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According to every source I've found the only advantage of Li-Po is their shape, but they are more expensive, have lower energy density and are more dangerous compared to Li-Ion.

Why do drones use Li-Po batteries instead of Li-Ion batteries for example 3s2p 18650 Li-Ion with 40A protection circuit.

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Vasil Kalchev
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    Isn't Li-ion the family of batteries, and lithium-polymer a member of the Li-ion family? Also, the currents are quite high, I don't know if many 18650 can pull it of (even if you do parallel them up) – Joren Vaes May 16 '17 at 10:43
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    Lithium-polymer **is** a type of lithium-ion, so the whole premise behind the question makes no sense. – Olin Lathrop Feb 27 '18 at 12:14
  • Watch this video. It mentions their advantages for example higher current draws :https://www.youtube.com/watch?v=jsqJn-3iDLI – F.Ahmed Feb 22 '19 at 09:38

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For this answer I'm going to define LiPo as flat packs, and Li-Ions as 18650s. The reality is more messy than that, but I believe this way of defining the terms will help answer your question as you seem to have intended it.

Traditionally, 18650s didn't support high current applications, a continuous current rating of 5A was considered to be high. 18650s that support 20A continuous current were more or less unheard of until high power cells like the Sony VTC3 came on the market.

(As a general rule, 18650s that claim to support more than 30A continuous, even today, are simply overrated. There is also a trade-off between capacity and continuous current capability; For example the 30A Sony VTC4 is "only" 2100 mAh.)

A lot of the reason why drones use lipos are probably tradition, due to this.

When you say that LiPos have a lower energy density, you mean a lower energy to volume ratio, right? I'm pretty sure they have a higher or at least as high energy to weight ratio, as Li-Ions. 18650 Li-Ions have a metal casing. That's unneccesary weight.

The flat form factor of many LiPos probably also helps with cooling.

Dampmaskin
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    I think the batteries used in RC drones might have worse volumetric capacity than 18650s since they have bigger copper sheets in them to sustain those high current capabilities, and lower the ESR. I haven't compared any numbers tho, so it could be that they are in fact better, but that's what I've heard as reason for it. – Joren Vaes May 16 '17 at 10:56
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    Yeah, the main reason is the current capabilities. The batteries used in drones can discharge at rates that can range from 10-40C( total discharge in 1.5-6 minutes), whereas a rate of more than 1-2C on a normal Li-Ion starts to negatively impact the cycle life. – Redja May 16 '17 at 21:16
  • Yes. To reiterate what Redja said, the main difference between RC aviation batteries and other lithium batteries (whether Lipo or Li-ion) is the higher C rating. No matter how you stack up 10C batteries in parallel or series, you will not be able to drain them faster than 6 minutes, and the burst power available for maneuvers can't go over this limit either. So drones usually use higher C batteries. For whatever reason, very high C batteries are not available in Li-Ion technology or 18650 form factor. – user57037 Nov 07 '17 at 08:50
  • @mkeith that may have been true a few years ago but Li-Ion are improving. The LG HB6 (PINK) is 1500mah is rated 30A continuous and in fact test benchmarks on the battery has no issue outputting 32A, with 4 cells that's 128A or 128,000mah which exceeds most setups. 4x LG HD2 2000mah 100A, 4x Samsung 25R 2500mah 80A, these are continuous rates and most can easily handle bursts of 5-10A EXTRA per a cell. The weight of a quality 18650 is around 40-50g which in total is around the same weight as a decent LiPO. – Simon Hayter Apr 12 '18 at 22:27
  • @Dampmaskin Neither LG, Panasonic, Samsung or Sony have ever sold a Lithium 18650 with false continuous rates, why? imagine all the the lawsuits with exploding laptops. 18650's that under perform are either fake or a crappy manufacture such as Ultrafire, Efest etc. See: https://i.imgur.com/O2ByACd.jpg – Simon Hayter Apr 12 '18 at 22:42
  • Problem with 18650 is they are easily faked because the batteries can be rewrapped... for example wrapping for [Ultrafire](https://www.ebay.co.uk/itm/30pcs18650-Battery-Wraps-Heat-Shrink-PVC-Pre-Cut-Sleeves-30MM-width-72MM-length/222781108480?hash=item33dec9f100:g:qUEAAOSwUBlZsXYk) and [Panasonic](https://www.ebay.co.uk/itm/10pcs18650-Battery-Wraps-Heat-Shrink-PVC-Pre-Cut-Sleeves-30MM-width-72MM-length/222605860330?hash=item33d457ddea:g:dg0AAOSwq1NZiK9x) – Simon Hayter Apr 12 '18 at 22:45
  • @SimonHayter 30A for a 1.5 Ah battery is still only 20C. That is impressive for an 18650, but LiPo is still higher. 30C packs are available for LiPo. And the main point I was trying to make is that the C rating of the cell and the C rating of the pack are always the same. Adding more cells in parallel increases the pack capacity, but does not increase the C rating. C rating is important for applications where the battery life is only a few minutes. – user57037 Apr 12 '18 at 23:42
  • @mkeith "Adding more cells in parallel increases the pack capacity, but does not increase the C rating." That is not true, if it were, the Tesla cars wouldn't even move an inch; they use packs of 18650 batteries in series and parallel. The benefit for LIPO over LI-ION is C rating over mass, LIPO provide higher C rating at a lower mass than LI-ION, otherwise they are pretty much equal in capacity / mass. – ShawnS Apr 17 '19 at 05:39
  • I would not say C is the measure of power density, density is the amp hours over mass, C is the measure of power delivery and it isn't sometimes, that is exactly what C rating means, it is an Amp/Hrs multiplier for how much current can flow "safely". You can get the same C rating from LI-Ion but the mass (or weight in Earth's gravity) would be to much for drones or most any RC for that matter. – ShawnS Apr 18 '19 at 08:48
  • We are using 18650 for an example, which is a standardized size that all 18650 "should" follow, there is not a standard package size for LIPO although there could. Going outside the 18650 size, there are LI-Ion battery packs that have the same voltage and C rating but they are monsters in comparable size. However, they have more Amp/Hrs but again the weight would not be practical or even possible for drones. – ShawnS Apr 18 '19 at 08:59
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    From a consumer point of view, C rating is a clunky and impractical unit anyway, as it depends on the capacity. Max continuous discharge amps per cell or for the whole battery is easier to relate to. Just like Ah is an impractical unit when using the battery with a buck and/or boost regulator, especially if you run multiple batteries in series, Then Wh is the way to go. Just convert the antiquated units to more usable ones sooner rather than later, and save yourself some grief. – Dampmaskin Apr 19 '19 at 11:09
  • @mkeith That is the point, you are referring to a specific battery, LIPO do not have a specific size, to get more amps or C they make the cells bigger, LI-Ion have the same capability. There is not just the 18650 battery, it is the most common and easier on a consumer point to make one standard size, but there are other LI-Ion battery sizes and others could be created if there were a practical need. Quite frankly I think we are getting off topic to the OP's question, bottom line is, vs Li_Ion, LIPO have the best Amp/hr and power delivery to mass ratio for drones. – ShawnS Apr 19 '19 at 15:19
  • @mkeith If you take 1 cell that has 2C discharge rate which would be, for 3400mAH, 6.8 amps of current flow, if you take 2 of those batteries in parallel it would be 2 x 3400mAH or 6800mAH x the 2C is 13.6 amps of flow. In another perspective, batteries have an internal resistance, current through resistance creates heat, which is why current has to be limited. What happens to resistance when you put resistors in parallel? What happens to current when you put resistors in parallel? Same applies to batteries and cells for that matter. – ShawnS Apr 19 '19 at 16:00
  • So it is 2C in both cases. So adding a parallel cell had no effect on the C rate. Thank you for proving my point. – user57037 Apr 19 '19 at 18:20
  • @mkeith I misunderstood your point, if you want to be correct that "C rating" does not change in parallel batteries of the same C rating, so be it, not quite sure how it is helpful, but C rating is not a measurement of power density. However, the same C rating batteries in parallel does multiply the current which is more helpful information and why I agree with Dampmaskin about C rating being clunky because it doesn't change even though the output current does change and it is misleading. – ShawnS Apr 19 '19 at 19:27
  • C rating is very closely tied to power density. It would be better to cite Watts/kg, I suppose. But C is a decent stand-in for that for the simple reason that, when comparing similar types of batteries, energy density is relatively constant. So, again, specifically for drones, the C rating is very important and very relevant. If you want to talk more, we can go to chat. I will probably delete most of these comments soon. I shouldn't have allowed such a long comment string in the first place. – user57037 Apr 19 '19 at 19:53
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the reason why people don't use 18650 as power source for drone is the C rate problem or discharging current.

for most 18650 battery packs, a 5C rate is sometimes hard to reach for most of the small manufacturers in china not to say 10C or 20C(technology is a problem, pricing is another problem, if more chinese companies can make batteries that reach 20C, pricing should be lower.)

but things are quite different for lipo batteries, 30C is common for most of the manufacturers

so again, lipo are more cheap and mature for high rate discharging applications.

Dave Tweed
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JojoD
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Weight and size also play a role. LiPo packs don't use a metal housing (=less weight) and the way they are build makes that use a lot less space compared to 18650 cells.

Kozn
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we can't use Li-ion for drones. at least i have tested 2 18650's 5800mAh in parallel: when you connect these to motors without board batteries get hot very soon and the speed of motors come down. but via a Li-poly motors work really better.

Ali Hashemi
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    "*i have tested 2 18650's 5800mAh in parallel*" FYI note that currently (in 2018) there is **no such thing** as a 5800mAh 18650 battery. The largest capacity genuine 18650 batteries are around 3000mAh (or a little more) from the *major* manufacturers - with limited availability on the retail market. Therefore the batteries you tried to use were fakes and could have had any *real* capacity, and were likely to be poor quality. :-( You should research fake 18650 batteries. One example of test results from fake 18650 batteries is here: http://danyk.cz/test18650_en.html – SamGibson Feb 27 '18 at 17:57
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discharge is no problem AT ALL. sanyo L ncR 20700b 6612 Li-ion cells have a 15c discharge, & parallell 4x, you get 60c. this practice is not uncommon in Lipo batteries either. and NO- the problem is not size vs. watt hours. the power density of a 20700 is almost double that of the most powerful Lipo, including the case. the only comparison that matters is power to weight density. and for 20700's this is 106wh per Lb. if you want to see how that stacks up against Lipo, look online at the battery weights/specs & do the math. you'll discover that Lipo is about 60wh per Lb... BUT- you have to build your own pack and brave the risk of no discharge protection (unless you know someone who can custom build the circuit for you).

jeff
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    Paralleling cells increases the max discharge current, measured in Amps. But it does not increase the maximum discharge when measured in C. If you have a 1Ah cell rated at for 10A discharge, that is 10C. If you add a second one in parallel, you have a 2Ah pack rated for 20A, which is still 10C. – user57037 Nov 07 '17 at 08:40
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    Basically, a pack made up of 20C cells will always be able to discharge more rapidly than a pack made up of 10C cells. The 20C cells will have higher power density (as opposed to energy density). – user57037 Nov 07 '17 at 08:42
  • @jeff: This is unreadable due to lack of capitalisation and random and missing punctuation. -1. – Transistor Feb 27 '18 at 18:15