Hi all,
I'm very much a beginner and have no engineering or electrical background. So please excuse any apparent incompetency (but do of course let me know

).
I'm currently working on a simple safe-shutdown implementation for a raspberry pi zero using two 10F 2.7V supercaps in series to provide reserve power while the pi shuts down.
Since I'm charging the caps in series from a 5V 1.5A USB power supply, I've gathered that i really should implement some level of protection. To do this I use this guide by Andreas Spies:
http://www.sensorsiot.org/simple-and-cheap-way-to-protect-your-super-caps-video-139/The relevant circuit diagram:

In the diagram two TL431X are used to shunt the caps in case of overvoltage (>2.7V), based on a reference voltage.
My main question is if it is correct that he uses two voltage dividers to generate separate reference voltages (R1-R2 and R4-R5) in stead of using just one voltage divider between 5V and ground?
And a side question. Wouldn't it be advisable to limit the current from the USB power supply with a current limiting resistor, or can one assume that the USB supply itself will limit the current?
I of course plan to add a diode to get rid of any backcurrent woes.
Oh, and in case you're curious. I plan on using a HT7044A-1 voltage detector with a pullup resistor to the pi's 3V rail connectet to its Vout go generate an active low signal for the pi to react to.
cheers
-JDP