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Re: [Phys-L] Endo/exo thermic battery



On 04/04/2017 10:03 AM, Forinash III, Kyle wrote:
Unless I am completely misunderstanding the chemistry (a
possibility), battery charging should be endothermic and discharging
exothermic because of the associated entropy changes (hence the
recent phones catching fire problem). Does the cooling while charging
really happen in (all?) real batteries or is there something in the
engineering that changes this picture? Can anyone point to any data
or discussion? Seems like my phone and laptop get hot when
discharging AND charging.

This involves some chemistry, but primarily it's basic thermo
concepts and terminology.

For starters, do not confuse H (the enthalpy) with heat energy.
Also do not confuse heat energy with temperature. Beware that
the word "heat" has at least six different perfectly reasonable
technical meanings, not to mention various vernacular, metaphorical,
and/or less-reasonable meanings.
https://www.av8n.com/physics/thermo/heat.html

Charging or discharging a battery is reversible, to a good
approximation, especially if it is not done too quickly (and
not done insanely slowly). It's not particularly different
from winding up a spring, or spinning up a flywheel.

If you charge or discharge it more quickly, then there will
be dissipation. This is second order in the rate of charge.
This explains why the laptop gets hot when charging AND
discharging.

Chemists very unwisely apply words like "exothermic" and "heat
of reaction" to anything that liberates enthalpy. It may be
that in the last 999 experiments they did, the enthalpy showed
up as heat, but this is not the whole story. Batteries and
fuel cells are a treeeeemendous exception.

Whenever you see a table of "heat of reaction" you should
cross it out and write /enthalpy of reaction/.

=======

If we (temporarily and hypothetically) model the battery as
an "air spring" (compressed gas in a cylinder) rather than
as an ordinary spring or flywheel, then yes, it would get
hotter when you add energy. However, this is not a good model.

Consider the fundamental relationship:

∂E
T = --------
∂S | V

You could take that as the /definition/ of temperature.

For a battery (unlike an ideal gas) the entropy is more-or-less
proportional to the energy, so we get T=constant.

=======

Also note that batteries and fuel cells are not heat engines.
Their efficiency is *not* limited by Carnot's law. A practical
fuel cell can have 70% efficiency without getting hot, in
situations where a nice hot automobile engine would be lucky
to achieve 30% efficiency.