E-RedondoNov 6, 2018

Hi!

You can get the #postprint version of my last article here:
https://hal.archives-ouvertes.fr/hal-01898906

This article is about #battery #ageing (degradation): how #capacity (amp-hours) and #efficiency decrease over time.

It provides quite simple equations, easy to implement if you want you simulate battery ageing by your own.

Two lithium-ion technologies are available: LFP and NMC

Any comments and questions are welcome!

Show threadEd SNov 6, 2018

Here's a thing, @eredondo - I read yesterday that the life-to-80%-capacity of a lithium ion cell depends on the cooling direction. Cooling using the tabs (end-cooling) gives significantly better life than immersion cooling (side-cooling.)

Is there anything to that?

Show threadE-Redondo
@EdS the inner temperature of a battery cell is very important in battery degradation. Depending of the cooling design and current flow on the battery temperature may vary in a very significant way and then battery degradation (because it is directly related to temperature)...
Nov 6, 2018 at 3:31pmWeb
Show threadE-RedondoNov 6, 2018
@EdS as a widely spread rule, 10°C of difference would change the battery lifetime in a factor of two (x2), that is, if you keep your battery 10°C colder it will last 2 times more.
This rule is not always true, but it could be considered here as an illustration.
In fact this is true when battery is at rest (not used). For example, when charging a battery, excessive cold temperatures can damage a battery (under 15°C).
Show threadEd SNov 6, 2018
I can't the reference right now, @eredondo, which is frustrating, but the idea I'd read was that the direction of the heat gradient makes a substantial difference. Knowing nothing of the physical chemistry, I found this surprising. Also, that it doesn't favour Tesla's chosen cooling architecture.
Show threadEd SNov 6, 2018

About Lithium cell life being seriously affected by cooling technique - I found the paper!

http://jes.ecsdl.org/content/163/9/A1846.full

via
https://news.ycombinator.com/item?id=18297090

In conditions of high discharge rate "surface cooling resulted in a rate of loss of useable capacity under load three times higher than cell tab cooling. We show that this is due to thermal gradients being perpendicular to the layers for surface cooling leading to higher local currents and faster degradation"

@eredondo

Surface Cooling Causes Accelerated Degradation Compared to Tab Cooling for Lithium-Ion Pouch Cells

Show threadE-RedondoNov 6, 2018
@EdS
in my last toot I talked about temperature gradients inside a pack. Here it is different, it is inside each cell, but it is also true. Temperature gradients inside a single cell lead to non uniform (and faster) ageing.
Show threadE-RedondoNov 6, 2018
@EdS
a battery is a series/parallel association of cells. In series you are adding voltages, and in parallel you are adding capacities (Amp-hours). For example, if you put in series 80 3.7V 10Ah cells you get a 300V 10Ah pack that is 3kWh . The pack capacity is equal to the capacity of the smallest cell (when fresh, they all are 10 Ah)...
Show threadE-RedondoNov 6, 2018
@EdS
The problem is when ageing is not the same in all cells, if one cell is at 8Ah but all other it is 9, your pack capacity is the minimum, that is 8Ah... Temperature gradients in a pack lead to non uniform ageing.