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Temperature control of LiFePO4 battery pack

ba38

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Aug 15, 2021
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Hi all
Looking at numerous post here or elsewhere it seems very important to avoid overtemperatures in battery pack.
I have made temperature simulation of an infinite battery pack (compacted cells closely tightened) in standard conditions.
A map of temperatures accross a quarter of a cell is appened where the center temperature is about 10°C higher than the side with air convection.
If we use a good thermal conductor between each cell (such as a copper sheet) of sufficient thickness (here 2 mm), another simulation of an infinite battery pack shows a great decrease of the temperature gradient between the center of a cell and the side which is about 2.5 °C in the same state : 4 fold decrease.
To well see the pictures, please click on them to display temperatures scale.
We can see that the hot area is also reduced to the right center of the cell.
It seems attractive to insert such passive cooler between batteries to reduce temperature stress and therefore improve battery life.
What do you think of ? I have also seen people showing polymer insulation sheets between cells in order to improve electric insulation (but increasing also core cell temperature) and even insulating threaded rod which compress cells together.
Your comments will be welcomed.
 

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Sorry, maybe I am having a blonde moment, but I have no idea what the vertical and horizontal axes are trying to represent... and what the low sticking out bit is to the right of the 2nd image ?‍♀️

Also not sure what you mean by 'A map of temperatures across a quarter of a cell" relates to. Is that a quarter of a LiFePO4 prismatic cell?
 
What do you think of ? I have also seen people showing polymer insulation sheets between cells in order to improve electric insulation (but increasing also core cell temperature) and even insulating threaded rod which compress cells together.

In general, cell temperature is mostly driven by ambient temperature. A LiFePO4 cell in solar applications rarely is pushed to C rates where the cell gets noticeably warm. In combination with the round trip efficiency, very little energy is actually there to heat up the cell even at elevated C rates.
 
Core temp of battery goes up with sustained high cell current. Biggest risk in a cascade meltdown where one bad cell blows and its heat caused neighbor cells to blow, cascading a thermal runaway down the whole battery pack.

A bad terminal connection can generate a lot of heat that gets transferred into cell core from the internal foil laminates connections.

You are pretty safe below about 0.25 C(A) discharge rate, but an older cell at 0.5 C(A) discharge rate can get fairly hot, especially when packed together restricting heat dissipation to ambient air.

There is a lot of mass in cell that must be heated to raise temperature so there is a long time delay.

Chart shows approximate internal heating watts for a new EVA 280AH cell at various discharge currents. Older cells have greater internal impedance, so their heating increases linearly with their impedance increase with aging. An older cell can be 2x to 3x the new cell impedance.

The manufacturers' spec sheet is a little deceptive on their results for cycle life at 1C discharge life cycle count. If you look at fine print it specifies cell held close to 25 degsC which can only be accomplished at 1C discharge rate with active fixture cooling.

Low temperature discharge curves at higher currents show the effects of internal heating. The curve has a funky shape, initially quickly slumping in terminal voltage, then rising again as cell internal heating improves performance, before dropping again as cell is exhausted of capacity.

LF280 AH battery dischg 0.1C-1.0C.png
 
Last edited:
Hi all
And thanks for your replies.
The figures maps an horizontal cut of a quarter of a cell. Ox maps the length of the cell and Oy the width. The cut is taken at the half height of a cell.
In this case it is a 304Ah cell (HLW=205*172*71 mm).
I agree that at low current the dissipated power stays low as the efficiency of LiFePO4 cells is high as indicated in EVE curves.
Thanks a lot for these curves. As you notice ambient temperature is maintained at 25°C in a lab environment, and that's why I ask myself of the fate of a cell in a pack tightly compacted.
I had not these curves and will redo computation.
But I ask myself for the whole life of cells and after 500, 1000,.., cycles and time passed the efficiency decreases and more losses could be generated and increase aging ?
Do you have the same curves at 1000, 2000,..., 6000 cycles ? I would be very interested indeed.
Thanks a lot.
 
Last edited:
Hi all
Looking at numerous post here or elsewhere it seems very important to avoid overtemperatures in battery pack.
I have made temperature simulation of an infinite battery pack (compacted cells closely tightened) in standard conditions.
A map of temperatures accross a quarter of a cell is appened where the center temperature is about 10°C higher than the side with air convection.
If we use a good thermal conductor between each cell (such as a copper sheet) of sufficient thickness (here 2 mm), another simulation of an infinite battery pack shows a great decrease of the temperature gradient between the center of a cell and the side which is about 2.5 °C in the same state : 4 fold decrease.
To well see the pictures, please click on them to display temperatures scale.
We can see that the hot area is also reduced to the right center of the cell.
It seems attractive to insert such passive cooler between batteries to reduce temperature stress and therefore improve battery life.
What do you think of ? I have also seen people showing polymer insulation sheets between cells in order to improve electric insulation (but increasing also core cell temperature) and even insulating threaded rod which compress cells together.
Your comments will be welcomed.
This makes me wonder whether it might be good practice to rotate cells within a compression mount so that, for example, each year the cells which are towards the centre are moved to the outer positions to give them a rest and even out the effects of temperature variations on overall pack life.
 
This makes me wonder whether it might be good practice to rotate cells within a compression mount so that, for example, each year the cells which are towards the centre are moved to the outer positions to give them a rest and even out the effects of temperature variations on overall pack life.
Hi Sverige
Unscrewing and screwing nuts on cells is probably the best way to destroy threads in aluminum which are soft and delicate !
Looking at industrial cells packs, they are always tightly packed with coolers (for instance Phase Change Material as I found in a thesis).
I found there LiFePO4 components thermal characteristics and redo the same computations.
Results are clear : at 10kW/m3 (which is equivalent to 0.33C for 300Ah cells) the maximum interior temperature is 47°C with packed cells only, and 36°C with a good cooler between every cells - 6°C over ambient -.
It is not worth trying : intercooler are necessary if you want a long cells life.
 
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