5 ms·

Why are most of the lithium cells cylindrical, while most of our devices are cuboid? (I think of phones, laptops, external battries... but phones and most lapto

by Kayou 12y ago

Why are most of the lithium cells cylindrical, while most of our devices are cuboid? (I think of phones, laptops, external battries... but phones and most laptops have flat lithium cells nowadays). Is cylindrical less expensive? Is it more secure? It seems like a waste of space to me.

jacquesm 12y ago
It's an efficiency matter. The cylinder gives good electrode surface relative to the amount of matter contained. For a cube that ratio would be much worse, for a sphere it would be optimal, but spheres are harder to manufacture and connect to, as well as harder to stack in arrays. So we take a medium between the sphere and the cube and end up with a sweet spot in the cylindrical shape.
- SeanLuke 12y ago
  This doesn't explain why LiPoly batteries are cuboids.
  - jacquesm 12y ago
    Indeed it does not, this is about Tesla's and Panasonics 'gigafactory' which will make Lithium-Ion batteries in a cylindrical form factor. Lithium Polymer batteries (or LiPo for short) are a special case of Lithium-Ion batteries. They are sold in a 'pouch' format (and they are not the first to use a format like that, for instance, in the polaroid cartridges there used to be a battery in a similar form factor (polapulse)). The reason why LiPo batteries are not so constrained in shape as their siblings is that the electrolyte is a liquid. Solid electrolyte batteries are typically much more constrained in terms of shapes. This is also why regular car batteries (lead-acid) can be block shaped, the acid is also a liquid. The polapulse was an interesting battery, it only had to fire the camera 10 times but was quite high current. The packaging constraints of the cartridge dictated the battery geometry and so a flat pouch like battery was born. LiPo batteries exist in all kinds of interesting shapes and sizes too.
    - tobz 12y ago
      Also, and this is entirely speculation and I'd love to hear a battery engineer confirm this, but: if you're packaging together cylindrical batteries, it inherently gives you more air channels between the batteries to promote cooling. When you're talking tens of kilowatts, both in discharge and charge rates, the percentage that ends up as waste heat suddenly becomes a whole lot of heat. You probably wouldn't want a maximally stuffed cavity with these packs turning into a single, massive chunk of thermal mass. EDIT: looks like I was firsted somewhere below, doh!
      - jacquesm 12y ago
        But you're 100% on point there. That's a side benefit from having those cylindrical cells.
    - TD-Linux 12y ago
      This is exactly backwards. Traditional Li-Ion batteries have a liquid electrolyte, and Li-Poly have a solid polymer electrolyte. Both batteries consist of anode/electrolyte/cathode rolled up. In the case of a "pouch" cell, they are rolled up flat, and a cylinderical cell has them rolled up in a cylinder. There is actually no limitation for the shape - you can totally make a Li-ion battery with liquid electrolyte in a prism - many cell phone batteries do this. Li-ion, due to the liquid, needs a hard metal case, whereas li-poly can often get away with a foil pouch. The surface area per volume of a cylinder is less, as well as easier to make structurally sturdy, which means less weight is spent in the casing. Tesla cares more about weight than volume, so this is an acceptable tradeoff.
      - jacquesm 12y ago
        Interesting! I just spent the better part of today reading up on all this because I found it hard to accept that I'd remember something so terribly wrong. Turns out that the names have changed over time to denote various packages and gel/liquid/solid combinations. Wikipedia has a whole page on the subject and it almost seems as if they made the naming of these battery classes confusing on purpose: http://en.wikipedia.org/wiki/Lithium_polymer_battery http://en.wikipedia.org/wiki/Lithium_polymer_battery Enjoy the read, I certainly did and it turns out the situation is a lot more complex (also historically) than I remembered it, but my memory wasn't quite as faulty as you made me feel initially (fortunately!).
- Retric 12y ago
  Why not stack flat plates like a car battery?
  - jacquesm 12y ago
    Because it would be more expensive and the space gain would be very small. Battery geometry is mostly dictated by physics and economics, even though a flat plate battery would be denser the gains in density would be off-set by the increase in cost and in the end the wasted space doubles as a cooling channel so you might end up having to separate your flat plates by periodic air channels anyway. The reason why flat plates are better in some applications (for instance, for starter batteries in cars) is because those batteries are optimized for very high current during very short bursts of time. 300 Amps @ 12V is 3.6KW out of a very compact little box. The power generated from that little package is extremely impressive (if you pull the distributor cap on your car (if you have an old car) you can actually move the car for a bit on the starter engine in first gear or reverse!) but because of the short duration the power absorbed by the battery is relatively small (current is absorbed by the internal resistance of the battery during charge and discharge leading to a warming up of the battery). But it does not last for very long so heat removal is not usually a problem, and the charge currents are reasonably low, so again no big problem in removing the heat (low current, longer time). With electric vehicles the situation is reversed, the discharge time is relatively long (hours) but the charge times are getting ever shorter which means packs have to be actively cooled to avoid overheating, and the easiest way to do that is to flow air through gaps in the cells.
catmanjan 12y ago
Don't cylinders have fewer potential points of failure? (2 edges at either face vs 8 corners). Not really sure!
ygra 12y ago
It would at least include a bit of space for expansion without immediately damaging surrounding cells or the enclosure, I guess.
dm2 12y ago
I think they use standard 18650 batteries (might be wrong) and there have been discussions from Tesla saying that the round batteries also have the benefit of allowing an air gap to insulate and help mitigate overheating problems. Plus they are mass produced, cheap, and have a relatively high energy density. The new factory might work on creating some new battery technologies though, not sure. Ideally yes, you want the battery as compact as possible and lots of round batteries aren't the most efficient for that, but when you factor in cost and the natural heat insulation, it might have been the best option at the time. If it works, roll with it and work on one of the numerous other problems they have to solve.
- TD-Linux 12y ago
  The Panasonic 18650B cell has the highest energy to weight ratio of any commercially available lithium-ion cell. This is by far the biggest factor. It's also the cell used by many solar car teams for the same reason.
deleted 12y ago
[deleted]
hemmer 12y ago
There was a recent discussion about cylinders on HN which might be of interest: https://news.ycombinator.com/item?id=8097125 https://news.ycombinator.com/item?id=8097125

by Kayou 12y ago

jacquesm 12y ago

It's an efficiency matter. The cylinder gives good electrode surface relative to the amount of matter contained. For a cube that ratio would be much worse, for a sphere it would be optimal, but spheres are harder to manufacture and connect to, as well as harder to stack in arrays. So we take a medium between the sphere and the cube and end up with a sweet spot in the cylindrical shape.

SeanLuke 12y ago
This doesn't explain why LiPoly batteries are cuboids.
- jacquesm 12y ago
  Indeed it does not, this is about Tesla's and Panasonics 'gigafactory' which will make Lithium-Ion batteries in a cylindrical form factor. Lithium Polymer batteries (or LiPo for short) are a special case of Lithium-Ion batteries. They are sold in a 'pouch' format (and they are not the first to use a format like that, for instance, in the polaroid cartridges there used to be a battery in a similar form factor (polapulse)). The reason why LiPo batteries are not so constrained in shape as their siblings is that the electrolyte is a liquid. Solid electrolyte batteries are typically much more constrained in terms of shapes. This is also why regular car batteries (lead-acid) can be block shaped, the acid is also a liquid. The polapulse was an interesting battery, it only had to fire the camera 10 times but was quite high current. The packaging constraints of the cartridge dictated the battery geometry and so a flat pouch like battery was born. LiPo batteries exist in all kinds of interesting shapes and sizes too.
  - tobz 12y ago
    Also, and this is entirely speculation and I'd love to hear a battery engineer confirm this, but: if you're packaging together cylindrical batteries, it inherently gives you more air channels between the batteries to promote cooling. When you're talking tens of kilowatts, both in discharge and charge rates, the percentage that ends up as waste heat suddenly becomes a whole lot of heat. You probably wouldn't want a maximally stuffed cavity with these packs turning into a single, massive chunk of thermal mass. EDIT: looks like I was firsted somewhere below, doh!
    - jacquesm 12y ago
      But you're 100% on point there. That's a side benefit from having those cylindrical cells.
  - TD-Linux 12y ago
    This is exactly backwards. Traditional Li-Ion batteries have a liquid electrolyte, and Li-Poly have a solid polymer electrolyte. Both batteries consist of anode/electrolyte/cathode rolled up. In the case of a "pouch" cell, they are rolled up flat, and a cylinderical cell has them rolled up in a cylinder. There is actually no limitation for the shape - you can totally make a Li-ion battery with liquid electrolyte in a prism - many cell phone batteries do this. Li-ion, due to the liquid, needs a hard metal case, whereas li-poly can often get away with a foil pouch. The surface area per volume of a cylinder is less, as well as easier to make structurally sturdy, which means less weight is spent in the casing. Tesla cares more about weight than volume, so this is an acceptable tradeoff.
    - jacquesm 12y ago
      Interesting! I just spent the better part of today reading up on all this because I found it hard to accept that I'd remember something so terribly wrong. Turns out that the names have changed over time to denote various packages and gel/liquid/solid combinations. Wikipedia has a whole page on the subject and it almost seems as if they made the naming of these battery classes confusing on purpose: http://en.wikipedia.org/wiki/Lithium_polymer_battery http://en.wikipedia.org/wiki/Lithium_polymer_battery Enjoy the read, I certainly did and it turns out the situation is a lot more complex (also historically) than I remembered it, but my memory wasn't quite as faulty as you made me feel initially (fortunately!).
Retric 12y ago
Why not stack flat plates like a car battery?
- jacquesm 12y ago
  Because it would be more expensive and the space gain would be very small. Battery geometry is mostly dictated by physics and economics, even though a flat plate battery would be denser the gains in density would be off-set by the increase in cost and in the end the wasted space doubles as a cooling channel so you might end up having to separate your flat plates by periodic air channels anyway. The reason why flat plates are better in some applications (for instance, for starter batteries in cars) is because those batteries are optimized for very high current during very short bursts of time. 300 Amps @ 12V is 3.6KW out of a very compact little box. The power generated from that little package is extremely impressive (if you pull the distributor cap on your car (if you have an old car) you can actually move the car for a bit on the starter engine in first gear or reverse!) but because of the short duration the power absorbed by the battery is relatively small (current is absorbed by the internal resistance of the battery during charge and discharge leading to a warming up of the battery). But it does not last for very long so heat removal is not usually a problem, and the charge currents are reasonably low, so again no big problem in removing the heat (low current, longer time). With electric vehicles the situation is reversed, the discharge time is relatively long (hours) but the charge times are getting ever shorter which means packs have to be actively cooled to avoid overheating, and the easiest way to do that is to flow air through gaps in the cells.

catmanjan 12y ago

Don't cylinders have fewer potential points of failure? (2 edges at either face vs 8 corners). Not really sure!

ygra 12y ago

It would at least include a bit of space for expansion without immediately damaging surrounding cells or the enclosure, I guess.

dm2 12y ago

I think they use standard 18650 batteries (might be wrong) and there have been discussions from Tesla saying that the round batteries also have the benefit of allowing an air gap to insulate and help mitigate overheating problems. Plus they are mass produced, cheap, and have a relatively high energy density. The new factory might work on creating some new battery technologies though, not sure. Ideally yes, you want the battery as compact as possible and lots of round batteries aren't the most efficient for that, but when you factor in cost and the natural heat insulation, it might have been the best option at the time. If it works, roll with it and work on one of the numerous other problems they have to solve.

TD-Linux 12y ago
The Panasonic 18650B cell has the highest energy to weight ratio of any commercially available lithium-ion cell. This is by far the biggest factor. It's also the cell used by many solar car teams for the same reason.

deleted 12y ago

[deleted]

hemmer 12y ago

There was a recent discussion about cylinders on HN which might be of interest: https://news.ycombinator.com/item?id=8097125 https://news.ycombinator.com/item?id=8097125