Heat conductivity properties of aluminum?

Hello!

I have 1 question I would like to ask about the heat conductivity of aluminum:

If I wrap aluminum foil around batteries that are charging, will the foil: a. insulate the heat, making the batteries hotter b. conduct a very small amount of heat away from the batteries into the air c. other (explain)

Aluminum will act as a heat sink, conducting heat away from the battery into the air. More foil is better.

d. Short the batteries out so they explode.

conducting heat away from the battery into the air.

As long as there is no air between the battery and the foil.

Aluminum foil tape maybe:
http://www.3m.com/product/information/Aluminum-Foil-Tape.html

Yes, but then you should remember that the glue doesn't do a very good job in conducting heat. And in order to radiate heat a bit efficiently , create a larger surface (a radiator) by folding the tape so you get some ridges.

@dkl65,

If I wrap aluminum foil around batteries that are charging, will the foil:

It depends!

Aluminum foil is shiny on one side for a reason- to reflect IR thermal energy ( the dull side reflects, too, but not as much.) As mentioned previously, if you create a sealed air gap, this "dead" space becomes an insulator and the battery mass may increase in temperature while the outside foil is closer to the ambient temperature. Of course, there will be some convection air currents flowing due to temperature differences.

There are only three ways to conduct heat away from an object: radiance, conduction, and convection. You may find that a small DC fan may serve your needs best.

  • Ray

Not sure why a person would want to do this, but I would advise against it. It is not likely to make a significant difference if the goal is to lower the temperature of the batteries. In fact, it may do the opposite. Some NiMH and NiCd chargers actually sense the temperature of the batteries and adjust charging current to keep within the recommended operating range. So if either (a) an insulation effect occurs because of imperfect contact between the foil and batteries, or (b) the foil successfully radiates the heat, and as a result the outer parts of the foil actually run cooler than battery, then the charger may in either instance think the batteries are cooler than they actually are. This could cause the charger to drive too much current into the batteries and overheat them with exciting results.

Personally I wouldn't do it because of possibility (d) that Nick raised. Some EE got paid a big salary to design the charger, I wouldn't muck with it.

overheat them with exciting results.

http://p4borg.net/startrek/data/tos/scottyallshesgot.wav

mrburnette:

overheat them with exciting results.

http://p4borg.net/startrek/data/tos/scottyallshesgot.wav

RIP Scotty! :(

mrburnette: Aluminum foil is shiny on one side for a reason- to reflect IR thermal energy ( the dull side reflects, too, but not as much.)

No, it's shiny on one side as a side-effect of the manufacturing process. The whole "shiny side out" thing was invented afterwards.

I thought it was probably a manufacturing thing. And I can never decide which side to put against the food. :)

[quote author=Nick Gammon link=topic=176167.msg1307549#msg1307549 date=1373261725] I thought it was probably a manufacturing thing. [/quote]

Basically: They roll it to final thickness two sheets at a time. One sheet is two thin to work well using rollers (it sticks to the rollers and tears or something like that).

The shiny side is where it was touching the roller, the matte side is where it was touching the other sheet.

[quote author=Nick Gammon link=topic=176167.msg1307549#msg1307549 date=1373261725] I thought it was probably a manufacturing thing. And I can never decide which side to put against the food. :) [/quote]

Shiny side out, just like I do with my hats. That way "they" won't know what you're eating.

Oh, aye.

From Wikipedia:

Aluminium foil has a shiny side and a matte side. The shiny side is produced when the aluminium is rolled during the final pass. It is difficult to produce rollers with a gap fine enough to cope with the foil gauge, therefore, for the final pass, two sheets are rolled at the same time, doubling the thickness of the gauge at entry to the rollers. When the sheets are later separated, the inside surface is dull, and the outside surface is shiny.

So, I was definitely mistaken in my post. Other elements of the post regarding the thermal cavity of “dead air” should remain correct and should be subject to easy proof by experimentation although I believe the analogy to the old (metalized) glass thermos bottles should be satisfactory as a brain exercise.

In rethinking my incorrect statement,

Aluminum foil is shiny on one side for a reason- to reflect IR thermal energy

it should have been obvious to me that the IR component should be nearly constant between the two sides of foil as a factor of material emissivity.

Thanks to all for the fast corrections… 40 years since college thermodynamics and sometimes common misconceptions surface in print before the neurotransmitters retrieve facts; I guess this is a human brain example of corrupted cache.

  • Ray

I bet if thicker aluminum was used and it was curved to match battery shape and then heat sink paste was used in the middle to help with thermal "connectivity", that cooling could be improved. Could get mighty messy, especially if the batteries were to be installed in another enclosure after charging. Probably get just as much improvement by dropping the batteries in a good airflow. Or in mineral oil. http://www.pcworld.com/article/262263/forget_liquid_cooling_give_your_pc_an_oil_bath.html

CrossRoads:
I bet if thicker aluminum was used and it was curved to match battery shape and then heat sink paste was used in the middle to help with thermal “connectivity”, that cooling could be improved.
Could get mighty messy, especially if the batteries were to be installed in another enclosure after charging.
Probably get just as much improvement by dropping the batteries in a good airflow.
Or in mineral oil.
http://www.pcworld.com/article/262263/forget_liquid_cooling_give_your_pc_an_oil_bath.html

You’d only get increased heat flow to the air by conduction if the outer diameter of the aluminium layer was substantially
larger than the battery/cell diameter - this would mean more surface area to transfer heat to the air.

Think in terms of thermal resistance - aluminium has basically no thermal resistance compared to the that of transfering
the heat from solid to air (in still air conditions), so the aluminium foil has no noticeable effect. The limiting factor is
that still air is an extremely poor conductor of heat.

However as others have pointed out the thermal radiation will depend critically on the nature of the surface - painting the
battery black (well with a paint that is “black” in infra-red spectrum) will maximize radiated heat. Aluminium foil that is
black on the outside will clearly have an effect.

Plain aluminium probably has very low emissivity and wrapping a battery with it would substantially decrease radiation losses.

However without knowing how much heat is lost by conduction to the air (compared to radiated into the surroundings), the
magnitude of the total effects is guesswork.

One thing is sure though, forced air cooling works really well and is simple and straightforward.

A fan will have far more effect than changing the radiative properties of the batteries.

If I wrap a bunch of aluminum foil around a plastic container, will it insulate the container, or make the ice melt faster, or something else?

ice melt faster.