🔋 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!

100 thoughts on “🔋 Battery amp-hour, watt-hour and C rating tutorial

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  2. So my question is this and I used to know the equation. In real life I have a battery operated WIFI camera it uses 4AA they last about 2weeks. Using good quality Akaline batteries.

    That is 6 volts in school we were told this is "pressure" so to speak.

    The batteries list 2200 mAh so this is capacity of electricity 2.2 amps of "current" will drain them in one hour.

    I want to remove the AA and wire up a 2 cell lipo 20c. 7-8volts. 3000 mAh.

    So do I figure the Watt hour to determine how long the lipo will last apposed to the AA.

    I have completely forgot how to figure the "time" it will last.

    Or would ysing a 3cell 12v lipo be the answer to look get use?

    can't believe I have forgot a lot of this. Back in 1994 when I earned my degree in applied science I could figure it my head.

  3. Hey i have done it practically according to you,
    i have 3 AAA 1.2V and 1100mAh batteries connected in series , i have connected LED across batteries in series with a 100ohm resister. what i can measure is current drawn by that LED is 40mA, so now to calculate the hours = 1100mAh/40mA== 27.5 hours.

    if i apply same thing for wAh, that is Total Wah=(1.2+1.2+1.2)*1100==3960mwh,
    and the voltage drop across LED is 2V and Current drawn is 40mA, so Total Watts=2*40=80mW.
    but if i want to calculate hours, i.e== 3960mWh/80mW==49.5 hours.

    In Both case Result is different, i.e one is 27.5 hours and 49.5 hours.
    which one we should Consider??

    please Replay

  4. If I have a 3.6 v 5000mAh battery and it has a discharge rate of 3Amps does that mean I get 3amps or 5000 mAh. I guess what I’m asking is I don’t want to go over my rated amps on my motor and I am curious to know how reliable these ratings are and what one I should use for my calculations. Thanks.

  5. Your videos are awesome. Could you please make a video explaining what it means for a electrical component to draw too much current?

  6. i have been looking at tons of batteries on the web….6v,12, assorted amps, etc. I have even looked at step-up dc/dc coverters….12>24 and 24>48…….i am looking to power a 48v 1000watt brushless,gearless ebike motor hub…..those lithium battery packs cost more than my trike and ebike conversion kit COMBINED…..so i have been exploring the SLA battery configs…… thankyou for this video….i dont have an apple to put on your desk but is it OK if i go to the playground a little early ?

  7. About the C rating, if the battery is 2.2Ah and its C rating is 44A, how long (time) can it supply at 44A?

  8. I have a question. If you would use two of those 20C batteries, would that mean you could provide safely the same current as that one 40C battery? It doesn't seem to me like you would. But I wanna be sure 😀

  9. took YouTube forever but it finally recommended me something useful, this is probably the best explaination of Ah, A, Wh and C I've ever come across. Definitely looking at batteries in a much more reasonable way now.

  10. this is a great video. thank you. I have a question. I have a cordless vacuum, its rechargeable battery pack needs to be replaced, however, it has been discontinued. It is 4.8v 1.3Ah. I wonder I can simply get 4 AA rechargeable batteries bundled up to replace this old one? thank you very much

  11. What is the c rating of a 18650 battery? Where can you find it on the battery? For example on the Samsung INR18650-25R 2500mAh battery

  12. Thanks for the valuable information you provided.
    Can you explain what does​ the "initial current less than 60A" written on the side of a Gel 200Ah battery mean?

  13. I getting into rc builds with my son and O'boy have we burned up batteries. I gotta watch this again 4 or 5 more time and take note…..again

  14. Afrotechmods, your videos are really cool, thanks for making them. Can you help me with advice on which ones to watch so that I might understand how to power my 50 Watt RV fridge and 62 Watt CPAP (breathing machine) through the night? I am trying to build a system that has the battery capacity to power those and then use Renogy solar panels to recharge it during the day.

  15. Love the video. Would like to know more about charging a SLA battery with solar panels and a charge control through an ammeter. How low should the ammeter show as the amperage drops when the batt. is charging ?When is a battery actually full? I am now subscribed to ur channel.

  16. 0:20 It's well established in electron theory that current (amps) travel from negative to positive, not positive to negative.

  17. You talked about impedance in batteries. Is that even possible? Impedance, as far as I know, is only valid for AC circuits; batteries create DC circuits.

  18. hello sir…
    i want i ask you a question

    suppose…

    can i change a wireless headphone battery. if original battery Voltage: 3.7V
    and Capacity: 180mAH and new battery voltage 3.7 and capacity 2500 to 3500 mAH

  19. As a neophyte battery pack builder, this was very helpful. Thanks! So, if the "C rating' is a marketing thing, should it be ignored?

  20. If you're confused, you should be because this has not been explained adequately. Particularly in relation to C ratings. And not a mention of batter packs construction relating to series and parallel connection and how this relates to Amps Mah and C.

  21. Hey good video…
    But ..
    I watched to get some insight into a battery I have….
    12v 46w 20 minutes.

    So what's that in Ah?

  22. Brother 1k ohm resister how many voltage i can use it??? Or have a any rule for using resister in how many voltage??? Please answer me

  23. + manufacturer bullshit info just to boost the sells. -10-20% from the manufacturers given amp hour and we are closer to reality.

  24. Why do they advertise battery capacity in mAh instead of mWh? Watt-hours is more general. And why do they use a rating in thousands of milli-units? That's just extra tongue-wagging. And instead of a "C" rating, why don't they just tell you how many Amps it can safely deliver?

  25. Great video.
    Looking at battery information is very confusing.
    As a thought experiment: Say I wanted to create a electric vehicle with the smallest possible battery bank.
    And charge the battery bank using a gas turbine generator.
    What should I be looking for in a battery bank. High volts with less amph?

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