| ampere-hour | |
|---|---|
 Rechargeable batteries Top: AA battery (2500 mA⋅h) Bottom: AAA battery (1000 mA⋅h) | |
| General information | |
| Unit system | Non-SI metric unit |
| Unit of | Electric charge |
| Symbol | A⋅h, A h |
| Conversions | |
| 1 A⋅h in ... | ... is equal to ... |
| SI units | 3600 C |
An ampere-hour or amp-hour (symbol: A⋅h or A h; often simplified as Ah) is a unit of electric charge, having dimensions of electric current multiplied by time, equal to the charge transferred by a steady current of one ampere flowing for one hour, or 3,600 coulombs.[1][2]
The commonly seen milliampere-hour (symbol: mA⋅h, mA h, often simplified as mAh) is one-thousandth of an ampere-hour (3.6 coulombs).
YouTube Encyclopedic
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Battery amp-hour, watt-hour and C rating tutorial
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Transcription
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!
Use
The ampere-hour is frequently used in measurements of electrochemical systems such as electroplating and for battery capacity where the commonly known nominal voltage is dropped.
A milliampere second (mA⋅s) is a unit of measurement used in X-ray imaging, diagnostic imaging, and radiation therapy. It is equivalent to a millicoulomb. This quantity is proportional to the total X-ray energy produced by a given X-ray tube operated at a particular voltage.[3] The same total dose can be delivered in different time periods depending on the X-ray tube current.
To help express energy, computation over charge values in ampere-hour requires precise data of voltage: in a battery system, for example, accurate calculation of the energy delivered requires integration of the power delivered (product of instantaneous voltage and instantaneous current) over the discharge interval.[4] Generally, the battery voltage varies during discharge; an average value or nominal value may be used to approximate the integration of power.[5]
When comparing the energy capacities of battery-based products that might have different internal cell chemistries or cell configurations, a simple ampere-hour rating is often insufficient. For example, at 3.2 V for a LiFePO4 battery cell, the perceived energy capacity of a small UPS product that has multiple DC outputs at different voltages but is simply listed with a single ampere-hour rating, e.g., 8800 mAh, would be exaggerated by a factor of 3.75 compared to that of a sealed 12-volt lead-acid battery where the ampere-hour rating, e.g., 7 Ah, is based on the total output voltage rather than the internal cell voltage, so the 12-volt output of the example UPS product can actually deliver only about a third of the energy of the example battery, not a quarter more energy. But a direct replacement product for the example battery, in the same form factor and comparable output voltage and energy capacity but based on LiFePO
4, might also be specified as 7 Ah, here based on output voltage rather than cell chemistry. For consumers without an engineering background, these difficulties would be avoided by a specification of the watt-hour rating instead (or additionally).[citation needed]
In other units of electric charge
One ampere-hour is equal to (up to 4 significant figures):
- 3,600 coulombs
- 2.247 × 1022 elementary charges
- 0.03731 faradays
- 1.079 × 1013 statcoulombs (CGS-ESU equivalent)
- 360 abcoulombs (CGS-EMU equivalent)
Examples
- An AA size dry cell has a capacity of about 2,000 to 3,000 milliampere-hours.
- An average smartphone battery usually has between 2,500 and 4,000 milliampere-hours of electric capacity.
- Automotive car batteries vary in capacity but a large automobile propelled by an internal combustion engine would have about a 50-ampere-hour battery capacity.
- Since one ampere-hour can produce 0.336 grams of aluminium from molten aluminium chloride, producing a ton of aluminium required transfer of at least 2.98 million ampere-hours.[6]
See also
References
- ^ "electric charge (Symbol Q). IEV 113-02-10". electropedia.org. International Electrotechnical Commission (IEC). 2020. Retrieved 2020-09-20.
Note 7 to entry: The coherent SI unit of electric charge is coulomb, C. The unit ampere-hour is used for electrolytic devices, such as storage batteries: 1 A·h = 3,6 kC.
- ^ Thompson, Ambler; Taylor, Barry N. (2008). Guide for the Use of the International System of Units (SI). NIST Special Publication 811 (PDF) (2nd ed.). Gaithersburg: National Institute of Standards and Technology. p. 45.
To convert from ampere hour (A·h) ... to coulomb (C) ... Multiply by 3.6 E+03
- ^ X-ray Safety Handbook, 9.0 Terms and Definitions, VirginiaTech Environmental, Health and Safety Services Archived July 23, 2007, at the Wayback Machine
- ^ Efty Abir, Najrul Islam (2016). "How to Calculate Amp Hours – Learn of Convert Watts to Amps". Leo Evans. Retrieved 8 December 2016.
- ^ National Research Council (U.S.) (2004). Meeting the energy needs of future warriors. National Academies Press. p. 27. ISBN 0-309-09261-2.
- ^ T. L. Brown, H. E. Lemay Jr, "Chemistry the Central Science", Prentice-Hall, 1977 ISBN 0-13-128769-9 page 562
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This page was last edited on 23 January 2024, at 04:07