# Super Capacitor with Real Time Clock

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#### Kian

##### Member
Hi all,

I am designing a real time clock circuit (RTC) and using a super capacitor as a back up power supply. The RTC I am using is PCF8563, and I am using a 3.3V super capacitor (DMS3R3224). The connection is as shown in the image below:

Is this the right way to connect up the super capacitor to the RTC? Any problems? VCC is 3.3V too. Also, I noticed that the super capacitor is drawing a lot of current from VCC. I presume that it is charging up. How do I calculate the time it takes to be fully charged? And how do I calculate the time it take to fully discharge if VCC is taken away and the super capacitor is powering up the RTC.

#### jjw

##### Member
Time to discharge t=U×C/ I
where U is voltage change, C is capacitance and I discharge current.
For example PCF8563 takes 0.25uA, works from 1V then with 0.22F capacitor
t=( 3.3V-0.6V-1V)×0.22F/0.25uA ~1.5 million seconds ~ 17 days

Edit: The leakage current of the capacitor also affects the discharge time.
With a 1uA leakage + 0.25uA the discharge time is about 3.5 days.

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#### Nigel Goodwin

##### Super Moderator
I really hate the name 'super' capacitor, as it encourages people to think they are particularly useful - when in reality it's just a very poor imitation of a battery.

Anyway, as far as this circuit goes it looks 'OK', but it will only give a fairly short backup time, and the capacitor will probably fail after a couple of years. They were used extensively in cheap VCR's for this same purpose, and it was an EXTREMELY common fault - I've replaced hundreds of them.

#### JonSea

##### Well-Known Member
Seems to me I read the recommended procedure to charge a supercap is through a current-limiting resistor to prevent excessive current flow and overheating.

#### schmitt trigger

##### Well-Known Member
Nigel
I would be very much interested in your experience related to these devices.
Why do you think was the reason for the failure rate? Improper charging!

#### Nigel Goodwin

##### Super Moderator
Nigel
I would be very much interested in your experience related to these devices.
Why do you think was the reason for the failure rate? Improper charging!
No, I think they are just fairly useless devices.