Harvest Energy
For Your Robots
with the EnerChip
by Fred Eady
The ancients invented today’s
battery. However, they didn’t
know that the sun they
worshipped held the real key to
portable electricity. Battery
power is something humans use
to enhance their daily lives. A
robot sees a battery in a much
different way. A battery may be
a robot’s life blood. Mobile
robotic devices must either pack
enough battery energy to
complete their mission or have
the ability to tap into energy
sources that surround them.
This month’s discussion revolves around energy harvesting. If the term “energy harvesting” is not in your everyday vocabulary, the Mars Spirit rover is a very good example of a robotic vehicle living “off of the land.” The Spirit was
only supposed to navigate the Martian surface
for a total of 90 days. Six years later, the Spirit rover failed
to communicate with its Earthly operators.
As of this writing, there are hopes that
communications with the Spirit rover will be re-established.
Meanwhile, the Opportunity rover — which is just a bit less
than seven years old — is still clocking miles and harvesting
Martian sunlight.
The EnerChip CBC050
Our energy harvesting discussion will revolve around
38 SERVO 03.2011
the Cymbet EnerChip CBC050 — a 50 µAh rechargeable
solid-state energy storage device. The EnerChip CBC050 is
not just another rechargeable battery. It is a low profile,
surface-mountable backup power source housed in an 8
mm x 8 mm QFN package. The CBC050 is designed to
replace coin cells, super capacitors, and batteries that are
normally used to provide backup power for embedded real
time clocks, non-volatile memory, and microcontrollers.
Don’t let the relatively tiny discharge rate of 50 µAh
negatively influence your opinion of the CBC050’s
performance. With the right supporting components and
some energy-aware host firmware, a pair of EnerChip
CBC050’s can pulse power embedded packet radios and
the microcontrollers that host them using nothing more
than ambient light.
The nominal output voltage of a CBC050 is 3. 8 volts.
The discharge cutoff voltage is 3.0 volts. If the discharge
cutoff voltage is adhered to, the CBC050 can be recharged
thousands of times. It is also able to retain a charge longer
than standard rechargeable batteries. No special current
limiting circuitry is required to charge one of these puppies.
One needs only to provide a 4.1 volt charging voltage for
20 minutes to take a CBC050 energy storage device to 80%
of capacity.
Fred Eady’s First Rule of Embedded Computing —
which states that “nothing is free” — applies to the
EnerChip CBC050. As the number of charge cycles exceeds
1,000, the CBC050’s internal resistance will increase. At
room temperature ( 25° C), a new CBC050’s internal
resistance lies between 750 and 2,000 ohms. A typical
CBC050 that has been recharged in excess of 1,000 times
exhibits an internal resistance of 4,200 to 7,000 ohms. An
increase in charge time can also be observed once the
number of charge cycles exceeds 1,000. At 1,000 charge
cycles, that 20 minute charge time to 80% of capacity
becomes 60 minutes. However, if proper design techniques
are followed, the load performance will be minimally
compromised.
