Forming a battery around a cylinder

[ASK1]

Are there any batteries on the market that would be able to wrap 360 degrees around a cylinder? Or for that matter, any fairly thin batteries that could comply to a curved surface without being damaged? Can any sufficiently thin LiPo cell do this, or do they have to be specifically designed for this?

Basically the application would be for mounting a battery pack on the inner hub of a bike wheel, so that you can supply power to lights (or whatever else you could think of) mounted within the wheel while maintaining a good weight distribution around the wheel. So this is a pretty small curvature radius, just a couple of inches at most.

Currently the best solution that most manufacturers of these bike lights are using are rectangular battery packs that have to awkwardly be zip tied near the hub, where they flop around and mess with the weight distribution of the wheel.

Any ideas?

 

[friendly1uk]

How much power do you want? Watch batteries are tiny and could be put in a row. Maybe 6 or 7 C cells of the 2/3rd variety. Many options that don't revolve around flexible batteries, but rather grouping of smaller cells. You can even get a holder for 18650s to go round a 20mm axle.

 

[ASK1]

So I haven't done the math on that yet. Some of the more popular wheel lights on the market use 2-3 AA batteries. I'm not sure if they are running in series or in parallel. In series that's 3.0-4.5 V, capacity of 2.5 Ah (7.5-11.3 Wh). In parallel that's 1.5 V, capacity of 5-7.5 Ah (also 7.5-11.3 Wh). Specifically on the Monkeylectric wheel lights, these have a run time of 20-60 hrs for the basic 4-LED setup off of 3 AA batteries, so I don't think the C-rate requirement is very high. If it is indeed running off of 1.5 V I wonder if I would run into issues since the LiPos would be at 3.7 V nominal.

I was also considering the grouping of smaller cells option, you could have like a flexible belt of narrow LiPos that are all in parallel and segmented so they can easily wrap around any size hub.

The 18650 holder sounds interesting, I think 18650s may be overkill here due to size and weight, but I'm interested to see what this holder looks like, do you have any links or info on that?

 

[Chalo]

Currently the best solution that most manufacturers of these bike lights are using are rectangular battery packs that have to awkwardly be zip tied near the hub, where they flop around and mess with the weight distribution of the wheel.

You're trying to solve a non-problem. The rotational moment of a battery tied to the hub center is negligible, significantly less than that of the pins in a rim seam, or the wheel lights such a battery is designed to power.

 

[ASK1]

Currently the best solution that most manufacturers of these bike lights are using are rectangular battery packs that have to awkwardly be zip tied near the hub, where they flop around and mess with the weight distribution of the wheel.

You're trying to solve a non-problem. The rotational moment of a battery tied to the hub center is negligible, significantly less than that of the pins in a rim seam, or the wheel lights such a battery is designed to power.

That's a good point. I should do the math on this. I haven't purchased any of these systems yet, but a fair number of users have reported that they can feel their wheels wobbling/shimmying from the imbalance. Also this has the effect of limiting the pack size because larger pack sizes will exacerbate this. However this imbalance could be from the lights themselves (though you can always mount your lighting system in a symmetrical manner to counteract this issue). I'll check on the math.

 

[Chalo]

My experience is limited to the 3 x AAA packs that come with the wheel lights my shop sells. Those are not noticeable when underway.

If you needed or wanted much more energy, it would probably be more useful to devise a symmetrical array as suggested earlier than to use an unconventional cell. Feeding it into a laced wheel and closing it securely and concentrically around the hub center would become the technical challenge in that case.

I remember when lithium polymer cells were first announced, and the takeaway for journalists at that time was the physical flexibility of the cells and the packaging possibilities they opened. In practice, nobody flexes lipo cells on purpose, and it would probably be a bad idea to do so.

 

[friendly1uk]

oh, my bad. It don't pivot around one center
https://bmsbattery.com/case/621-18650-cells-protection-cover-case.html

Sounds like you just need two cells opposite each other.

See what the competition are offering.

 

[Toorbough ULL-Zeveigh]

In practice, nobody flexes lipo cells on purpose, and it would probably be a bad idea to do so.

http://www.powerstream.com/curved-lithium-polymer.htm
http://www.pdbattery.com/ultra-curved-battery.html
http://www.ufo-battery.com/detail/small-capacity-battery/76
http://www.gpcbattery.com/english/Products_show.aspx?id=157
http://www.electronicsunlimited.com/curved-lithium-polymer/
http://www.aliexpress.com/promotion/promotion_curve-battery-promotion.html
http://news.panasonic.com/global/press/data/2016/09/en160929-8/en160929-8.html

 

[Chalo]

From one of the above links:

The maximum thickness of a curved cell is about 2 mm, though 2.5mm cells can be curved if the radius is large enough.

The above are all watch batteries. That seems like it would be fine if your idea of bike lighting is one or two 5mm LEDs.

 

[Toorbough ULL-Zeveigh]

From one of the above links:

The maximum thickness of a curved cell is about 2 mm, though 2.5mm cells can be curved if the radius is large enough.

The above are all watch batteries. That seems like it would be fine if your idea of bike lighting is one or two 5mm LEDs.

http://batteryuniversity.com/learn/article/serial_and_parallel_battery_configurations

 

[Punx0r]

I have a feeling commercial curved cells are constructed that way - not made flat then bent.

 

[Toorbough ULL-Zeveigh]

I have a feeling commercial curved cells are constructed that way - not made flat then bent...

The following cells are bendable, but shipped flat.

http://www.powerstream.com/lip/PGEB201212C.pdf

 

[Punx0r]

I stand corrected!