Milwaukee tools used the 18650 cells for years in there M18,18 volt line. They recently started using 21700 cells in their High Output battery packs. There are three differences. 1) the new packs are wider, but still fit all of their tools.
2) Even the smallest capacity new battery (the one layer 3.0CP - 3 Ah) can easily power the highest current draw tools. This is the reason for the “High Output” designation. 3) New is almost always more expensive. This is also the case here.
Now back to the subject at hand, the difference between 18650 and 21700 cells. The 18650 cells are 18 x 65 mm, and the 21700 cells are 21 x 70 mm. To keep things fair, let’s discus these batteries as though they used the same formula, so that we can compare apples to apples. The larger size of the 21700 cells has a few advantages. 1) 21700 packs are safer and more reliable because a battery with a wider pathway between electrodes, can handle more current. Using this larger cell, we don’t have to worry as much about the pack overheating in high draw conditions. Cool running batteries last longer
2) A battery with more density between electrodes, can hold more energy. On all same sizes batteries, the higher the Ah rating, the lower the current rating. On the smaller batteries, you have to ether chose between capacity or current, or find a way around this problem. The “work around” favored by most pack manufacturers, is to wire more than one layer of cells in parallel. Volts add in series, Amps and Ah add in parallel. Using 21700 cells, to build a similar pack often takes less cells and less complexity.
3) Milwaukee makes 7, 18650 cell packs and 4, 21700 cell packs. There is very little size and weight difference between 18650 and 21700 packs of similar storage capacities, but the ones with 21700 cells do have a superior current handling capacity.
I choose Milwaukee M18 tool batteries as a platform in order to give real world examples. The same rules apply to building a skateboard pack.
The chart on top use pounds for weight, and inches for the dimensions.
The second diagram shows how layering works. Volts add in series, Amps and Ah add in parallel. Layering is a great way to get more Amps and Ah out of a battery pack.
I use to race 1/8th scale RC trucks. A stock motor pulls over 100 Amps starting off and coming out of corners. Don’t under estimate these small motors, high loads can draw some serious amps. Build packs that can easily handle what you ask of them.
Thanks for the input but I actually think this is mostly common knowledge at this point in time.
Most high end builds here are running 12S4P 30Q packs and above. In terms of current/capacity/weight, the extra space occupied by larger cells offsets the benefits incurred. In short, battery chemistry is the limitation here and the dollar cost per performance is in equilibrium these days. In some cases moving to larger sized cells may be beneficial due to enclosure limitations but the same can be said for smaller sizing as well.
Thanks for the reply. I found this thread in Google, and for me, it came up short. Batteries have been around for a long time, and the rules have not changed. It may be common knowledge to you now, as it was to others several years ago. My point is, a new group comes into school, as another group leaves.
12S4P 30Q packs are expensive, and the ones I saw use 18650 cells. Do you think that this pack would benefit from a cell that runs cooler and longer, and is more reliable? At this point in our technology, the 12S4P 30Q pack you use is an antique.
What voltages were these packs running at? [email protected]=720w whereas [email protected]= 3552w (in the case of the 30Q 12S4P pack). So a completely different league in terms of power output.
What “non-antique” cells are comparable to the Samsung 30Q in terms of discharge capability, capacity and cost? They really do seem like a good balance, hence their popularity with builders.
You make it sound so simple, numbers are a good start. I also fly RC prop powered jets. Take a look at those battery’s (flat packs).
The only reason they still use 18650 cells is the cost of retooling. 18650 cells are OK and work acceptable, but the 21700 is unquestionably a better cell. And the weight and size difference are no problem for a skate board.
I’m sorry but you are completely wrong. I fly race quads from a 65mm prop to 5" and I esk8 with battery that I built myself. Flying a quad or a plane and riding a skateboard are very different things.
It seems you are quite literally comparing LiPo to Li-Ion and have no understanding of the chemistry as well as the advantages and disadvantages of them both. I highly suggest you google them before responding further or using the search function on these forums.
I don’t need to look it up. Skateboards consistently draw high loads, RC planes and vehicles draw high loads in burst. In RC planes, there is an art to choosing the right battery. To have a warm battery at the end of a flight, you need a balance of overall weight, speed, motor, aerodynamics, flight duration, along with how often and hard the pilot turns the aircraft (too many to list all). A Skateboard’s batteries largest restraint is load. The higher the voltage, the less Amps are needed to do the same amount of work. The one you talked about uses 43 volts, 60 18650 cells in two layers. I believe you would need only one layer of 21700 cells, and take a small reduction on the Ah rating.
Lithium-Ion, Nickel-metal hydride, Nickel-Cadmium, Lead-acid, Alkaline, I don’t care. You can power a Skateboard with any type of battery you want. Each has their advantages and disadvantages. Some may favor a different motor (voltage & specs). Layer technology lets you use 18650 cells; this technology can be used in any pack. The best scenario is to build a pack that will easily fit on a skateboard, but belt and back packs have been used in the past. Commercial configurations are always a compromise. If you’ve got the money, there are always better choices available.
@LukeCool if you do the math… youll find that for a given area(enclosure), the 30q 18650 almost always beats 21700 in terms of energy density AND price.
Energy density is close, price of a newer cell will be higher while it’s new, but the 21700 cells have superior current handling capacity, and will be more reliable because the packs can be built simpler and run cooler. I’m not saying that 18650 cells are junk. Milwaukee M18 5Ah batteries are $53 at Amazon, so I bought two. A month ago, the same battery was $85. So out goes the old, and in comes the new. The M18 5Ah are old tech, but are still great batteries for most tools. And I can use them in my saws, but they get a little too warm if I run them consistently.
And Friskies; I’d like to be able to agree with you and say the 12S4P 30Q pack is the best all around choice. but its not. Its only superior traits are that they are tucked neatly under the board, and the ride last a while (25-30 miles). But lets see you ride this board down a rail. The choices we make are relative. Can you imagine 21700 cells inside the board. It wont travel as far, but it will be lite and thin enough to where you can still do some tricks, Wouldn’t it be nice to do more than just go for a ride. Or maybe I’m wrong, a 12S4P 30Q pack skateboard may not be as fragile and heavy as it appears.
You lost me. The 18650 cells are 18 x 65 mm, and the 21700 cells are 21 x 70 mm. 21mm is a larger diameter than 18mm. In the same formula, more length increases storage capacity, and a larger diameter increases storage and current capacity. Given that the 21700 cell are newer, the formula used will be the newer and more advanced of the two. But like wire of the same type; the larger the diameter, the higher the current capacity. “How hot a battery gets” is directly related to how often, how long, and how close you get to its current capacity. This is why a larger diameter cell will run cooler. Storage capacity and heat are not directly related. I repair navigation systems in jets (all the way down to the circuit board); I know a thing or two about electricity.
i’ve got a 12s2p, yes 2p, battery in my board ade out of 21700 cells. i can asuure you it’s one heavy boy of a pack compared to 18650. you’re spouting a lot of bullcrappery mate
@LukeCool If you’re wondering why people seem hostile it’s because your tone sounds like you are presenting yourself as an expert in something where you have not done enough research. This creates misinformation on a forum intended to provide accurate information to builders and so people want to discredit what you are saying so as not to mislead our users. That said we all could do a better job being more polite to each other, state facts and explain (cite sources is even better) rather than telling people they are dumb and to use the search function (I know it takes extra effort and time).
There’s lots of number crunching done by the members of this forum that show dollar per Wh, power density and energy density of 30q packs sit at a really happy equilibrium. Especially when you consider the prices of cells that consumers can easily access.
The main argument for diy pack 21700 is compartmentalized enclosures on some emtb’s etc. that could fit either a row of 21700 or 18650 cells can get more range, and also a lower number of required cells means slightly less work welding (maybe). Otherwise the 30q just makes more sense. Calling it an “antique” is just inaccurate, they are all still leveraging the same basic chemistry and changing the dimensions of the cannisters slightly isn’t some amazing new generation of battery technology. Granted it’s quite a feat in terms of infrastructure to manufacture.
Until the price per Wh of 21700s drop they aren’t really worth attention except with certain enclosure dimensions or if you don’t care as much about cost.
I agree with almost every thing you said. People seem hostile because they’re passionate about there belief’s. But I don’t mind; more is learned when we actually have to think things through. And 18650 cells being more bang for the buck is another point I have repeated several times. We can also agree that "antique” is an overstatement, 18650 cells are widely used for good reason.The 30q pack uses 43 volts, 60 18650 cells. 60 times 1.2 volts is equal 72 volts. If used in two layers, the pack would be 36 volts. I’d love to see a schematic for this pack, this makes no sense to me. I am an expert in some aspects, but like everyone else, my education does have some holes in it. If I’m wrong, please school me, I’m egger to learn.
And also no, I’ve “never ridden a direct drive at 10 miles per hour”; but I’ve build motors from scratch, and repaired more than I can count. The motors in bikes that do the same thing are big. The size restraint of a skateboard has to demand compromises. So the higher performance the motor, the more human help it will need to keep it at a reasonable temperature. This little motor is often moving well over 100 pounds. Few 50 pound kids can afford a skateboard like this.
I do admit that this is reasoning conjecture on my part, NOT skateboard experience. But none of this is hard to figure out. Heat is a biproduct of the resistance of work, tempered by how close and how often we visit the top end of the specs.
