LifePO4 GBS Amp Hour Testing 2.5v to 3.6v per cell

[Sunking]

No heat is not going to be an issue for you. You will not be able to pull 200 amps from your batteries for any meaningful amount of time for them to get hot enough to worry about. Using 1% rule on battery voltage sag, your batteries are only good for 20 amps (C/5) or 1/10th what you want.

Good luck, time for me to go play

[dax]

Ahh, peltier coolers are about 10% efficient. i'd sure like to be shown otherwise.

Of course they are but in a properly set up and insulated system they use very little energy and operate less than 2% of the time.

[karrak]

Lets do some analysis of using a peltier effect device to regulate the temperature.

From my logged data the average power efficiency of my ~ 10kWh battery is 94.5% and the average amount of power that is cycled through it on a daily basis is ~ 1.6kWh. The amount of heat generated in my battery is equal to 88Wh per day ((1-0.945)*1600). This equates to a average power of 3.7W (88/24).

My understanding from the very brief research that I have done is that the power efficiency of a peltier effect device is around 50%, the 10% quoted earlier is the Carnot cycle efficiency. This means that for every Watt of thermal power transfered it takes 2 Watts of electrical power. This means that to transfer the 3.7W of heat generated by my battery would take a huge 7.4W (177Wh/day) of draw from my power system.

If the battery is placed in a well insulated box and there is enough thermal mass in the box and the average ambient temperature over a 24 hour period is around 20C-25C one would not have to provide any electrical power above what is needed to remove the heat generated by the battery to keep it around 20C-25C.

I must say that before doing this rough analysis I was a little skeptical about using Peltier effect devices to keep the battery temperature regulated but not any more.

Hopefully there isn't anything wrong with my logic. I have never done any design work with Peltier effect devices.

Simon

[karrak]

I'm not as well versed and knowledgeable about batteries as some, but from the little I think I may know about heat transfer, that may not be a very efficient or workable method of heat dissipation.

It is not so much about heat dissiption but heat distribution. My understanding is if there is nothing to transfer the heat away from the cells in the center of the battery and they have to rely on the cells around then to do this that they will end up being hotter than the cells on the ends of the battery. This difference in cell temperatures will mean that the cells in the center of the pack will age faster. It will also mean that the cell "leakage" in the hotter cells will be more so the battery will go out of balance faster.

Simon

[karrak]

No heat is not going to be an issue for you. You will not be able to pull 200 amps from your batteries for any meaningful amount of time for them to get hot enough to worry about. Using 1% rule on battery voltage sag, your batteries are only good for 20 amps (C/5) or 1/10th what you want.

Is the 1% voltage sag rule from "Sunking's Little Red Rule Book". Which rule number is it? Maybe you could give some technical reasons for it.

[J.P.M.]

It is not so much about heat dissiption but heat distribution. My understanding is if there is nothing to transfer the heat away from the cells in the center of the battery and they have to rely on the cells around then to do this that they will end up being hotter than the cells on the ends of the battery. This difference in cell temperatures will mean that the cells in the center of the pack will age faster. It will also mean that the cell "leakage" in the hotter cells will be more so the battery will go out of balance faster.

Simon

I've done some applications involving power transformer cooling, but I've got no hard #'s on thermal design of batteries. However, my experience is such that conductive heat transfer - the type probably responsible for most of the heat transfer within a battery outward to the cover/case, combined with what's probably some convective heat transfer from the contained fluids will be an order of magnitude or so greater than the convective heat transfer that takes place between the outside of the case and the surrounding air. If so, the max. temp. difference between the interior of the battery and the cover will be a fair amount less than the temp. difference between the cover and ambient surroundings. If so, I'd suggest that if the battery is too hot to touch, there might be a need for some internal aux. cooling, otherwise, not.

My gut guess might be that the highest interior battery temp. will be no more than a few degrees warmer than the inside surface of the battery case.

As for efficiency, thermoelectric cooling is indeed only about 10 - 15 % as efficient as a Carnot cycle, with Carnot being the limit in terms of highest efficiency for a cycle operating between two temperatures. the thermoelectric effect is limited by the thermal and electrical properties of the materials being coupled to produce the effect. See the open literature for details.

Thermoelectric internal cooling of a battery is an interesting idea, but perhaps more as an academic investigation than practical. Seems more like a solution looking for a problem to me.

[dax]

The carnot cycle is not efficient, it's really only theoretical and the more I read from the suppose experts here, the more I realise they are clueless when it come to lifepo4. The biggest laugh is they continue to use li-ion as an example of how lifepo4 works and completely misunderstand them, meaning they just babble on making claims which don't represent the reality. Lifepo4 are totally different to li-ion in ever way when dealing with charge discharge.

Unless you have experienced lifepo4 systems where outside 24hr temps ranges from -3 to 35degC in winter and 25-50degC in summer, you're just whistling in the wind. It's also clear they have no idea of how to set up a lifepo4 system, relying again on last century lead acid approaches. If you set up lifepo4 in 500ah banks, using 50ah cells, there is very little increase in cell temps during charging. The only temps you have to worry about are changes to the ambient temperatures and that's when peltiers come in handy and as the majority of their usage would be in the day time, the energy lose is negligible in a decent system. Those in th USA are years behind Aus and probably decades in understanding.

[J.P.M.]

The carnot cycle is not efficient, it's really only theoretical .

Of course it's theoretical, but to say it's not efficient is off the mark. The Carnot cycle is a way of describing the upper limit of any thermodynamic cycle's efficiency of any device when operating between 2 temperature limits. That (theoretical) Carnot efficiency will vary as the two temp. limits vary.

A Peltier device can be described as a thermodynamic system that operates in a series of consecutive states, sort of in a circular fashion that's called a cycle. It's efficiency can be determined and compared to that of a Carnot cycle device which also operates in a cycle. When that comparison is made, it is found that the Peltier cooling mechanism is usually about 10-15 % as efficient as a device operating on a (theoretical) Carnot cycle. See the open literature if interested or an undergraduate text in Thermodynamics.

Seems to me that I'm about as clueless about lifepo4 as you are about Thermodynamics. I try to stay out of areas where I'm clueless. Fair enough ?

[dax]

Seems to me that I'm about as clueless about lifepo4 as you are about Thermodynamics. I try to stay out of areas where I'm clueless. Fair enough ?

Then why are you posting in a lifepo4 thread. Don't need to learn about thermodynamics, got 3 engineers for that. Know enough to be able to set up and test in a practical situation and I am not clueless, you are. Our engineers do the sums, make some changes, test under real conditions and not in a lab or home workshop with controlled environments and then we introduce them to our systems. Went through a few engineers until we found those who actually listened and were prepared to step outside their programming into the real world an really experiment.

I understand supposed experts have this deluded belief only their programming is right and things like lateral approaches are wrong. In the real world, lateral approaches actually work and that's what counts. Not some blind claim by over educated clones unable to see past their simple programming and determined to be right against the reality.

Our systems using peltiers works and have had many supposed experts claim otherwise, even after seeing the working data. I put up with this for decades, having experts tell me what and how I build is wrong and won't work. But it does and has made us a lot of money, so to me it means nothing when experts run down our approach and systems. Especially when the experts, are just employed slaves and not real innovators or developers of new approaches. Fair enough.

[J.P.M.]

Then why are you posting in a lifepo4 thread.

I left it alone trying to learn stuff until I saw something inaccurate with respect to a subject I may actually know something about. Simply commenting on your prior inaccurate statements with respect to the Carnot cycle and perhaps helping others avoid getting it as wrong as you seem to have it.

I try to stay out of areas where I'm ignorant. My comment about my cluelessness was meant as a non volatile attempt at politely suggesting you do the same with respect to Thermodynamics so as to perhaps help avoid spreading misinformation to others as ignorant as, IMO only, you seem to be on the subject of Thermodynamics.

Good luck in your future endeavors.

[SunEagle]

The carnot cycle is not efficient, it's really only theoretical and the more I read from the suppose experts here, the more I realise they are clueless when it come to lifepo4. The biggest laugh is they continue to use li-ion as an example of how lifepo4 works and completely misunderstand them, meaning they just babble on making claims which don't represent the reality. Lifepo4 are totally different to li-ion in ever way when dealing with charge discharge.

Unless you have experienced lifepo4 systems where outside 24hr temps ranges from -3 to 35degC in winter and 25-50degC in summer, you're just whistling in the wind. It's also clear they have no idea of how to set up a lifepo4 system, relying again on last century lead acid approaches. If you set up lifepo4 in 500ah banks, using 50ah cells, there is very little increase in cell temps during charging. The only temps you have to worry about are changes to the ambient temperatures and that's when peltiers come in handy and as the majority of their usage would be in the day time, the energy lose is negligible in a decent system. Those in th USA are years behind Aus and probably decades in understanding.

I wouldn't say that the USA is necessarily years behind Aus in understanding peltier technology. It is just so inefficient (about 1/4 compared to conventional compression cooling technology) it isn't considered economical. It might be worth while in Aus to cool batteries but not so much in other places.

[dax]

Sorry, my comments of "years behind",were regarding the use of lifepo4 and not peltiers. For general cooling, peltiers aren't efficient at all, unless you have a constant energy supply and they are even less efficient when compared to inverter technology cooling/freezing, which is what we recommend our customers use in their off grid systems. It's very worthwhile keeping your lifepo4 pack at an even temp, they perform much better especially in the mornings when conditions are cool and charge inputs are high. You only need a 3-5 degree shift in temps for it to have a small effect, which over time reduces life.

[SunEagle]

Sorry, my comments of "years behind",were regarding the use of lifepo4 and not peltiers. For general cooling, peltiers aren't efficient at all, unless you have a constant energy supply and they are even less efficient when compared to inverter technology cooling/freezing, which is what we recommend our customers use in their off grid systems. It's very worthwhile keeping your lifepo4 pack at an even temp, they perform much better especially in the mornings when conditions are cool and charge inputs are high. You only need a 3-5 degree shift in temps for it to have a small effect, which over time reduces life.

OK. I understand that. We certainly seem to be behind the curve understanding the usage of LiFe batteries.

Hopefully it will become more mainstream here in the US but IMO still has to become a lot cheaper for most people to justify using that type of energy storage.

[dax]

OK. I understand that. We certainly seem to be behind the curve understanding the usage of LiFe batteries.

Hopefully it will become more mainstream here in the US but IMO still has to become a lot cheaper for most people to justify using that type of energy storage.

Not knowing what you pay per ah for lifepo4 in the USA, can't comment. The majority of our residential customers get their money back within 5 years, based on grid connect costs. Some do better and commercial installations very quickly, the majority are all off grid diesel driven and get their costs back within the first year with the cost of diesel here.

[Mike90250]

Just because Peltier coolers do not consume much of the day's production (if you count a couple KWh as incidental) does not make them more efficient.

I've seen dozens of destroyed Peltier devices from unequal torque on the mounts and many others fail from poor thermal coupling to the hot & cold sinks.
I actually suspect that many of your Peltier coolers are ineffective after being cycled for 6 months, because it's really hard to perfectly make the environmental sandwich that they need to have a long lifetime, unless you purchase them as a pre-fab assembly from a specialty vendor.
I'd bet a nickel that just a dozen watts of forced air circulation will out perform the Peltier coolers in a LFP battery pack. But we'll never know because you "cant disclose anything".