welcome to my battery capacity tutorial
I'm going to talk about amps and powers
what hours see ratings and other
important things that you need to know
when working with batteries first I'd
like to talk about a common question
people have let's say a battery like
this is capable of delivering one amp
just because a battery or any power
source is capable of delivering one amp
it doesn't mean that if you connect it
to something it will supply one app
voltage sources like batteries will only
deliver as much current as the load
needs the amount that the load draws
depends on the load it could be a low
resistance load that draws a lot of
current a high resistance load that
draws barely any current or it could be
a complicated digital device like this
microcontroller which draws a different
amount of current depending on what it's
doing this 2 amp hour battery is capable
of delivering over 4 amps but with the
motor I have connected to it it's only
delivering 2 milliamps and it can do it
for hundreds of hours if you're confused
so far keep watching the first thing
that you need to understand is that amps
and amp hours are two completely
different things
you've heard of amps or amp ere's before
that's a unit to describe how much
electrical current is flowing but what
is an amp hour an amp hour is a
completely different unit it's a measure
of capacity and it's a way to help
estimate the amount of energy that a
battery can hold for example here we
have a rechargeable double-a battery the
capacity is 2,000 milliamp hours or 2
amp hours the simple explanation of what
this means is that it can supply two
amps for one hour until the battery runs
out of energy two amps multiplied by one
hour
is 2 amp hours if we draw less current
the battery lasts longer it could
deliver 1 amp for 2 hours and if we draw
more current the battery gets drained
faster it can deliver 4 amps for half an
hour so amp hours are a simple way of
estimating battery life and in general
capacity in amp hours divided by the
load in amps gives you the battery life
in hours so does that mean that this
battery can deliver a hundred and twenty
apps for one minute
let's try hmm it seems the battery is
only able to supply 9 amps
and it's heating up a lot let's Google
the datasheet of the battery and see
what the limitation is take a look at
this this battery has an internal
impedance of 25 milli ohms so it's kind
of like there's a little resistor inside
the battery but in reality it's going to
be a limitation of the batteries
chemical reaction and electrodes this
internal impedance limits the amount of
current that the battery can deliver and
from an electronics perspective it
effectively becomes the source of heat
when the batteries delivering current
this explains why very few batteries can
actually deliver a hundred and twenty
amps and it raises the question how much
current can a battery safely deliver a
little lower in the datasheet we can see
the discharge curves of the battery
ranging from 400 milliamps to 4 amps so
it's implied that we probably shouldn't
be discharging this battery at a rate
higher than 4 amps oops
also take a look at this notice how the
effective capacity changes depending on
how fast we discharge the battery this
is only a 2 amp hour battery when we
discharge it at under 400 milliamps if
we discharge it at 4 amps the effective
capacity is only 1.7 amp hours because
now we're losing a lot more energy in
the form of internal heating and the
overall trend is that the more current
we draw the lower the output voltage
will be because we're dropping voltage
across the internal resistance of the
battery
so this 1.2 volt 2 amp hour rating is
only a guideline of what you can expect
to see under ideal conditions ok
that's amp hours now here's something to
get you thinking this is a 1.2 volt 2
amp hour battery over here we have a 9.6
volt 2 amp hour battery pack so if these
are both 2 amp hour batteries do they
both hold the same amount of energy of
course not
the 1.2 volt battery will theoretically
deliver 2 amps for 1 hour with a voltage
of around 1.2 volts the 9.6 volt battery
pack will also theoretically deliver 2
amps for 1 hour but with a voltage of
around 9 point 6 volts so one way we can
compare the stored energy of these two
batteries is to use another unit called
watt hours volts times amps equals watts
so you can probably guess that volts
times
powers equals watt-hours the single cell
has a capacity of 2.4 watt hours and the
larger battery pack has a capacity of
19.2 watt hours now it's more obvious
which battery stores more energy because
we're comparing apples to apples and
watt hours to watt hours now let's talk
about C ratings here are two batteries
that seem identical they both have a
nominal voltage of 11 point one volts
and a capacity of 2,200 milliamp hours
they look the same but one of them has a
20 C rating and the other is rated at 40
C but what is a C rating the C rating is
an informal way of describing how much
current the battery can safely deliver
if you show a battery discharge curve
like this to most people they'll have no
idea what it means and it's not very
exciting marketing material so marketers
like to use C ratings instead the C
refers to the battery's capacity in amp
hours so this 20 C battery can deliver
20 times C or 20 times 2.2 amp hours so
this battery can safely deliver up to 44
amps and this 40 C battery can safely
deliver 88 amps now are you confused
because you should be
remember that amps and amp hours are
completely different units C ratings are
confusing because they screw up the
units you multiply the a power capacity
by the C rating and then you pretend the
result is amps finally I can't make a
video about amp hours without mentioning
the amp hour a podcast for electrical
engineers check out the link in the
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