Below is from the paper.
"At around 6.5 hours, the capacitor voltage exceeds the charging threshold
of 4.4V and the battery is
below 3.5V, resulting in a charge pulse (as seen by the sharp drop in
capacitor voltage in A and increase
in battery voltage in B). This rapidly transfers most of the supercapacitors'
energy to the battery.
The solar panel quickly replenishes the supercapacitors and battery voltage
stabilizes at around 3.54V"
The 4.4v is a pulse voltage. The stable voltage is 3.54 that is under the
normal max voltage 3.6v. 3.8v is a little bit high than 3.6v. But I think
3.0v regulator is the most safe way.
Incidentally, you would think that a you'd need a regulator to drop
3.8V down to something more suitable for Telos/Tmote, but if you take
a look at Section V, Paragraph 3 in the Prometheus paper (co-authored
by one of the Telos/Tmote designers), you'll find that the system
voltage is allowed to rise up to 4.4V so it's *conceivable* that you
could avoid a regulator for 3.8V:
Prometheus: http://www.polastre.com/papers/spots05-prometheus.pdf
WARNING: I have no idea what a safe threshold is, so don't take my
word for it. Check with the hardware manufacturer to make sure you're
not doing something that will damage your mote.
My experience indicates that 3.8V should not damage the node, but if
you operate the mote outside the rated conditions you will void the
warranty.
Cheers,
Rob
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