8

LiPo is often the first choice of battery due to its high discharge rate and high specific energy (100 - 265 Wh/kg), defined as its energy per unit mass.

In terms of these two characteristics, what type of batteries are next best alternatives to LiPo?

  • You are not entirely correct as to the reasons LiPo batteries are the first choice and how the key characteristics are measured. I'm not entirely sure how this should be resolved, as I don't feel that the text of this question as it is will fit the community, but the main point of the question itself (e.g. discussing the various battery types for RC aircraft) is a good one, so it shouldn't be closed. I would have edited it but I feel it might be rude to you as the author. Maybe we can work together on rephrasing it so that it will be a good reference question for drones.SE in the future? – FlashCactus Apr 16 at 12:50
  • @FlashCactus Alright, though I don't understand why it is closed due to its being opinion-based. I chose the weight/lifespan characteristics so that comparisons can be made quantitatively and not subjectively. – TheSimpliFire Apr 16 at 13:35
  • It's not closed yet (or again; I don't know whether the question has been closed and reopened.) The reason I think you chose the wrong characteristics is that weight is primarily dependent on the battery's capacity (so energy density, i.e. how much energy a cell stores per gram is more relevant), and lifespan depends on a huge amount of factors. Also, the main reason for choosing lipos is that of all the (mostly lithium-based) cell types with high energy density they have the highest usable drain current, and electric aircraft are very current-hungry. – FlashCactus Apr 16 at 15:00
  • So would "energy density" and "depth of discharge" respectively be more suitable? – TheSimpliFire Apr 16 at 15:07
  • Energy density and rate of discharge, but yes. – FlashCactus Apr 16 at 15:09
7

Many people choose to use Lithium Ion batteries on long range builds.

They have a very large capacity and I have seen them give in excess of 20 minutes of flight on a quadcopter.

The reason that they aren’t often used for other purposes is that their current output is less than LiPo batteries.

This means that whilst they are good for long range flight, they lack the power for aerobatics and racing.

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4

Also take a look at LiFePO/LiFe chemistry batteries.

https://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery

Some advantages are:

  • More capacity per gram, meaning lighter weight
  • 1,500 - 2,000 cycles
  • More stable chemistry, still dangerous but less volatile than Li-Po
  • Almost no "self discharge", ready to go even weeks after charging
  • Superior voltage performance under load
  • Slightly faster charge time

Disadvantages:

  • Harder to find and in the sizes you want
  • More expensive
  • LiFePo compatible battery charger required
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3

Li-Ion batteries are problematic because they do not deliver the currents needed for FPV flying. Take a look at this diagram: https://batteryuniversity.com/learn/article/discharge_characteristics_li

It tells you that the specific 3000mAh cell tested there loses around 30% of its capacity when discharged at 2C, which corresponds to around 6A, which is not much in terms of FPV flying. If you draw more current, the loss will be even greater. This is also true for LiPo batteries, but they're made for tens of amps of discharge, and the relevant effect will only manifest itself noticeably at discharge rates of, say, 30C.

Saying that, you can discharge Li-Ion to much lower voltages than LiPo, typically to slightly below 3V instead of the typical 3.5V for Lipos (assuming you don't want to damage the cells or accept capacity loss). So you'd have to adjust your copter warnings/beepers accordingly. Data sheets and available discharge plots at https://lygte-info.dk and https://batteryuniversity.com mostly suggest to not go below 2.5V - 2.7V, if the lifetime of the cell is to be achieved.

Then weight is an issue. A typical 4S 1800 mAh LiPo weighs in around 200g, so you get 9 mAh/g. A modern high-current Li-Ion 18650 cell such as the Sony US18650VTC5A with 2600 mAh weighs 48g, so you get 2600 mAh / (4 * 48g) = 13.5 mAh/g. Now if you take into account a small capacity loss because you're discharging the Li-Ion at 10A ~ 4C (flying very carefully!) you'd probably end up with 2200 mAh or 11.5 mAh / g.

A potential way out of this is, of course, to use more 18650 cells to make a 4S2p pack, which would half the discharge rate per cell and therefore give you not only more capacity in total (2 x 2600 mAh = 5200 mAh), but you'd also get closer to the nominal cell capacity because the 10A you're discharging at are ~ 2C and not ~4C as for a 4S1p pack. But then again you'd carry around a 8 x 48g = 384g battery, which would probably feel noticeably more sluggish than a 200g battery.

The Samsung INR 21700-40T (this is not a 18650, but it's slightly larger!) has a capacity of 4000 mAh, can deliver 40A and weighs 70g, resulting in 4000 mAh / 280g = 14.3 mAh / g when assembled in a 4S1p pack. Discharging this at 20A to 3V results in a capacity of 3300 mAh, or 11.8 mAh/g. This might be a viable option for a long-range copter.

So while Li-Ion batteries probably are the next best option compared to LiPo, they can't compete in racing or acrobatics, but they may outperform LiPos in long-range situations.

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