a direct read from the battery terminals with the voltmeter. I installed my new Rover yesterday and same thing. Using two flooded 12V lead acid batteries in parallel. I have had the same problem through the years...... That seems to be normal............ I also have lead acid batteries...............
I don't think it's the type of battery...........
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Post by tattoo on Oct 26, 2019 12:30:08 GMT
I wouldn't care that much except for that this seems like it would make my low voltage cutoff wildly inaccurate, as well as possibly the other charging stages (boost, equalization, etc.) When controller read 14.4 yesterday for boost, battery was actually 14.56 Have you went into your CC and changed the values manually to make up for the difference?
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Post by tattoo on Oct 26, 2019 12:45:49 GMT
When controller read 14.4 yesterday for boost, battery was actually 14.56 Thanks, Marcus 14.56 isn't that high you should be just fine..... What KIND of batteries do you have? You know who made them? Thanks
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russ
Solar Advocate
Posts: 63
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Post by russ on Oct 27, 2019 5:34:52 GMT
I don't think the voltmeter in the charge controllers are very accurate. I've noticed the same thing. A couple of tenths of a volt is not going to be a problem.
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Post by tattoo on Oct 27, 2019 12:52:13 GMT
I don't think the voltmeter in the charge controllers are very accurate. I've noticed the same thing. A couple of tenths of a volt is not going to be a problem. Thanks for saying that, I had forgotten about that...........
I believe I responded to a thread about a year ago about this problem. What I can remember, I checked several times a day to help the guy just for comparison....... I think I seen as much a .7 volts...... I couldn't see why it just was.....
What we came up with it has a lot to do with the Load and what the CC is seeing in real time..............
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Post by mediadogg on Oct 27, 2019 14:54:26 GMT
I don't think the voltmeter in the charge controllers are very accurate. I've noticed the same thing. A couple of tenths of a volt is not going to be a problem. You can get a few fractions of a volt difference based on where the measurement is taken, the amount of current flowing and your wires. That's one of the reasons that some equipment that monitors battery voltage actually has a separate low current wire that connects directly to the + battery terminal to avoid that error. The difference is due to the voltage drop across the wires leading from the CC to the battery, and will be larger for large currents or smaller gauge wires. I think that is basically what tattoo is referring to in the previous post.
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Post by tattoo on Oct 27, 2019 15:58:45 GMT
I think that is basically what tattoo is referring to in the previous post. Exactly.......... I was amazed at the difference I seen at different times of the day.......... At night it was closer to the actual reading on the CC to the batteries themselves........
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Post by playersz28 on Oct 29, 2019 14:23:54 GMT
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Post by tattoo on Oct 29, 2019 14:52:43 GMT
Yep you are correct it sure did........ And that is the thread I remembered................
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Post by mediadogg on Oct 29, 2019 17:52:28 GMT
It s not hard to understand if you postulate the opposite. Suppose you had some imaginary current flowing between points A and B, where A and B are connected by wire with a resistance > 0. If you measured the voltage at A and B, and they were exactly the same, you would say "Oh, it must be an open circuit, i.e. no current is flowing."
So, if A is the charge controller terminals, and B is at the battery terminals, and you have any significant current flowing, then the voltage at A CANNOT be equal to the voltage at B. It would defy the laws of physics and electronics (at least at the non-quantum level).
Example: If the charge controller is supplying 10A to the battery, it would take only 0.02 ohms to create a 0.2V voltage drop. And since you have two wires going to the battery (+ and -), each wire (combined with terminal resistance) could have as little as 0.01 ohm resistance to cause a 0.2V drop. AWG 12 has a resistance of 1.59 ohm / 1000 ft. So, it would take about 6.5 feet (each of + and - wires) to cause a 0.2v drop at 10A.
Let me know if you find errors in my calculations. Of course, use proportional adjustments based on your actual configuration.
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Post by tattoo on Oct 29, 2019 19:19:37 GMT
Let me know if you find errors in my calculations. Of course, use proportional adjustments based on your actual configuration. It sounds good to me........
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russ
Solar Advocate
Posts: 63
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Post by russ on Oct 30, 2019 5:42:39 GMT
I don't think the voltmeter in the charge controllers are very accurate. I've noticed the same thing. A couple of tenths of a volt is not going to be a problem. You can get a few fractions of a volt difference based on where the measurement is taken, the amount of current flowing and your wires. That's one of the reasons that some equipment that monitors battery voltage actually has a separate low current wire that connects directly to the + battery terminal to avoid that error. The difference is due to the voltage drop across the wires leading from the CC to the battery, and will be larger for large currents or smaller gauge wires. I think that is basically what tattoo is referring to in the previous post. When checking the accuracy of the charge controller voltage I use a fluke multimeter and measure the voltage at the battery terminals on the charge controller in order to eliminate voltage drop from the battery cables. I want to see the voltage at the charge controller itself and compare that with the voltage displayed on the charge controller. The charge controller measurements are always off by at least a couple tenths of a volt.
When I measure the voltage at the battery it is always a little different from the measurement at the charge controller battery connection, when panels are charging the battery. As you said, due to voltage drop.
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Post by mediadogg on Oct 30, 2019 11:57:04 GMT
You can get a few fractions of a volt difference based on where the measurement is taken, the amount of current flowing and your wires. That's one of the reasons that some equipment that monitors battery voltage actually has a separate low current wire that connects directly to the + battery terminal to avoid that error. The difference is due to the voltage drop across the wires leading from the CC to the battery, and will be larger for large currents or smaller gauge wires. I think that is basically what tattoo is referring to in the previous post. When checking the accuracy of the charge controller voltage I use a fluke multimeter and measure the voltage at the battery terminals on the charge controller in order to eliminate voltage drop from the battery cables. I want to see the voltage at the charge controller itself and compare that with the voltage displayed on the charge controller. The charge controller measurements are always off by at least a couple tenths of a volt.
When I measure the voltage at the battery it is always a little different from the measurement at the charge controller battery connection, when panels are charging the battery. As you said, due to voltage drop.
Ah ha! Interesting. That's what I would do. But there must be a reason other than poor circuit design, especially if it is pervasive? A single shottky diode could account for it, if there is a polarity protection circuit. Or maybe rather than using a shunt and very sensitive current sensing circuit, they simply use a resistor (now that would be really poor design)? What do you think? Seems to me if there was a fixed error, they could calibrate the meter to give the correct reading.
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Post by tattoo on Oct 30, 2019 12:56:56 GMT
Seems to me if there was a fixed error, they could calibrate the meter to give the correct reading. During the day my voltage is contently going up and down because of it charging. Have you checked your readings at night to see how far they are off?
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