often viewers ask in the comments
why do you still use batteries and not
super capacitors for your projects
good question
do you know the answer
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microcontrollers
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super capacitors are super like superman
or superwoman
otherwise they would only be called
capacitors right
where are they super and where not
where can we use them and how and the
most important question
are they better than batteries
all questions which have to be answered
in this video
let's start with the first question what
are supercapacitors
capacitors have their name from the
capacity to store energy in an
electrical field
the unit is farad
and this is the formula
i have no clue why they use kilograms
and meters in this definition but i'm
only a poor engineer who knows how to
use capacitors not a physicist who knows
how they work so let's get started
usually we only use tiny capacitors from
picofarad in radio frequency
applications to a few thousand
microfarad in power supplies
the latter usually are called
electrolytic capacitors
today i do not want to go into the
details on how they are built
instead i leave you a link in the
description if you are interested
all capacitors have in common that they
store the energy in an electrical field
and do not change any chemical structure
therefore they do not wear out with
charging cycles
standard capacitors have no polarity and
last extremely long
on the other hand electrolytic caps have
two disadvantages they have a polarity
and at least the non-solid ones dry out
after a few years lose capacity and
eventually have to be replaced
if we go on the way towards higher
capacities we need the super or ultra
capacitors
they still are capacitors and still
store the energy in a field
so they theoretically support an
infinite number of charging cycles they
also use electrolytes but differently
from the electrolytic caps they have a
membrane in the middle so their
construction is similar to batteries
and as electrolytic caps or batteries
they also can leak
they are also polarized and have much
more capacity than electrolytic caps
their capacitance is measured in farads
my biggest ones have 100 farads which is
quite a lot compared with electrolytic
caps
this electrolytic cap for example has
3300 microfarad or 3.3 millifarad and
this supercap with the same size has 30
farads or 10 000 times more capacity
a huge difference
so the super or even ultra is well
deserved
however there is a caveat the minimum
voltage of this supercap is only 2.7
volts
this electrolytic cap supports 25 volts
so we need 9 super caps in series to get
up to the 25 volts
this is the formula for capacitors in
series and the result is that we only
get nine times less capacity and need
nine times more space
so the density factor between the two is
only 123
still something
maybe still super but for sure no more
ultra if you ask me
what is then the difference between
super capacitors and batteries
as we saw capacitors store energy in a
field
that means they do not do any chemical
reaction and do not lose capacity with
the number of charging cycles
batteries on the other hand store energy
by chemically changing their materials
this is why they wear out and lose
capacity over the number of charging
cycles
let's check out how good supercaps are
for powering our small projects we do
not need 25 volts for that we only need
3.3 or maybe 5 volts
let's concentrate on 3.3 for this video
so we need two capacities in series to
get up to 5.4 volts
here i have two supercaps in series with
a total capacity of 15 farad
i charge them to 5.4 volts and discharge
them with 100 milliampere a typical load
of an esp32
for the test i use this simple
electronic load because it has software
to report the results
0.1 ampere is a very low current for
this load this is why it fluctuates a
little but the average is okay
here is the voltage curve it starts at
5.4 volts and decreases constantly
not what i was hoping for
why is that
this is the formula for capacitors
v0 in our case is 5.4 volts
c is 15 farad and i is minus 0.1 ampere
so it is clear that the voltage will
drop constantly
not very good for our purpose to say the
least
our mcu's are built to work with
constant voltages
so what to do
we would need a buck converter for this
area and a boost converter for that area
extremely complicated and my frequent
viewers know that i do not like
complicated things
so what would be a better solution
we could only use a buck converter and
do without the energy delivered below
3.3 volts
not very efficient
fortunately there is a better solution
we know from before that inside these 15
farad caps are two 30 farad 2.7 volt
supercaps
so if we connect them in parallel we
have a maximum of 2.7 volts and we only
need a boost converter
here is my setup
two 30 farad caps in parallel and this
small boost converter and here is the
voltage curve
much better
we get our more or less constant 3.3
volts and the current still is around
100 milliampere
but how long will the setup feed in
esp32
nearly 5 minutes if we trust this test
not a long time
the two supercapacitors have about the
same volume as this battery with a
capacity of 650 milliampere hours
the battery would last about five hours
not only five minutes
not good but how is it if we sleep the
esp32
to draw 10 microampere i connect a 330
kilo ohm resistor at the boost
converter's output and measure the
voltage across the supercap
as we see it keeps its voltage much
longer
very good if we keep in mind that the
supercapacitors have an internal leakage
current and the boost converter also has
some losses
so we see why supercapacitors are used
in computers as backup batteries for
memory chips
of course we could add capacity we get
supercaps up to 3000 farad and we can
buy them bundled in larger packs
but this gets quickly big and expensive
and because we must not overcharge them
we have to add balancing and protection
hardware if we connect them in series
fortunately for our setup with a boost
converter we can connect them in
parallel and therefore do not need to
care for the balancing we just have to
protect them against over voltage
but even if we use a 3 000 farad
capacitor we get 100 times 5 minutes or
around 8 hours of continuous operation
similar to this small battery
these tests clearly show that batteries
are much better for standard projects
however for sensors with long periods of
deep sleep supercaps could be an option
if we could reduce the current 100 times
to 1 milliampere average we could get 8
hours from these 30 farad caps
if we combine it with a solar cell
we could nearly survive the night
with a 3 000 farad capacitor and a solar
panel we probably could survive the
whole winter if it is not up north where
we hardly get any sun during a few
months
a typical system looks like that a solar
panel a buck converted to 2.7 volts a
super capacitor a boost converter to 3.3
or 5 volts and the mcu board with
sensors
not simple but also not much more effort
than with a solar charge battery
but one question remains why is this
hype around supercapacitors
did i make a mistake in my measurements
or calculations
i hope not but anyway super capacitors
are not made to replace batteries they
are made for different purposes because
they can be used for an infinite number
of charging cycles they are ideal for
short-term storage
for example to store energy created from
de-accelerating a car
this energy can be used a few seconds
later without bothering the battery
so the battery will last much longer or
in this bus which is charged at every
station like that the stored capacity
can be low but the number of charging
cycles is high
supercaps are usually also built to
support very high charging and
discharging currents
even small capacitors can create large
currents for a short time
most of those advantages are not helpful
for our small projects where we need
energy for a long time
even the ultra low power in ocean
sensors use tiny batteries as we saw in
video number 406.
however there is one use case for small
projects harsh environmental conditions
lithium batteries should not be charged
below zero degrees centigrade
so if your device has to be charged at
such low temperatures you might consider
using supercaps
but as we saw you probably need space
and money
at least if your device is not extremely
low power
so we found that all capacitors store
energy in electrostatic fields they keep
their chemical structure all the time
supercapacitors can store much more
energy than ordinary capacitors or even
electrolytic capacitors
most super capacitors can only be used
up to 2.7 volts if we need higher
voltages we have to connect them in
series unfortunately the capacity in
this configuration is reduced but they
can store much less energy than
batteries which change their chemical
structure to store energy
capacitors have a less than ideal
discharge curve
discharged with a constant current they
drop the voltage linearly
utterly different to batteries which
have a much more stable voltage across a
long time
we can solve this issue if we use a low
voltage capacitor and a boost converter
tests show that the capacity of small
super capacitors are too low for an
always on esp-32 or arduino board
so super capacitors are not a
replacement for batteries
they are good in scenarios with high
currents and or heavy charging or
discharging
because they can also be charged at low
temperatures
they could be used in harsh environments
and with solar panels
will the capacity gap between
supercapacitors and batteries become
smaller
i do not think so chemical reactions
have more potential to store energy than
electrical fields
so the chance that the game-changing
material will be discovered is small
increasing the maximum voltage would
help a lot of course
that was all for today as always you
find the relevant links in the
description
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