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I'm currently building an autonomous drone as an experimental project, because my high school classes are getting quite boring.

I've already ordered all the parts, but upon further inspection of the 18650 cells I chose, it seems as though the C rating may be considerably lower than what my motors can pull.

The cells are Panasonic NCR18650B cells which are rated for 2C continuous discharge. I'm going to be running a 4S3P configuration of cells, so that would leave my total safe continuous discharge rate at around 20.4 A.

The problem with this is, the motors can pull up to 25 amperes EACH at 100% throttle. I went a bit overboard on the motors, but after looking at the specifications, it is completely possible for the drone to fly at around 60% throttle (5A per motor.)

Now my question is, would it be safe to exceed the battery's C rating, lets say to 3 or 4 C for short bursts (10 seconds?) What about 10C for a fraction of a second?

I'm not super experienced with electronics, seeing as this is my first "real" electronics project, any help would be appreciated.

TLDR: Can I exceed 2C rating of a battery for short bursts safely?

datasheet

winny
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Eryk Halicki
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    Those are "laptop" batteries. Not a great choice for a drone. I don't know that it would be dangerous, but it will be unsatisfactory because the cells will get too hot during discharge and have a large voltage sag under load. – user57037 Apr 28 '21 at 17:42
  • There are 18650 cells specifically rated for high C rate discharge, and no obvious reason not to use them. –  Apr 28 '21 at 21:41

2 Answers2

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Broadly speaking, there are two categories of 18650 cells. The first is "rapid discharge" or "power tool" type cells and the second is "laptop" or "storage" type cells. The panasonics you mention are laptop cells. They are optimized for large capacity and good cycle life.

Rapid discharge cells are optimized for the ability to deliver very high power. Generally the capacity and cycle life are lower than laptop cells. Rapid discharge cells are used for vaping, power tools, and small vehicles or really any application where the cells go from full to empty in less than an hour (at full load).

What you will notice when rapidly discharging the cells is that "power tool" cells will build up much less heat during discharge at high rates. To my knowledge, laptop cells are never used for quadcopters (or hex- or octo-copters either). You might be able to use them for a long endurance fixed wing aircraft. I am not sure.

So long story short, you should find different cells with a much higher discharge rating.

user57037
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The datasheet doesn't specify a maximum discharge rate, but all of the graphs have a discharge rate of 2C or less. It looks like if you choose to exceed 2C for any length of time whatsoever then you are on your own. The battery might be instantly destroyed or it might not be harmed at all.

Elliot Alderson
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  • I know there's no quantitative way to measure, but would you say its at all likely that the cells would fry at anything over 2C? – Eryk Halicki Apr 28 '21 at 16:58
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    Based on the available information, it would be foolish of me to make any prediction at all about what would happen if you exceed a discharge rate of 2C. I'm certainly not willing to risk your safety, are you? – Elliot Alderson Apr 28 '21 at 17:07
  • Nope not at all, thankfully by some miracle, the cells haven't shipped yet so I refunded them and I'm going to find cells with a better discharge rate. Thanks for the replys – Eryk Halicki Apr 28 '21 at 17:29