Hey Guys,
I have a 10mF Electrolytic Capacitor rated 25V.
I want to measure how charged the capacitor is.
How can I achieve this?
I use a Arduino Uno.
Gonna have to do some measuring
How is the cap being charged up? From a 5V source, or something higher?
It is charged from a 5V USB Adapter [The ones used to charge Smartphones].
Measure the voltage. Total charge Q = CV.
Whose voltage?
Cap's? or Adapter's?
By the by, CrossRoads the link you have provided does not exist.
Crossroads' link:
Q = CV is basic physics, and is also the definition of a capacitor.
A 10uF capacitor connected to a 5V USB power supply will fully-charge to 5V in a matter of milliseconds.
Whose voltage?
Cap's? or Adapter's?
Measure the voltage across the capacitor. But, 10uF will discharge fairly quickly into your multimeter. i.e. If your multimeter has a 10M input resistance, the time constant is 1 second, so it will be more than half-discharged after 1 second.
It will hold the charge longer with nothing connected because it only discharges through it's own internal leakage resistance.
It might be more "fun" to try a 1000uF or greater capacitor.
10mF, not 10uF - read the post carefully!!
MarkT:
10mF, not 10uF - read the post carefully!!
What is 10mf? I am used to 10nf or 10 uf or 10 ufd, but not mf?
I must be getting old. Spent 40 years in circuit design and have never come across mf (even nf is not very common here in the US)
Here's a time constant calculator from Digi-Key.
@gpsmikey:
Yep, I remember when pF was mmF (micro-micro Farad) aka "micky-mikes".
Now guys,
If I charge the capacitor with a 5V 2A adapter and discharge the thing via a joule thief and resistors and inductors can I improve the discharge time?
If yes, by how much?
A joule thief is usually used to charge a capacitor, not to discharge. But it might work. It will certainly do something.
By "improve" do you mean faster or slower? Why do you want to make it faster or slower?
outsider:
@gpsmikey:
Yep, I remember when pF was mmF (micro-micro Farad) aka "micky-mikes".
Or mighty-mikes - those kilovolt disc capacitors for high-voltages with sparks, right?
We've got one Farad and up these days. Are batteries obsolete yet? 
gpsmikey:
What is 10mf? I am used to 10nf or 10 uf or 10 ufd, but not mf?
"u" is not the correct designator for "micro" The correct designator is the Greek letter "mu" µ or Ascii character 230 or "alt m" on my apple keyboard
gpsmikey:
I must be getting old. Spent 40 years in circuit design and have never come across mf (even nf is not very common here in the US)
Not my picture hence a link: nice cap; 40 mF. Made in USA 
//Edit OK, I get it; you never saw mF, always N000 uF
jackrae:
"u" is not the correct designator for "micro" The correct designator is the Greek letter "mu" µ or Ascii character 230 or "alt m" on my apple keyboard
Schematics usually use u instead of µ (mu). Same reason I wrote (mu), greek letters are less common, harder to remember, and sometimes don't work. The letter u always works, and we can figure out that it's mu but it's tail got cut off, so it spun around to see who did it.
MorganS, I mean SLOWER.
And I don't require 40mF. It is just a cheap 10mF Cap what I need