Understand. Well mostly: I didn't lookup Arrhenius.
But my question still remains. What is the electrical result when the
reaction is slower. I am presuming there is a lower voltage across the
cell. And, if that's so, more current must come from the battery to
provide the same driving experience. Is this correct ?
And if it's correct, then the cell's internal resistance must come into
play. Is it simply not significant enough to warm up the cell ?
Peri
------ Original Message ------
From: "Bill Dube via EV" <[email protected]>
To: [email protected]
Cc: "Bill Dube" <[email protected]>
Sent: 09-Sep-19 4:59:04 PM
Subject: Re: [EVDL] li ion battery in cold weather
Ions are not the same as electrons. Ions are what moves _inside_ a battery.
Electrons are what moves _outside_ a battery.
Electrons travel at near the speed of light. Ions travel at far, far lower
speeds, (less than the speed of sound.) Ion must diffuse through the solid or
liquid in which they are surrounded.
Ions are indeed governed by the chemistry around them. Essentially, a battery
internally rusts, or corrodes, and you exploit that electro-chemistry to get
electricity.
The Arrhenius equation is what governs the chemical reactions that drive the
ions from one plate to the other inside a cell. There is a strong temperature
component to this equation.
https://en.wikipedia.org/wiki/Arrhenius_equation
The higher the temperature, the faster the reaction would like to occur. In a
cell, what holds back the chemical reaction (corrosion) is the lack of
electrons to feed the ionic reaction. If you give the reaction all the
electrons it wants, then the Arrhenius equation governs the speed of the ion
production, and the geometry of the cell governs the distance that the ions
must diffuse.
Bill D.
On 9/10/2019 10:23 AM, Peri Hartman via EV wrote:
Yes, it's the chemistry that results in slower ion transfer. But that results
in an electrical effect. I believe that effect is lower voltage across the
cell. Thus, how do you explain the fact that higher current, I**R, means more
heat ?
Peri
------ Original Message ------
From: "paul dove" <[email protected]>
To: "Peri Hartman" <[email protected]>; "Electric Vehicle Discussion List"
<[email protected]>
Sent: 09-Sep-19 2:59:54 PM
Subject: Re: [EVDL] li ion battery in cold weather
You are confusing electric current with a chemical process.
Ohms law does not apply in a chemical process.
Low temperatures slow the chemical reaction and charge transfer velocity, which leads
to the decrease of ionic conductivity in the electrolytes and lithium-ion diffusion
within the electrodes
<https://www.sciencedirect.com/science/article/pii/S1002007118307536#bib53>.
Such decrease will result in the reduction of energy and power capability, and
sometimes even performance failure.
Sent from my iPhone
On Sep 9, 2019, at 12:38 PM, Peri Hartman via EV <[email protected]> wrote:
Hi,
Does anyone have a more in-depth explanation why li ion cells perform poorly in
cold weather ? It's well known that your range goes down when the battery is
cold. Lots of writers talk about the phenomena and advise you about it but fail
to give any real explanations. I think this conversation may have come up a
long while ago but it's hard to search for.
I did a bit of research. Some authors talk about "intercalation", which happens
below freezing. Intercalation is when the anode becomes plated with lithium ions instead
of the ions being absorbed into the anode. Obviously, that permanently ruins the battery.
So, this is irrelevant regarding poor range in cold weather.
I did find one article that might be relevant, though.
https://cen.acs.org/articles/96/i10/Rechargeable-battery-weathers-extreme-cold-conditions.html
They claim the electrolyte becomes viscous, slowing down the ion movement.
That's about all they say. But, I'll add my thoughts.
If the electrolyte is viscous, then I suspect the voltage potential drops.
Since watts-hours (energy) is V * Ah, you have less energy available, assuming
that the Ah is constant. But, what about internal resistance ? For the same
driving behavior, you'll need higher amps at a lower voltage. Resistance loss
is I^2 * R, so that loss should go up significantly. I'm getting into a trap
here: why doesn't the increased resistance loss heat up the battery ? The
nominal resistance is extremely low, so maybe even at an elevated state there
isn't enough heat to affect the electrolyte's viscosity. Don't know.
Here's another article that talks about electrolyte. They talk about preventing
decomposition of electrolyte.
https://spectrum.ieee.org/energywise/green-tech/fuel-cells/lithium-additives
Obviously, the other major factors are cabin heat and defrost.
Peri
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