Wednesday, October 28, 2015

FAQ-1 Battery Capacity mAh, Ah, Wh

Good evening everyone, although it is about 3 a.m. but whatever I will be starting a new segment where I will try to answer questions that are often asked. So the first one is a bit about battery capacity. Many people ask me about it and I see many people ask someone else about it so I thought why not just try and answer it simply! So here it goes!


Let's talk about mAh first because you will see this unit very often. Your mobile phone's battery, laptop's battery or portable media player's battery might have this rating printed on it. So what does that mean? Well mAh has two parts one is the mA or Milli-ampere and the second part is hour so in total it means how much current can the battery provide for an hour. For example if you have a cell that has a rating of 1000mAh it means if you draw 1000mA from it it will last for an hour. Theoretically it also suggests that if you draw more, say 2000mA from it, it will just give you 30 minutes and if you draw less, say 500mA it will give you 2 hours. So, I said theoretically why? Because battery has internal resistance which can be realized as a resistor in series with a power source, so more current you draw more power will be wasted on the resistor as we know that power dissipated on the resistor is equal to the value of resistance multiplied by the square of the current that is flowing through it. So more current, more waste and that's the reason why batteries heat up with higher current draw. There is one more thing, as I said earlier a 1000mAh cell can deliver 1000mA for one hour, does that mean it can provide 60000mA/60A for a minute? No, not really. Why? Because as I said there is an internal resistance that can be assumed as a series resistance with the source, so it will limit the current and each battery chemistry and construction has it's limit so 1000mAh does not indicate it can provide 60000mA/60A for even a minute.


Next stop Ah, there is not much to say about it really because it is almost the same as mAh which you might have guessed. The reason for using this unit is sometimes for powerful batteries if you need to express it in mAh, the numerical digit might become huge for example if you think about a car battery it is much easier to write 100Ah rather than writing 100000mAh. So to make life a little easier this unit is used with batteries that has higher capacity. Even for slightly smaller cells too like a normal 1200VA computer UPS usually comes with two 7Ah battery.


So far I have talked about mAh and Ah which are used all over the world for understanding battery capacity but there is one more unit, Wh or Watt hour. Why do we need another one? Well let's break it down, this unit has two parts like the other two, one the Watt or unit of power and other being the hour or unit of time. So Wh will simply tell you how much power you can get out of a battery for one hour but why do we need that? Because the mAh/Ah doesn't give you any indication of power. Let's think about it, how much power can a 1000mAh battery provide? Sadly we can't answer that question because power is the product of voltage and current and in this case we don't know the voltage so we can not answer this question. Another problem is if someone ask you that are all 1000mAh battery the same? Again we don't know the Wh so we can not tell. Let's look at an example, say a NiMH cell has a capacity of 2000mAh and a lithium ion with the same capacity, 2000mAh, are they same? No because the lithium ion can deliver more power than the NiMH because lithium ion has a nominal voltage of 3.7V where the NiMH has 1.2V so the Lithium ion will have 7.4Wh as opposed to the 2.4Wh of the NiMH. Let's look at an interesting thing here, even if I assume the lithium ion to be of 1000mAh capacity, it will still have 3.7Wh rating where even with 2000mAh rating the NiMH has only 2.4Wh rating. So recently many devices comes with Wh rating too, like the Lenovo laptop that I'm using right now has a 54Wh battery.

Like mAh and Ah there is a smaller unit of Wh too, which is mWh essentially the similar thing and I guess you know the simple math already, 1000mWh is equal to 1Wh.

If you don't have the information about voltage say you only know about the mAh rating and type of the battery, can you determine the Wh rating? Yes in many cases you can. Let's look at the chart to understand this better.

The table itself is self explanatory I think, so if you know the battery type and mAh/Ah rating you can easily find the Wh rating using this table. There are many other types of batteries out there but these are the most common ones so to keep things simple I have only mentioned them.

If you want to learn more about this I would highly suggest you to check out the following links below which might help you clear a lot of confusions about this.

Wikipedia Link:

1. Ampere Hour
2. kilowatt Hour
3. Battery type Comparison

Quora Link:

1. Meaning of mAh

Youtube Link;

1. Ah and Wh rating.

If you are interested in more of my posts take a look at my other posts here.

About me and Index of my blog.

Hope You have enjoyed the post. Have a good day.