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Supercapacitor as backup source for RTC?

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asrock70

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On ebay and any stores is offered a lot supercapacitor with capacity 1-10F, for example **broken link removed**

Have you some experience with it?
Is good idea use this part as backup source for RTC with consumption 1,5uA?
What is the longlife lifespan of how current charge it?
 

Hi,

with the capacitor formula C = I x t / V
you can calculate the backup time:

t_backup = (V_charged - V_RTC_min) * C / (I_RTC + I_selfdischarge)

Klaus
 

Theoretical backup time can be easily calculated, but leakage current is not necessarily specified.

In practice, small super caps, e.g. Panasonic Gold Caps are only good for one or several days backup time. That may be O.K. for devices with permanent mains connection to bypass power fails, but not for portable devices. A usual requirement is also that devices don't loose there date and time information between production and commissioning. That's only achievable with long life batteries.
 

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