Why are Lithium Cells Cylindrical in Shape?

[6zfshdb]

This may be a stupid question and I could be showing my ignorance but please enlighten me.

I opened the case of my e-bike battery recently to replace a burned contact. I was amazed by the amount of empty space which is seemingly wasted between the individual cylindrical cells. Yes, the ratio of interior volume compared to surface area is greater with a cylinder but it seems this efficiency is lost due to wasted space when the cylinders are packed together. Why can't a much more energy dense battery be constructed using rectangular cells?

I understand heat dissipation may be an issue but is it really that much of a problem inside a sealed battery case? Is it to allow room for swelling? Does all this wasted space serve another purpose?

With the popularity of e-bikes and the many thousands of batteries being manufactured, I'm surprised there isn't a market for rectangular or similarly shaped cells which could pack more energy into a smaller space.

Why can't someone design a battery specifically for ebikes that doesn't use cylindrical cells meant for flashlights and other small electronic devices?

@Ravi Kempiah, perhaps you could weigh in here?

 

[tomjasz]

wasted between the individual cylindrical cells.

That’s a good thing rather then cells glued together.

 

[JRA]

Why can't someone design a battery specifically for ebikes that doesn't use cylindrical cells meant for flashlights and other small electronic devices?

You are aware that electric cars use the same cylinder type cells and in fact has bred the tech of our batteries to date and will into the future?

Other than that I got nothing......

 

[ruffruff]

1. Dissipate pressure evenly.
2. Dissipate heat more evenly.
3. easier to handle in manufacturing?

 

[theemartymac]

And cheaper to use existing cell profiles and manufacturing processes/equipment. Custom costs money.

 

[rich c]

All manufacturing driven is my assumption. The lithium sheet is so thin that trying to roll it on a square mandrel would result in tears.

 

[Pulsar]

There are lithium polymer batteries that have different shapes with much less wasted space between cells. I have several drones who use high output lithium polymer batteries. Lipo cells can expand/swell over time which is when they start to crap out. I have no doubt a Lipo ebike battery could be built, just not sure how long they would last and dissipating heat could be a challenge. If there was a benefit of Lipo vs lithium ion on cost, form factor or longevity for our ebikes, I’m sure a manufacturer would have already jumped on it.

 

[tomjasz]

I have no doubt a Lipo ebike battery could be built,

See endless_sphere !
Riders have been running their eBikes on RC packs for at least a decade. But they are dangerous in the hands of most eBikers. A pack large enough for decent speed and mileage will be 10 times the size of common actual RC users batteries. Over charge BOOM.
Drain to low BOOM.

 

[JRA]

See endless_sphere !
Riders have been running their eBikes on RC packs for at least a decade. But they are dangerous in the hands of most eBikers. A pack large enough for decent speed and mileage will be 10 times the size of common actual RC users batteries. Over charge BOOM.
Drain to low BOOM.

You forgot the BURN part Tom.

 

[fauconnier]

Li-po batteries are very efficient in discharge, they will certainly won't heat, puff nor explode with a typical commercial E-bike discharge rate of 0.5C to 3C . The best one can be discharge 10 to 20 times faster without any danger, which is not necessary for a commercial E-bike, who need the safest, protected by a case and by a circuit board, low discharge battery. Power density of Li-ions batteries is very high.

My bike on my round thumbnail is powered by two Li-pos. I charge them with an intelligent balance charger. Regular E-bike rely on a BMS to do it.

 

[m@Robertson]

Everything I have heard has been that the cylindrical shape - in conjunction with pack design that keeps a hair of separation between the cells - is better for heat dissipation. I'm sure there is more to it than just that (i.e. manufacturing efficiencies). You can't expect a battery design optimized for a single platform, so it has to be the least-bad fit for all platforms combined.

 

[tomjasz]

My bike on my round thumbnail is powered by two Li-pos.

And you're not alone. I get that. I'll take the advice of the brain trust of LiPo users and former users and avoid RC packs.

 

[m@Robertson]

Yeah if you are using Lipo's you need to know exactly what you are about or bad things can happen, and they often do because there are inexperienced 'builders' out there who slap stuff together. If I had a dollar for every bubble gum/baling wire build I've seen with a suitcase sized bunch of lipo's duct taped - or shrink wrapped - directly to a frame ... I'd have my bar tab paid off.

 

[harryS]

In another thread, I mentioned I had three Walmart car starter accessories. Each had a 12V rectangular pouch Lipo inside, strong enough to crank my car. Three in series gave me 36V, enough for 15-20 miles on the ebike and cost less than $100. I left them out in the rain and the charge/protection circuits died. They were still under warranty, but I don't ding others for my screwups. Lipo's scared me anyway.

NIssan and Chevy, among other car makers, do run their e-cars on rectangular prismatic batteries. These have hard cases instead of using pouches. I bought a surplus prismatic cell, 3.6V 63AH, for $12, just to see what they're all about, It's huge, and if I had another 39 of them, maybe I could run a golf cart,