0
\$\begingroup\$

I already know the 't=RC' . But what happens if I start charging a capacitor via a same voltage battery? When will it be fully charged?

\$\endgroup\$
  • 3
    \$\begingroup\$ The battery will have an internal resistance, so that's the missing part of the equation. Also the time constant above isn't for it to be fully charged. \$\endgroup\$ – PeterJ Aug 3 '14 at 9:07
  • \$\begingroup\$ Hmmmm.... I heard something about internal resistance but I didn't believe it , thanks anyway! \$\endgroup\$ – user3029101 Aug 3 '14 at 9:09
  • \$\begingroup\$ What do you mean "I didn't believe it"? That's not something you can't believe in... \$\endgroup\$ – Vladimir Cravero Aug 3 '14 at 9:14
  • \$\begingroup\$ I mean , batteries , with resistance that is wasting energy? \$\endgroup\$ – user3029101 Aug 3 '14 at 9:20
  • \$\begingroup\$ Also the wires have a resistance, of course. There is no such thing as a perfect conductor (R=0). You can read up on internal resistance here. \$\endgroup\$ – user17592 Aug 3 '14 at 9:33
3
\$\begingroup\$

If you know that T = RC, then you know that when T gets to RC the capacitor will only be charged up to about 63% of the battery voltage.

Then, after another RC passes the capacitor will have charged up to about 63% of the difference between the battery voltage and the voltage across the capacitor at the end of the last RC.

Then, after another RC passes the capacitor will have charged up to about 63% of the difference between the battery voltage and the voltage across the capacitor at the end of the last RC.

And so on, forever, so you can see that since the charge is exponential the capacitor never really charges up to the battery voltage completely.

This sketch pretty much says it all for the first RC, and if there's no intended resistance in there, then R is equal to the sum of the battery's internal resistance, the wiring resistance, and the capacitor's equivalent series resistance:

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.