Super capacitor selection and Charging & discharging

[vignesh_shan]

Hi enthusiasts,
I just need a super capacitor for one of battery operated application, the super cap will be a back up power supply for the design. But i didn't use super cap before,. I need answer for below queries,

1) why most of super capacitors are available in 2.3V to 2.7V range ?
2) how to charge a super capacitor? Can we apply some minimal voltage at discharged condition and increase the voltage? Is there any plot or explanation about charging?
3) please confirm below calculation is correct ?
I= 200mA, V= 2.8v to 2.5V need to be discharged at t = 30s
 C = I * dt/dv
= 0.2 * 30/0.3
=20F
 So required C is 20F ??

Please answer the queries
Thanks,
Vicky