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.
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| amp Hour and Watt Hour Explaination |
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.