# Temperature effects



## jumpjack (Sep 9, 2012)

There are various types of headway cells; look at the datasheet of yours and check the voltage curves related to C rate at room temperature.

For example, 38120S standard cells show 2.6 volts at 10C, w.r.t. 3.2 at 1C, which, for a 40S pack, means 40*(3.2-2.6)= 24V sag of the whole pack at 10C (your are getting 600A from a 64Ah pack).

I think you should add some batteries in parallel to reduce both voltage sag and battery fatigue (they won't last so much at 10C).


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## Yabert (Feb 7, 2010)

Duncan said:


> Now its down about 8C - and I seem to be getting 30v sag
> Is that what you would expect?


Yeap... and you can't imagine how bad is the voltage drop at -10°C.
I personally think than headway cells are worst than some other brands.


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## dougingraham (Jul 26, 2011)

Duncan said:


> Hi guys
> Just how much does temperature effect Lithium Batteries?
> I had assumed that when people talked about low temperatures they meant like very cold


Today I noticed how much better my batteries behaved. It was the first day this year when the overnight lows were not that low and the batteries did not get cold. And by cold I mean below 40F or 4C. The batteries were probably at 50F or 10C. Sag is quite a bit more below that. They still work, just have a lot of sag. The coldest I have driven the car was -10F which is -23C. It was pretty dreadful at that temperature. I had to wait almost a week to recharge after that.

So what is very cold? To people living in southern California 40F is cold. To people living Minnesota, -40 is when they close the bathroom window.


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## azdeltawye (Dec 30, 2008)

Duncan said:


> Hi guys
> Just how much does temperature effect Lithium Batteries?
> ...


Here is a chart of the measured performance of Thundersky LiFePO4 Prismatic cells in my Jetta conversion:


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## palmer_md (Jul 22, 2011)

azdeltawye said:


> Here is a chart of the measured performance of Thundersky LiFePO4 Prismatic cells in my Jetta conversion:



awesome data. thanks for sharing.


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## dougingraham (Jul 26, 2011)

azdeltawye said:


> Here is a chart of the measured performance of Thundersky LiFePO4 Prismatic cells in my Jetta conversion:


That is an interesting scatter plot. To what do you attribute the noise in the measurement? If you draw a median line it is clearly a gradually declining curve as the temp increases. I wonder where exactly it levels off. Probably around 120 degrees and near 97%. And being where you are you don't actually have any really cold numbers.


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## kennybobby (Aug 10, 2012)

It doesn't look like any noise to me, plot it from 0 to 100% and it would look like a solid line. Only have +/- 1% at the higher temps and +/- 2% as temp drops--looks like excellent measurement procedures were used to get such good grouping and repeatable data.

Aero and rolling friction loads go up as the temperature goes down, so consumption will increase. Chemical reactions such as is in battery cells slow down with colder temps and can't keep up with high demand loads, hence the higher voltage sag at low temperatures.

Thanks for sharing.


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## NabilAhmad (Feb 26, 2011)

Indeed noise is low, but there are several outliers from the norm. That said, we only see a total deviation that would be obscured by the noise of other variables. Most notable capacity noise in my experience is parasitic loads that fly under the radar of my ammeter. As such, the amount of time between charges would easily explain those outliers.


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## wb9k (Apr 9, 2015)

Driving conditions are also a good potential explanation--specifically, wind and precipitation, big mileage-killers.


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## GerhardRP (Nov 17, 2009)

azdeltawye said:


> Here is a chart of the measured performance of Thundersky LiFePO4 Prismatic cells in my Jetta conversion:


That is good information, probably reflecting internal impedance vs. temperature. I wonder if you could make a similar plot for Coulombic efficiency: (Ahr out/Ahr in) x100. That might even slope the other way.


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## Moltenmetal (Mar 20, 2014)

It's a plot of Wh in via charging vs Wh out of the battery to the controller. What am I missing guys? It seems to have nothing to do with rolling resistance etc.- the efficiency of the cells in charge-discharge is all this is showing. Reasons for poorer efficiency at colder temperatures? I'd guess you're seeing a mass transfer limitation which manifests itself as apparent internal resistance- it takes some energy to force those ions to move through a sluggish, viscous cold electrolyte. That's a guess on my part, not supported by study of the battery literature, but would seem to be consistent with the little I know of the electrochemistry.


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## azdeltawye (Dec 30, 2008)

Moltenmetal said:


> It's a plot of Wh in via charging vs Wh out of the battery….- the efficiency of the cells in charge-discharge is all this is showing…..


 This is a correct statement. All the energy used for moving the vehicle and heating/cooling is removed from this comparison. It only compares the energy in and out of the battery. Graphing the coulombic efficiency (Ah out/Ah in) of the battery is of little value since it is so close to unity (99.9%) and any variance thereof is beyond the accuracy of my Ah meter. I have other plots that show Wh per mile vs. ambient temperature and they are somewhat skewed since a portion of that energy is used for heating and cooling instead of moving the vehicle. These plots tend to favor the warmer temperatures since the heater in my EV is an inefficient resistive unit (Coefficient of performance of 1.0) vs. the A/C compressor which is much more efficient (COP of approx. 4). They are further skewed because I am a desert rat with thin blood and am more likely to run the heater on a 50° day than run the A/C on a 90° day…


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