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" <dov...@bellsouth.net>
To: "Peri Hartman" <pe...@kotatko.com>; "Electric Vehicle Discussion
List" <ev@lists.evdl.org>
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 <ev@lists.evdl.org>
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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