Adding freeze protection to Batteries.

I'm new to solar, but I have some electronics background.
I found several references and utube videos online regarding the lack of freeze protection installed on LIFEPO4 batteries.
It is often considered an important factor in grading battery quality.
Most systems do however have high temperature protection.
There is a simple way to make the high temp protection also do freeze protection without sacrificing the high temp protection.
Most if not all temp sensor used in HVAC, batteries, appliances and solar devices, are NTCs (Negative Temperature Coefficient) resistors.
The quickest way to explain how the hack works is by example.
Assume your system (Battery, Charge controller, inverter charger etc.) uses a 10 k nominal ohm NTC. The 10 K is the value of resistance at 25 C (77 F).
This is just an example and the method described here will work with any NTC with any nominal resistance.
Assume you device shuts down at a temperature of 135 degrees F (about 55 C). At 55 C the resistance of the 10 K NTC will be about 3.5 kohms.
At 0 C (32 F) the resistance will be about 27 kohms.
Paralleling a 4 kohm resistor with the temp sensor would reduce the total resistance at 32 F to around 3.5 K and the unit should shut down from what it perceives as high temp cut off.
There are many ways of doing this and one can get as creative as one likes.
One of the simplest is a cheap over the counter freeze protect snap disc thermostat in series with a 4 kohm resistor, This series combination is then paralleled with the existing temp sensor.
When the temperature is above freezing the snap disc is open and the hack has no effect on the performance. In freezing temperatures the thermostat snap (disc) closes and the resistance suddenly drops, triggering the high temp shut off circuit. If the temperature rises the thermostat opens and the unit functions as normal.

What you are describing will work in that it will prevent charging the LiFePO4 cells when they are near or below freezing. There are some other considerations you may want to include in your thinking.

I may have to think about it some more, but I'm not aware of that many charge controllers and/or inverters that already do a battery disconnect at high temperature. Maybe there are many out there, and I just haven't had any experience with them.

For some of the items you may have connected to your battery, having a sudden complete disconnect from the battery may have catastrophic consequences. Many of the older or less expensive charge controllers will fry immediately if there is PV power coming in with no battery connected. An inverter may survive a sudden disconnect of the battery, but for most of us having the power suddenly go out is not good.

FET-based common-port BMS's mostly have low-temp cutoff these days, and it will only cut off charging, not discharging. This makes it so the charge controller still sees a battery, but no charging current goes into the battery. Saves the SCC. Likewise the inverter can keep going, although the battery eventually needs to get warmed up and take a charge.

I chose to make a thermostat-controlled heating circuit to keep my battery warm. I posted the details here. It's probably not an approach for everyone, and your idea is much simpler. Choices depend on the situation and requirements.

Horsefly said:

For some of the items you may have connected to your battery, having a sudden complete disconnect from the battery may have catastrophic consequences. Many of the older or less expensive charge controllers will fry immediately if there is PV power coming in with no battery connected. An inverter may survive a sudden disconnect of the battery, but for most of us having the power suddenly go out is not good.

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This fix does not in of itself shut down the charging. It simply tricks the existing high temp shutdown circuitry to do so.
I would hope that existing high temp shutdown circuits are built to avoid the damage you are reffering to.

I chose to make a thermostat-controlled heating circuit to keep my battery warm. I posted the details here. It's probably not an approach for everyone, and your idea ...

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I prefer your approach as well and am going to do something similar.
The reason I made this post is because there so many inexpensive batteries available these days and many Utube videos are discouraging people from purchasing them simply because they don't have freeze protection.
The point is: If you have access to the inside of the battery, the issue can be fixed for a few bucks and a little work.
The same is true if your charge controller lacks freeze protection, there is an inexpensive fix.

McKravitts said:

Paralleling a 4 kohm resistor with the temp sensor would reduce the total resistance at 32 F to around 3.5 K and the unit should shut down from what it perceives as high temp cut off.

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Except it will shut down above 32 degrees rendering your battery useless in normal temperatures.

The 4k resistor is in series with a NO thermostat. The thermostat will open when the temp rises, taking itself out of the circuit and the charging will restart.
I've used this idea or a modified version of it several times in my life on various pieces of HVAC equipment.
It has always worked.

McKravitts said:

The 4k resistor is in series with a NO thermostat.

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That is not what your original post said. You are trying to parallel a resistor with the high temp cutoff sensor. Fine, but the firmware is set to disconnect as the temp rises above that set-point. Fooling it into thinking the temp is lower than it is will still cause it to trip on a temp rise (above 32). In other words it will be cut off during a normal temp range. And the high temp cutoff is for discharge. What Lifepo4 needs is a low temp cutoff for charging.

That is not what your original post said. You are trying to parallel a resistor with the high temp cutoff sensor. Fine, but the firmware is set to disconnect as the temp rises above that set-point. Fooling it into thinking the temp is lower than it is will still cause it to trip on a temp rise (above 32).

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What I said in my original post was:
"One of the simplest is a cheap over the counter freeze protect snap disc thermostat in series with a 4 kohm resistor, This series combination is then paralleled with the existing temp sensor."

The resistance of NTC's ( negative temperature coefficient) resistors increases as the temperature decreases.
I trust i don't need to say more.

Lifepo4 only needs to be prevented from charging at low temps. Even if the BMS's high temp sensor did cut off charging on a high temp alarm (not sure if they all do) your method would still render the battery useless when tripped.

I used to teach electronics years ago, so I have experience with stuborn students. I lost much of my patience in my old age.
I also admit that my technical abilities far exceed my communication skills so I'll try to figure out how to incorporate a schematic and a chart of what happens at various temperatures in a future conversation with you.
For now let me just add that I've already experimented with just such a circuit and it works.

BTW: the heading of this section is titled; "Danger Zone! (Advanced User Experiments Only)"

McKravitts said:

I used to teach electronics years ago, so I have experience with stuborn students. I lost much of my patience in my old age.
I also admit that my technical abilities far exceed my communication skills so I'll try to figure out how to incorporate a schematic and a chart of what happens at various temperatures in a future conversation with you.
For now let me just add that I've already experimented with just such a circuit and it works.

BTW: the heading of this section is titled; "Danger Zone! (Advanced User Experiments Only)"

Click to expand...

later on you commented about a snap disc thermal relay.. i think some of the folks might have missed that. that or they and you are both hard headed... being a teacher/professor is not much of a recommendation these days... Literally, professor derives from Latin as a "person who professes". just because you profess does not mean diddly squat.

ken morgan said:

later on you commented about a snap disc thermal relay.. i think some of the folks might have missed that. that or they and you are both hard headed... being a teacher/professor is not much of a recommendation these days... Literally, professor derives from Latin as a "person who professes". just because you profess does not mean diddly squat.

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Touché ken.

The internet is an interesting place in that everyone gets abused regardless of what they may have achieved in the past. You’re actually better off playing dumb.

Thanks for the advise.

I got out of teaching for that very reason. I was also a graduate student at the same time that I was teaching.

McKravitts said:

Touché ken.

The internet is an interesting place in that everyone gets abused regardless of what they may have achieved in the past. You’re actually better off playing dumb.

Thanks for the advise.

I got out of teaching for that very reason. I was also a graduate student at the same time that I was teaching.

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regardless both parties are missing either accidentally or on purpose.. I noted your post about a snap disc. you never noted that afterwords while the other party seemed to miss it. then you both continue to bicker like a couple of 5 year old children. then the whole spiel about how I am a teacher/professor/doctor/ whatever someone thinks will impress the other.... That's just my observation of the entire situation... improvise, adapt, overcome.

Your point is well taken Ken and I find it interesting on how forcibly you "profess" your points and how well you mastered the art of not reading between the lines but rather skipping lines entirely.

I did miss the part about the disc, but the rest remains valid. If one is going to use an external "thermostat" might as well just use it to stop the charging process, and not lose the ability of the battery to provide power in the process.

McKravitts said:

Your point is well taken Ken and I find it interesting on how forcibly you "profess" your points and how well you mastered the art of not reading between the lines but rather skipping lines entirely.

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I'm sorry which do you prefer? Preparation H or Vagisil? whichever it is I will be happy to send you some. that sums up my involvement in this bit of internet stupidity.

This seems to have turned pretty ugly in short order. I'm not sure why.

Horsefly said:

This seems to have turned pretty ugly in short order. I'm not sure why.

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Not sure why either. Did not appreciate being accused of "bickering" for trying to explain some of the flaws with this approach.

Snap disc freeze protec thermostats are not a one time thermostat. They work just like normal thermostats.
The turn off/open when the temp rises above freezing.
when open the circuit I use is no longer doing anything. Everything goes back to what it was.
If you are proposing that just having been in the circuit caused other circuits to get damaged please eloborate.

McKravitts said:

If you are proposing that just having been in the circuit caused other circuits to get damaged please eloborate

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Who said that?

But once again (last time), if you use the high temp cutoff as a low temp cutoff, it may or may not stop the charging MOS (which is what you want to do at low temp). And most likely you will also cut off the discharge MOS which would render the battery useless until the disc resets. I would say having no power at all at low temp is undesirable.

And as I also said, if one is going to use a thermal disc or other type of thermostat for low temps, it would be easier to just use it to stop the charging circuit instead of trying to trick the BMS. Sorry if that was not clear.

Am I missing something here? Why not just use a BMS which has low-temp cutoff (in addition to the high-temp protection), as the last line of defense, and wire in the heater pad with digital thermostat like so many others here have done (with the posted how-to threads and whatnot)? ...So you can continue to charge in freezing environment, but will still have last line of defense in case the heater circuit fails.

Why the need to re-invent the wheel?


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W1209 Thermostat With Case




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