Old question, but another insight I'd like to share

I assume people will want to charge the supercap when normal power supply is on, and that will bring diodes to the curcuit, (see image) reducing the effective stored energy your RTC can use, leaking significant amount nobody mentioned so far here.

I found it on particle.io They also mention that STM micro don't tolerate sinking current back from VBAT input when: you start charging the cap that is below Vin-0.6V. Most of the discrete RTCC's are like that too, that's way you need D2. Reason D1 is pretty obvious, you want only the RTCC to use the supercap's stored energy.

Choosing Schottkys will be a tradeoff (yet again). The lower the forward voltage you choose, most probably the higher the reverse leakage current will be. For example a BAS-40 (you can find two in series configuration in SOT-23, with "S" postfix) will have 0.4V voltage drop if you charge the cap with 10mA at 25C (see datasheet), AND will leak on the order of 0.1 uA at normal temperatures. If you choose another schottky, leakage can easily be ten-hundred times higher. That can be explain the value Kasi measured in the previous answer.

Old question, but another insight I'd like to share

I assume people will want to charge the supercap when normal power supply is on, and that will bring diodes to the curcuit, (see image) reducing the effective stored energy your RTC can use, leaking significant amount nobody mentioned so far here.

I found it on particle.io They also mention that STM micro don't tolerate sinking current back from VBAT input when: you start charging the cap that is below Vin-0.6V. Most of the discrete RTCC's are like that too, that's way you need D2. Reason D1 is pretty obvious, you want only the RTCC to use the supercap's stored energy.

Choosing Schottky for D1 will be - as often-times in engineering - a trade-off: The lower the forward voltage you choose, most probably the higher the reverse leakage current will be. OTOH D2 should always be Shottky as that doesn't provide a leakage path, and you want the lowest drop there. For example a BAS-40 (you can find two in series configuration in SOT-23, with "S" postfix) will have 0.4V voltage drop if you charge the cap with 10mA at 25C (see datasheet), AND will leak on the order of 0.1 uA at normal temperatures. If you choose another Schottky, leakage can easily be ten-hundred times higher. That can be explain the value Kasi measured in the previous answer. If component/assembly cost is not a big concern, but you want the longest backup time, different Schottkys for D1 and D2 can be the best answer...

Old question, but another insight I'd like to share

I assume people will want to charge the supercap when normal power supply is on, and that will bring diodes to the curcuit, (see image) reducing the effective stored energy your RTC can use, leaking significant amount nobody mentioned so far here.

I found it on particle.io They also mention that STM micro don't tolerate sinking current back from VBAT input when: you start charging the cap that is below Vin-0.6V. Most of the discrete RTCC's are like that too, that's way you need D2. Reason D1 is pretty obvious, you want only the RTCC to use the supercap's stored energy.

Choosing Schottkys will be a tradeoff (yet again). The lower the forward voltage you choose, most probably the higher the reverse leakage current will be. For example a BAS-40 (you can find two in series configuration in SOT-23, with "S" postfix) will have 0.4V voltage drop if you charge the cap with 10mA at 25C (see datasheet), AND will leak on the order of 0.1 uA at normal temperatures. If you choose another schottky, leakage can easily be ten-hundred times higher. That can be explain the value Kasi measured in the previous answer.

Old question, but another insight I'd like to share

I assume people will want to charge the supercap when normal power supply is on, and that will bring diodes to the curcuit, (see image) reducing the effective stored energy your RTC can use, leaking significant amount nobody mentioned so far here.

I found it on particle.io They also mention that STM micro don't tolerate sinking current back from VBAT input when: you start charging the cap that is below Vin-0.6V. Most of the discrete RTCC's are like that too, that's way you need D2. Reason D1 is pretty obvious, you want only the RTCC to use the supercap's stored energy.

Choosing Schottkys will be a tradeoff (yet again). The lower the forward voltage you choose, most probably the higher the reverse leakage current will be. For example a BAS-40 (you can find two in series configuration in SOT-23, with "S" postfix) will have 0.4V voltage drop if you charge the cap with 10mA at 25C (see datasheet), AND will leak on the order of 0.1 uA at normal temperatures. If you choose another schottky, leakage can easily be ten-hundred times higher. That can be explain the value Kasi measured in the previous answer.