LFP Internal Resistance model for heat loss calculation

[Johan]

Does anybody have a simple generic analytical model to quantify the internal resistance of an LFP cell as a function of, for example, the state of charge (and possibly a.f.o. other key parameters like age)? Or do cells differ too much between manufacturers and models so any generic model is likely to mismatch? The goal is to calculate heat loss during (dis)charge using various C-rates and design appropriate cell cooling if needed. Sometimes manufacturers only give a single value like the "internal impedance" in [1], which may (?) be a worst case number that, for example, applies to nearly discharged state [2, p. 2-2] that is not used in practise.

References
[1] https://www.electriccarpartscompany.com/Lithium-Battery-206Ah-EV-LiFePO4-XCell
[2] https://www.ti.com/download/trng/docs/seminar/Topic 2 - Battery Cell Balancing - What to Balance and How.pdf

 

[electric]

Unless you are building an EV where C rate can be higher than 1C, you do not need to worry about this subject at all. Ambient temperature is a much larger concern than internal heat rise. When continuous C rate is less than C/2-C/3 , the LFP chemistry does not create notable temp rise, maybe a few degrees F inside a well insulated box.

 

[grizzzman]

Unless you are building an EV where C rate can be higher than 1C, you do not need to worry about this subject at all. Ambient temperature is a much larger concern than internal heat rise. When continuous C rate is less than C/2-C/3 , the LFP chemistry does not create notable temp rise, maybe a few degrees F inside a well insulated box.

So aluminum type prismatic cells are not built that way for increased cooling and lighter weight?

 

[electric]

If you really want to play with numbers, assume 95% power efficiency, or 5% loss over full discharge cycle, which is probably even lower in real life with low C rate application. So, 5% of your pack energy lost in heat over whatever number of hours of discharge. Now you'd have to estimate how effective you insulation is, i.e. how fast this heat is dissipated into ambient before it contributes to internal temp rise inside the box.
I think thermal mass of the pack also plays a role. I'm not an expert in thermodynamics, it's a bit over my head.

 

[electric]

So aluminum type prismatic cells are not built that way for increased cooling and lighter weight?

They are built for worst case scenario, which is 1C in most datasheets. At 1C you need to shed heat, but at fractions of 1C you do not need to. Aluminum cases are better than plastic for several reasons, so it's all a benefit, but final design decision depends on your planned C rate.

 

[grizzzman]

They are built for worst case scenario, which is 1C in most datasheets. At 1C you need to shed heat, but at fractions of 1C you do not need to. Aluminum cases are better than plastic for several reasons, so it's all a benefit, but final design decision depends on your planned C rate.

Thank you

 

[Johan]

@electric, @grizzzman, see also my related new post about the maximum C-rate here.