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#### Zoe313

##### New Member
Tech-question:
At present, UAVs use lithium batteries as the power supply, and supercapacitors on the market also say they can charge and discharge quickly to replace batteries at some fields, can supercapacitors be used in UAVs?

#### Nigel Goodwin

##### Super Moderator
No, supercapacitors are massively inferior to batteries in almost every way, it's only in VERY rare circumstances where you can use them in place of batteries.

#### ronsimpson

##### Well-Known Member
supercapacitors
Look at the discharge graph on a battery. After the charge is over a car battery is near 12 volts. The voltage remains near 12 for almost all the time. A capacitor's voltage drops linearly. Starts at full voltage and drops to zero.

#### Nigel Goodwin

##### Super Moderator
A capacitor's voltage drops linearly.

No, no Ron - go to the back of the class!

It drops exponentially, if it was linear it would be of more use.

#### bh4017

##### New Member
The energy that can be stored in a capacitor is limited to the amount of charge that can be stored in its electric field. This is enhanced by dielectric materials but it still can't match the amount of energy that can be stored chemically, as in a battery.

Furthermore, the useful energy in a capacitor is limited (in the context of powering equipment) because the potential difference falls as the electric field collapses and charges move away. In the case of a battery the potential difference across its terminals is continually maintained via chemical reactions.

So with a capacitor the amount of energy that can be stored in the first place is less and the amount of that energy which is useful for powering equipment is also less.

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#### unclejed613

##### Well-Known Member
can supercapacitors be used in UAVs?
yes, but not in place of batteries. they sometimes get used with batteries to add short term high current capability that might be slightly over what the battery itself can provide.

#### ronsimpson

##### Well-Known Member
No, no Ron - go to the back of the class!
It drops exponentially, if it was linear it would be of more use.
If you have a resistive load I agree. I should have used more words.
If you are using a 7805 linear regulator to power a u-computer, a 12V battery/capacitor sees a constant current load. Thus linear.

#### Zoe313

##### New Member
er...you all make me more confused, last day, I have read an article about supercapacitor, the instant discharge of the capacitor is stronger than that of the battery, but the energy density of the battery is far less than that of the li-battery.

#### dknguyen

##### Well-Known Member
er...you all make me more confused, last day, I have read an article about supercapacitor, the instant discharge of the capacitor is stronger than that of the battery, but the energy density of the battery is far less than that of the li-battery.
That still make sense. One is a marathon runner (battery), the other is a sprinter (capacitor). Just because a sprinter can run faster doesn't mean they can run for longer than a marathon runner, even if they ran at a slower speed.

A capacitor is a small tank with a very large opening you can pour electrons out of faster, but the total available is less. A battery is a much larger tank but has a smaller opening so you have more total electrons you can pour out, but you the speed at which you can pour them out is slower.

Or it's like a bank account with a $10k daily withdrawl limit but only has$100k in it versus a bank account that has a \$1k daily withdrawl limit but has a billion dollars in it.

Nitromethane and gasoline are the same. Nitromethane produces less energy per mass burnt but, for a given combustion chamber volume, you can burn so much more of it in a single combustion cycle that you can make up for the lower energy per mass burnt in order to produce higher power than gas. So high power, low run time.

Gasoline produces more energy per mass burnt but chemistry prevents you from burning too much at once, so lower power but much much longer runtime. That's why many small model airplanes use nitro engines. More power with a smaller, lighter engine. More acrobatic. Gasoline engine planes of the same size have less power, but run for much, much longer on the same amount of fuel.

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