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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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    \$\begingroup\$ 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) \$\endgroup\$
    – Joren Vaes
    May 16 '17 at 10:43
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    \$\begingroup\$ Lithium-polymer is a type of lithium-ion, so the whole premise behind the question makes no sense. \$\endgroup\$ Feb 27 '18 at 12:14
  • \$\begingroup\$ Watch this video. It mentions their advantages for example higher current draws :youtube.com/watch?v=jsqJn-3iDLI \$\endgroup\$
    – F.Ahmed
    Feb 22 '19 at 9:38
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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.

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    \$\begingroup\$ 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. \$\endgroup\$
    – Joren Vaes
    May 16 '17 at 10:56
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    \$\begingroup\$ 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. \$\endgroup\$
    – Redja
    May 16 '17 at 21:16
  • \$\begingroup\$ 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. \$\endgroup\$
    – mkeith
    Nov 7 '17 at 8:50
  • \$\begingroup\$ @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. \$\endgroup\$ Apr 12 '18 at 22:27
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    \$\begingroup\$ 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. \$\endgroup\$
    – Dampmaskin
    Apr 19 '19 at 11:09
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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.

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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.

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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.

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    \$\begingroup\$ "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: danyk.cz/test18650_en.html \$\endgroup\$
    – 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).

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    \$\begingroup\$ 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. \$\endgroup\$
    – mkeith
    Nov 7 '17 at 8:40
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    \$\begingroup\$ 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). \$\endgroup\$
    – mkeith
    Nov 7 '17 at 8:42
  • \$\begingroup\$ @jeff: This is unreadable due to lack of capitalisation and random and missing punctuation. -1. \$\endgroup\$
    – Transistor
    Feb 27 '18 at 18:15

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