🔋 Battery amp-hour, watt-hour and C rating tutorial

Welcome to my battery capacity tutorial.
I’m going to talk about amps, amp-hours, watt-hours, C 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 definitely supply one amp. 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 2Ah battery is capable of
delivering over 4 amps, but with the motor I have connected to
it, it’s only delivering 2mA. 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 about amps or amperes
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
AA battery. The capacity is 2000mAh, or 2Ah (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 one amp for two 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 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 120 amps for one minute? Let’s try! Hmmm it seems the battery is
only able to supply 9 amps. And its heating up a lot. Let’s Google
the data sheet of the battery and see what the limitation is. Take a look at this. This battery has an
internal impedance of 25 milliohms. 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 battery’s chemical reaction and electrodes. This internal impedance limits the
amount of current that the battery can deliver and from electronics perspective
it effectively becomes the source of heat when the battery is delivering current. This explains why very few batteries can
actually deliver 120 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 400mA to 4A.
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 400mA. 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. Okay 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 two amps for one hour with a voltage of around 1.2 volts. The
9.6 volt battery pack will also theoretically deliver two amps for one
hour but with a voltage around 9.6 volts. So one way we can compare the stored
energy of these two batteries is to use another unit called watt-hours. Volts x amps=watts. So you
can probably guess that volts x amp-hours=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 11.1 volts and a capacity of 2200mAh. They look the same but one of them has a
20C rating and the other is rated at 40C. 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 use C ratings instead.
The “C” refers to the battery’s capacity in amp-hours. So this 20C battery can deliver 20xC, or 20 x 2.2Ah, so this
battery can safely deliver up to 44 amps. And this 40C 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 amp-hour capacity by the
C rating and then you pretend the result is in 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 video description! Thank you for watching and check out the
video description section for a link to see how you can support the channel. I’ve got some new merchandise like
t-shirts and mugs that you can buy!