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How to Calculate the Number of Cells in a Li-Ion Laptop Battery

You may find that most of laptop manufacturers will state how many cells in their Li-ion laptop battery, such as Dell, Sony, ACER and so on. Further more, many laptop battery reseller also say the number of cells of their Lithium-ion Laptop battery.

But, do you know to calculator the number of cells?

First of all, let’s know the inside of a Li-ion Laptop battery, it is made with outside plastic case or aluminum case, circuit boards, 18650 battery Cells. These are the main three parts of a Li-ion notebook battery.

So, what are 18650 battery cells?

It is called by its size, the diameter of it is 18.6 mm and the length is 65.2 mm, so the name is combined with the first two numbers of the diameter and length. This cell type is used in many laptop batteries, the Tesla Roadster, Tesla Model S, and LED flashlights.

Because most of the Li-ion notebook batteries are made with 18650 battery cells, and the capacity of it is 1800 mAh to 3400 mAh, this depends one what cells the battery use.

The voltage is 3.7 V, but, some battery cell manufacturer may say 3.6 V, in fact, 3.7 V and 3.6 V rechargeable is the same thing. Those are the mean/average voltages under load and it has to do with fully-charged-open-circuit-voltage. 10 years ago Li-ions were only charged up to 4.1 V and the mean/average voltage under load was 3.6 V. A few years later the manufacturers changed this to 4.2 V to increase the capacity of the batteries. Obviously this else made the mean voltage under load increase.

Even today, though it still makes sense to only charge your batteries (18650 for example) only up to 4.1 V as this drastically increases the life expectancy (number of cycles) of the batteries while only reducing the capacity by ~10%. Many hobby chargers allow doing this.

Now, we know what a Li-ion laptop battery is made of, and let’s calculate the number of cells.

As we know, series in physics makes current the same, but voltage will plug one by one, parallel makes the voltage the same, but Current will plug one by one.

For example, a 10.8 V 4400 mAh Lithium-ion laptop battery, in order to have the 10.8 V, it must take 3 battery cells, and in order to have 4400 mAh, it must have 2 cells. So, this needs 6 cells, and this is the number of the Li-Ion Laptop battery.

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