Are the number on the left side of the graph Amps? If so, you often go over 80.
Amps and volts.
Very interesting, thanks for the post.
Iād expect the voltage to stay fairly consistent. With that current draw, this is a pretty nice rig. A skate board graph should look similar, only much smoother less dramatic peeks. Do you have one you could show us?
Ill try to explain. My point is that a pack constructed of 18650 cells will run cooler than one of 21700 cells, even if the chemistry and construction is identical (same energy and power density). This is because 18650 cells have a higher surface area to capacity ratio. This is similar to the way that a mouse requires high metabolism/weight to maintain itās body temperature than an elephant.
Of course it is not possible to build two such packs, because Samsung does not make a ā30Q+ā in 21700 size.
You have a fundamental misunderstanding of the way in which these types of battery packs should be analysed. Itās true as you say in this post that the larger 21700 cells will have higher capacity and power, but of course they do. Itās the equivalent of me saying my 100kg 300Ah lead acid battery will run cooler than an 18650, its larger and heavier. However it will not run cooler than an equivalent (capacity/weight/size) 18650 based pack.
If you build two identical packs, not identical in the number of cells obviously, but in the capacity. Maybe you end up with 10s5p(18650) and 10s4p(21700). The packs have identical power capability and energy. The 18650 pack will run cooler because it has higher surface area and will therefore conduct heat away more easily.
There are a lot of people on these forums who are far more qualified than you. My advice is that if someone disagrees with you about something you have relatively little experience in, assume you are wrong and politely ask them to clarify.
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I am sorry but this sentence has disqualified you for giving any advice about liion cells currently. You should have at least the basics down - like that a 18650 cell has a nominal voltage of 3.6V and is not like a AA battery.
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If you consider one cell in free air? Sure. Lots of cells glued to each other inside a enclosure that in most cases is made of a material with low thermal conductivity? Not so much difference
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Using words like ānot so muchā isnāt very specific. Back of the envelope comparing two rectangular enclosures each containing an equal cell volume. So comparing the surface area of the battery enclosures in common eskate form factor.
18650 -> 10s6p -> 2 x 30 cells = 1645cm2 21700 -> 10s4p -> 2x20 cells = 1411cm2
So the 18650 enclosure has 16% more surface area. Maybe that is not so much, but my point STILL stands. An 18650 based battery pack will run cooler than 21700.
I have limited width on my board that I canāt go for 5P 18650 cells, but I can go for 4P 21700. Now there is a clear advantage of using 21700 in keeping it simple yet packing in a lot of capacity.
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Of course Your right, but thatās why I ask to see a schematic for this pack. Iām on 60 hour shifts at work right now.
12cells X 3.6 Volts = 43.2 volts, and a 60cell pack divided by 12cells = 5 layers . So that pack is 5 layers @ 12 cells per layer. This explains the high amp rating.
Taz; thanks for posting the graph. I see the skateboardās max current is about 70Amps, but this high is rare, where as 10 to 30 Amps is fairly common. An RC helicopter is a bit different than RC planes and ground based vehicles. Both can glide and use little to no current ā¦ a lot like your skateboard. If this was what youāre trying to prove, you succeeded.
we have a scheme to describe packs like this: 12S5P It gives you the number of cells in series (in this case 12) as well as the number of cells in parallel (in this case 5). Other typical configurations are 10S3P, 13S4P and so on. You can find plenty of schematics for these configurations in the battery build threads