Understanding Laptop Battery Power Ratings

Laptop battery power rating

Your laptop battery power rating can be found  labelled on the battery. A typical laptop battery rating is as stated below;

Laptop battery

11.1V, 5.2Ah, 57Wh.

This means that the battery supplies 11.1 volts, a current of 5.2 Ampere hours and a power of 57 Watts hour.

What does this information really tell? 5.2Ah means the battery can supply a current of 5.2A in one hour before it runs out . Or 57 Watts in one hour before it depletes provided the voltage supplied remains constant.

So let’s say your laptop power rating is 30Watts then on a battery capacity of 57Wh, your laptop will run for( 57Wh÷30W)= 1.9h or almost 2 hours. In reality it may be more or less as a result of several factors.

Both Watts hour( Wh) and Ampere hour( Ah) are measures of battery capacity. However, Wh is more reliable.

A laptop battery is usually made up of small battery cells. These battery cells are similar in shape to AA alkaline batteries but bigger..

6 cell laptop battery

 Laptop battery usually consists of at least 3 or 4 battery cells arranged in series. To increase the laptop battery capacity, 2 or more sets of 3 or 4 battery cells in series are arranged in parallel.

The smallest laptop batteries I have come across are the 3 cell batteries. That is, consisting of 3 battery cells in series. These types of batteries are usually made for netbooks.

The standard laptop battery is made up of 6 cells. It consists of 2 sets of 3 battery cells in series, connected in parallel. 

Similarly, a 9 cell laptop battery consists of 3 sets of 3 battery cells in series, connected in parallel. Of course there are laptop batteries. comprising sets of 4 battery cells in series, connected in parallel. 

Laptop battery cells  are connected in series and then in parallel to fulfill 2 purposes;

  1. To meet the voltage rating required for the laptop to operate. This condition is met when the battery cells are connected in series.
  2. To increase the time the laptop can run on batteries. This condition is met when sets of battery cells in series are connected in parallel.

Decoding laptop battery power ratings

Laptop battery cell

A single battery cell can supply 3.6 or 3.7 volts and a current of over 2Ah. Since in electrical terms, power = current × volts, a battery cell can supply ( 3.6V× 2Ah = 7.2Wh) a power of over 7.2 Wh.

Battery power ratings for your laptop are labelled on the battery. 

For example assuming the battery power ratings are;

11.1V, 5.2Ah and 57Wh.

11.1V is the result of three, 3.7V battery cells in series. That is (3.7 + 3.7 +3.7)V= 11.1V. If the battery rating was 10.8V, then it implies that three 3.6V battery are connected in series, that is (3.6+3.6+3.6)V= 10.8.

5.2Ah is the result of two sets of three 3.7V batteries in series, connected in parallel. It means each of the 3.7V battery supplies a current of 2.6Ah. Even though 3cells are in series, the same 2.6Ah flows through the series connection. But because 2 sets of 3 battery cells in series are connected together in parallel, then the current supplied becomes 2× 2.6Ah = 5.2Ah.

From the calculations given above, you will realise that the laptop battery comprises 6 cells. Since power = current × volts, then each battery cell produces  ( 2.6Ah×3.7V) = 9.62Wh ( when fully charged)

57Wh is the result of multiplying the power produced by one battery cell by 6.( since there are six cells in the laptop battery).

I.e. 9.62×6= 57.72Wh rounded down to 57Wh by the manufacturers.