Capacitor charge time, two conflicting answers

[rad_controls]

TL;DR Summary

Help me correctly determine cap charge time

While attempting to calculate capacitor charging time I came across two conflicting concepts. I think my confusion can be pretty much summed up by the top two responses to this quora post:

https://www.quora.com/How-can-I-cal...-the-charging-current-voltage-and-capacitance
Pasted below for convenience.

I understand the first guy's response results in a time value that is half of the answer provided by the assistant bot. That comes from the energy loss of charging the capacitor being a factor of 2(we lose half our power to heat and other losses when charging a cap). But I am having trouble coming up with the why/how through the use of a proof. There is something that he is missing when integrating the current with respect to time but I cannot figure out what.

Answer 1:

Answer 2:

 

[Baluncore]

Help me correctly determine cap charge time

It depends on how you charge the capacitor.
You need a circuit diagram for context.

Given constant i and constant C.
C = Q / V ; definition of capacitance.
Q = i ⋅ t ; charge.
C = i ⋅ t / V ; ∴ t = C ⋅ V / i .
V / t = i / C ; ∴ dv/dt = i / C .

 

[Rive]

You need a circuit diagram for context.

Especially since it's often about a voltage source with a resistor or: with an impedance...

 

[phyzguy]

Yes. Typically you will not be charging a capacitor with a constant current.

 

[Gordianus]

Charging a capacitor with constant current has been used many times in timing circuits.

 

[Averagesupernova]

Charging a capacitor with constant current has been used many times in timing circuits.

Yep. CRT scopes did this to generate the sweep voltage. I've been involved in a few of those designs. Relatively slow sweep speeds for NTSC waveform monitors. Although I did troubleshoot higher speed scopes for a few years.

 

[willem2]

TL;DR Summary: Help me correctly determine cap charge time

While attempting to calculate capacitor charging time I came across two conflicting concepts. I think my confusion can be pretty much summed up by the top two responses to this quora post:

The answers are the same. both give t = C*V/I. I don't understand what the problem is