Supercap for RTC?

[tymbusku]

Has anyone looked into using a supercapacitor to run the Teensy 3.x RTC instead of a coin cell?

I'm working on a design where physical access to the finished circuit will be a problem but it would be normally powered by mains AC at all times so I think the RTC backup would only need to be capable of running for a few hours to cover power outages etc.

Thanks in advance for any guidance.

 

[overthrow]

Supercaps charge fast and release their charge quick but they don't hold anywhere near as much energy as other batteries like lead acid, lithium, etc.

The other issue is when they are near 0V and you apply voltage to them they basically dead short so you need a way to limit inrush current on them. They are also low voltage 2.4 to 2.7 so usually they are run in series and then you have to worry about charging all the cells evenly otherwise you could overcharge one and undercharge the other. That means you need a balancing circuit which is an entire thread by itself.

Supercaps are awesome for short duration high current applications but can be a bitch to make work. Not plug and play.

 

[overthrow]

There's a good image in this article on energy density of supercaps vs. lithium ion. It's an order of magnitude off, something to keep in mind. http://berc.berkeley.edu/storage-wars-batteries-vs-supercapacitors/

 

[Eka]

I've only recently done it for a Teensy 3.6, but haven't tested how long it will maintain the RTC. I originally did it for a DS3231, and would get 4 to 7 days with temp corrections, and a bunch more without. I'm using a 1F super cap with it's positive terminal tied to the Vbat. Between the positive terminal and Teensy 3.3 VDC I have in series a resistor for current limiting, and a very low reverse leakage current diode to prevent back discharge to the power supply. Obviously the negative terminal is tied to the Teensy ground. It charges up to around 3V over an hour or so. It has kept the RTC clock going for short periods without power, but I haven't had time to do testing to see how long it will last. Size the resistor for a reasonable current you think you can afford to charge at when the super capacitor is fully discharged. The very low reverse leakage current diode is to prevent the super capacitor from powering the rest of the circuit after power failure. It needs to be very low leakage current to reduce the discharge rate through the diode and resistor.