Air gap between cells
- Thread starter BenjAZ
- Start date
no, why would they?
that would be a job for the designer / maker familiar with the use case, expected current rates, ambient temps, ventilation specs, etc
the cell mfg just provides the "do not exceed" temp rating
which should be stayed well away from, even occasionally and for short times
that would be a job for the designer / maker familiar with the use case, expected current rates, ambient temps, ventilation specs, etc
the cell mfg just provides the "do not exceed" temp rating
which should be stayed well away from, even occasionally and for short times
MJSfoto1956
10 kW
Stated differently, if they did specify a minimal distance, then they would be liable. Instead, the generally-understood goal is to prevent extreme heat/cold from affecting your pack. There are many ways to accomplish that but no "one size fits all" approach that works for all use-cases.
M
M
They can't know the pack usage/etc, so you have to determine this by conditions you expect the pack to undergo.
If there is a lot of vibration you probably want a gap to ensure no cell rubbing.
If there is a lot of heat you want a gap large enough to allow airflow from your cooling system to keep cells from overheating.
Etc.
If there is a lot of vibration you probably want a gap to ensure no cell rubbing.
If there is a lot of heat you want a gap large enough to allow airflow from your cooling system to keep cells from overheating.
Etc.
liveforphysics
100 TW
BenjAZ said:
Unless you've got some amazing position fixture for your cells, 2mm minimum (and ideally no cell wrappers) is a good place to start.
As you validate a design with respect to tested propagation behaviors, you may find your cell and your application lets you bring that spacing in, or maybe find you need to push it out further.
Even when nearly the whole case surface is the cathode / negative ?liveforphysics said:
I would have thought electrical insulation would be useful there, and
I believe most wrap materials are not too thermally insulating?
And my understanding is that heat rises first from the positive / anode end, not in the middle?
If you have cell holders that guarantee no cell contact, the insulation wrap is useless and unnecessary as electrical insulation, and only serves to trap heat inside.john61ct said:
If you have no cell holders and a design that puts different groups in contact with each other, the wrap is somewhat useful, but any vibration that can move cells relative to each other even a tiny bit will cause rubbing that will eventually wear thru that wrap, and cause a short and possibly a fire.
So the wrap isn't all that useful in packs for EVs, and can be detrimental. (especially if it encourages pack design based around it being the sole insulation between groups).
if it were bad enough to do that, it'd also probably damage the wrap, and short teh cells anyway. Or cause the interconnects to touch each other or the cell cans / etc, and short them.john61ct said:
You have to design and build the pack casing sufficiently well so the pack inside doens't receive such shocks/vibrations if the usage scenario includes the risk, to preclude the possibility of such damage. Or else live with the risk.
liveforphysics
100 TW
Shrink creeps steadily under pressure and relative motion.
Before it creeps out of insulating contact points for you, it serves as a vapor/humidity trap to accelerate the cell corroding.
Real mechanical structure for the cells to control spacing and relieve all mechanical stress from interconnect assembly, then fully encapsulate in something slightly elastomeric and non-corrosive.
Before it creeps out of insulating contact points for you, it serves as a vapor/humidity trap to accelerate the cell corroding.
Real mechanical structure for the cells to control spacing and relieve all mechanical stress from interconnect assembly, then fully encapsulate in something slightly elastomeric and non-corrosive.
liveforphysics
100 TW
john61ct said:
You can make a good pack that has shrink on the cells if you fully encapsulate the cell/shrink assembly, but at that point the shrink is only decorative for your pack anyways.
The most popular option for DIY batteries is to make something that fails after a short service life, sometimes spectacularly. I recommend testing for yourself with whatever cell/pack you're working with in corrosive environments and high vibration environments to draw your own conclusions about what will last in your applications needs.
liveforphysics
100 TW
john61ct said:
If you're going to use active ventilation, then you need to have a solution to deal with what happens to the dew/humidity/condensation and corrosive vapors (if you're near the ocean or a city that salts the roads in winter).
RootedSuperuser
10 W
Your really are trying to prevent thermal runaway, I'm assuming. So anything, any thickness, even the use of toothpick wedges will help mitigate neighbouring thermal runaway.
Any pressure on a can causing a dent will cause a hot spot in the battery. Any battery that cause heat to build in any 1 spot has the potential to cause thermal runaway.
You be the judge for your build, error on the side of caution here.
Any pressure on a can causing a dent will cause a hot spot in the battery. Any battery that cause heat to build in any 1 spot has the potential to cause thermal runaway.
You be the judge for your build, error on the side of caution here.
