How laptop battery life is computed?

Abe Olandres Editor-in-chief March 12, 2010 · 2 min read

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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.

Notebook is active, WiFi is On, VLC playing a movie (1/4 screen) 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.

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Abe Olandres

Editor-in-chief

Abe is the founder and Editor-in-Chief of YugaTech with over 20 years of experience in the technology industry. He is one of the pioneers of blogging in the country and is considered by many as the Father of Tech Blogging in the Philippines.

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39 Comments

Loree Gingerich · 15 years ago

Hi all! I recently bought a Asus notepad but the battery burned out, sucks but now I have a new 6 cell, lasts ages! If you recently bought one I recommend getting one !

jose · 16 years ago

Always have been they under deliver what they over promised. Sort of marketing gimmickry.

jv paz · 16 years ago

i’m bleeding. help!

ocean16 · 16 years ago

consumers oftenly mistake that the number of cell in the battery is the number of hours the laptop or netbook can go.

but actually, it’s divided by two. and that will be for a standby mode, or a few typing here & there.

basically it will have to depend on the usage of the user.

StrayArrow · 16 years ago

Thanks gotta try this today..

Sanctuarian06 · 16 years ago

In my friend’s case kasi, sabi niya OK lang daw.

Kasi sa tray Icon niya may power meter, It says:

When plugged in to AC ADPATOR:

CURRENT POWER SOURCE: AC POWER
TOTAL BATTERY POWER REMAINING: 94% (CHARGING)

When NOT plugged in to AC ADPATOR:

CURRENT POWER SOURCE: BATTERY POWER

So it means, ok lang talaga kahit 100% charge and it will not cause any battery over charging or battery defects?

Thae Battery by the way is Lion from HP.

1timnatupa · 16 years ago

I’ve read that lithium-based batteries, must keep the electrons in it moving occasionally. So it is not recommend to leave your laptop plugged in all the time.

Theoretically, it is ideal to use it for mobility and plug it in when you come to your destination (e.g using it at school then plugging it when you see an outlet at a coffee shop). This keeps the battery juices flowing.

If you often use a desktop and only use your laptop infrequently, it is highly recommended that you go through a cycle (charging and discharging its battery) at least once per month.

You may also want to know the recommended temperature for your laptop and its batteries. This will keep your batteries from overheating and losing the capacity.

John Ray Cabrera · 16 years ago

my battery wear(which is suprisingly 7% already for a 2month old laptop) and the BatteryCare for the number of discharge cycles.

note that laptops ideally should only have 10% of battery wear for the first year of use. if your battery wear is more than that in less than a year, then most likely you are putting too much stress on your battery.

my advice:

1) remove the battery if you’re using your unit plugged in to the power outlet

2) donâ€™t discharge it up until 20-30%, battery will get stressed if you drained it too often. battery draining should only happen every 30discharge cycles only.

3) do not expose your battery to extreme heat. heat is batteryâ€™s mortal enemy.

John Ray Cabrera · 16 years ago

i’ve been using this app along side with the BatteryCare. Battery Bar to know my battery wear(which is suprisingly 7% already for a 2month old laptop) and the BatteryCare for the number of discharge cycles.

note that laptops ideally should only have 10% of battery wear for the first year of use. if you’re battery wear is more than that in less than a year, then most likely you are putting too much stress on your battery.

my advice:

1) remove the battery if you’re using your unit plugged in to the power outlet

2) don’t discharge it up until 20-30%, battery will get stressed if you drained it too often. battery draining should only happen every 30discharge cycles only.

3) do not expose your battery to extreme heat. heat is battery’s mortal enemy.

david · 16 years ago

The battery voltage is 10.8V.

Abe Olandres Editor-in-chief · 16 years ago

@junp – same tayo ng results. Edited the milliamps to mAh sa taas.

@david – what’s the voltage of the battery?

@Sanctuarian06 – newer Li-Ion batteries have built-in protection not to over-charge.

Sanctuarian06 · 16 years ago

Just a question sir, sometimes my friend uses laptop connecting on a AC POWER or Charger, at sabi ko baka masira batt. mo? It is advisable ba na gamitin yun in that way? Hindi po ba masisira yung batt.? He uses HP COMPAQ.

david · 16 years ago

With my HP laptop (not a netbook), with power saver on and screen brightness reduced to around 40%, with light browsing and streaming audio, the discharge rate is around -29,451 mW.

Sanctuarian06 · 16 years ago

Whoa nice. :)

junp · 16 years ago

Hi Yuga,

I think the calculation should have been battery capacity (total in mAh) divided by discharge rate per hour (also expressed in mAh; see below)

Wh / V * 1000 or

mWh / V

= 4,400 mAh /(7,986 /10.8) or 5.95 (6 hours)

just my 2 cents worth,
jun

Abe Olandres Editor-in-chief · 16 years ago

@calvin – BatteryBar is that app actually. But it computes the historical data which includes fluctuations in discharge. This one assumes you’re going to be on the same state all the time.

Calvin · 16 years ago

wala bang easier app without having to manually compute?

Abe Olandres Editor-in-chief · 16 years ago

@wha – download BatteryBar and run it while clocking down your laptop (reduce brightness, turn off Bluetooth, WiFi, etc) and closing background processes.