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Old 08-05-2016, 01:35 PM   #1
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Default Battery temperature sensor

The new victron monitor I ordered had an option for a temp sensor. I assume the intent is to adjust SOC based on degradation at cold temps.

That's fine I suppose for lead acids, but not necessarily tells you anything with lithiums.

I ordered it anyways because it was fairly cheap and I'd like a good idea of the battery temp just for monitoring purposes. I also ordered the bluetooth transmitter so I can get all the data on my iphone.

So the question I have is, how good a method is this to get an internal battery temp? The sensor is mounted on the positive post on the battery. I would think this would just be the same as the outside ambient temp. Or does it really work?
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Old 08-05-2016, 01:56 PM   #2
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The new victron monitor I ordered had an option for a temp sensor. I assume the intent is to adjust SOC based on degradation at cold temps.

That's fine I suppose for lead acids, but not necessarily tells you anything with lithiums.

I ordered it anyways because it was fairly cheap and I'd like a good idea of the battery temp just for monitoring purposes. I also ordered the bluetooth transmitter so I can get all the data on my iphone.

So the question I have is, how good a method is this to get an internal battery temp? The sensor is mounted on the positive post on the battery. I would think this would just be the same as the outside ambient temp. Or does it really work?
I think the battery post mounted temp sensors are fairly imprecise. It is hard to imagine that they could accurately read the internal temp of the battery while hanging in the air. On our old system, we had one sensor that was on the post, and one that was on the battery case, and they matched fairly well at low speed temp changes, but got further different as the temp changed quickly or the air around the batteries got hot. It is probably time for me to take some IR heat gun readings and compare them to the sensors on our new setup. I have found the two sensors read a few degrees different even though they are on the same post.

On your lithium batteries, it is likely that you only have a wire to the battery post, so less heat transfer path than normal batteries, so you probably would be even less accurate. Using a temp gun to compare would be interesting, but even then the case isn't intimately in contact with the guts of the battery like in a wet cell or AGM.

I do find it interesting that folks like Magnum and some others don't use the standard temperature compensation factor that most of the battery manufacturers recommend. I think it is to try to compensate for the less than great battery temperature readings.
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Old 08-05-2016, 02:09 PM   #3
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There is a hefty solid metal path into the battery from the terminals. It conducts heat far better then does air. I am pretty sure that terminal-mounted temp sensors work just fine.
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Old 08-05-2016, 02:16 PM   #4
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As you might imagine, my concern would be for my lithiums getting down close to 32 F so I can take the necessary precautions on charging.
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Old 08-05-2016, 03:02 PM   #5
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There is a hefty solid metal path into the battery from the terminals. It conducts heat far better then does air. I am pretty sure that terminal-mounted temp sensors work just fine.
I think the biggest problem is that you have a sensor that is hanging out in the air that can change temperature a whole lot, as well as the just the post cooling off, from ambient changes. It is also connects to, in many cases, a huge heat sink battery cable that will either be hotter or colder than the battery as it is charged or discharged, and that cable will conduct heat just as well as the battery post to internals connection.

Pretty simple to test with the IR temp gun. Shoot the post/sensor, the battery case a couple of places, and the battery cable. In our case I would expect the cable to be adding heat to the sensor when the engine is running, as it is closer to the exhaust.

I think it would be interesting to move one of our sensors to the case, taping or tie wrapping it on so good contact, and then some insulation over the sensor.
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Old 08-05-2016, 03:29 PM   #6
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I think the biggest problem is that you have a sensor that is hanging out in the air that can change temperature a whole lot, as well as the just the post cooling off, from ambient changes. It is also connects to, in many cases, a huge heat sink battery cable that will either be hotter or colder than the battery as it is charged or discharged, and that cable will conduct heat just as well as the battery post to internals connection.

Pretty simple to test with the IR temp gun. Shoot the post/sensor, the battery case a couple of places, and the battery cable. In our case I would expect the cable to be adding heat to the sensor when the engine is running, as it is closer to the exhaust.

I think it would be interesting to move one of our sensors to the case, taping or tie wrapping it on so good contact, and then some insulation over the sensor.
Your thermodynamic intuitions are very different from mine. In this kind of analysis, the thermal conductivity of the various materials makes all the difference. Metal conducts heat far, far better than either air or plastic. The ambient air is going to make a much smaller contribution to the sensed temperature than the metal terminal, which leads directly into the heart of the battery. The plastic case wouldn't be great, either. There is a reason the nurse sticks the thermometer into an bodily orifice rather than on your skin. The terminals are the closest we have to a bodily orifice in this case.

You DO have a point about the large battery cables, but they are insulated and so are likely to serve more as a ballast than as a bias (i.e., they will tend to slow down the temperature changes at the terminal). Also, this is one of many reasons to use MRBF fuses right on the battery terminal. They act as a thermal insulator, at least to some extent:

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Old 08-05-2016, 04:00 PM   #7
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Your thermodynamic intuitions are very different from mine.
I agree, and that is why some testing is needed to see what is real, or not. I wish I had a thermal camera, as they show everything at once.

Three main ways to move heat. Conduction, convection, radiation.

The battery guts to post is pure conduction and is temp diff, distance, and coefficient dependent.

At the post you have some of everything. Conduction to get the heat to the surface, conduction of the heat to the cable, conduction at the air interface, convection to carry the heat away with moving air, and radiation off the surfaces. The air moving is the critical part because it maximizes the temp difference between the surface of the post and sensor and the air. Insulating the post and sensor would certainly help this.

The cable will get or give heat to the post and sensor with by conduction, and it's materials will transfer that heat very quickly, just like the heat coming from or to the battery guts. The cable will also heat up by conduction and convection from the surrounding air temp, but a bit slower because of the insulation. There is so much heat transfer area in the cable, though, so still could be a big factor. It could also heat, or cool, by radiation from it's, or other, surfaces like hot asphalt, exhaust pipes, etc.

All this is pretty readily modeled with right software, but requires a lot of work and expertise. On non-critical stuff like this, it is common to build a prototype and just look at it with a thermal video camera over a period of use. We used to do that all the time with control panels, motors, ovens, etc, before we would put them into use on our capital equipment projects. On the consumer stuff we made, that was all done by UL and CSA in the approval process, so we didn't have to be as accurate initially. They have super accurate testing available.

My guess is that the cable could turn out to be the biggest factor.
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Old 08-05-2016, 04:10 PM   #8
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Totally agree about the need for data.

One question, though: How do you determine the "correct" temperature? What really matters is the "core" temperature of the battery, which is itself a heat source. Not obvious to me how you measure that. I guess you can make inferences from the deltas. But that would get complicated.
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Old 08-05-2016, 04:47 PM   #9
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Well the thing about lithium batteries is that their heat generation is practically nil. At least these prismatic type cells in the drop-in type cases.

The lipos generate alot more heat from my experience.

What I wonder is that translates into taking a long time to get cold, unlike metal plates which get cold quickly.
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Old 08-05-2016, 05:26 PM   #10
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While I have your attention, here is a pic of my wiring setup. Unsure of the best placement to install the shunt for the Victron. Does it matter if it's on the cabling to the inverter? or the Charger? Would it be better if it was on the neg cable connecting the batteries to each other?

Also, I'm thinking it's wise to upgrade the wiring between the batteries to match the 4/0 cable I'm using for the inverter. Correct?

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