How laptop battery life is computed?

For some time now, claims of more than 10 hours of battery life on netbooks are often times unbelievable and not really achievable in real world tests. So, where do these numbers come from? We look at one possible computation.

Laptops and netbooks consume a certain amount of power depending on which peripherals are turned-on or active. The CPU, graphics, hard drive and LED display constitute the biggest share of that consumption.

On the other end, the battery pack supplies the power and the capacity is measured in watt-hour (Wh) or more popularly as miliamp-hour (mAh). For our purposes, we’ll use the mAh which you can see in the sticker in your laptop’s battery (to convert Wh to mAh, just follow this formula — mAh = Wh / V * 1000).

Depending on the settings and usage of the laptop, the discharge rate varies. What I did was set several condition on the laptop and check the discharge rate. I used BatteryBar to monitor the discharge rate.

Notebook is idle, WiFi is On and LED brightness at lowest setting.

Notebook is active, WiFi is On, iTunes playing and LED brightness at highest setting.

You will notice the discharge rate increases from -7,986mW on idle to -13,068mW on audio playback and -14,498mW on video playback.

The 6-cell battery of the laptop I used is 4,400mAh (47,300mWh at 10.8V rating). Converted, the discharge rates are -739 mAh, -1,210 mAh and -1,342 mAh.

So, on idle the maximum battery life of the laptop is 4,400/739 or close to 6 hours while on audio playback it’s 3.6 hours and 3.25 hours on video playback.

Using the same formula, is a netbook manufacturer alleges 12 hours on a single charge, we look at the battery capacity and figure out how low the discharge rate can go. For example, Samsung promises 12 hours on its N220 with a 6-cell 5,900mAh battery. To reach 12 hours, the discharge rate should be -492 milliamps.

You should try this on your laptop or netbook too and share with us here how low you can get the discharge rate.