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Just out of curiosity, I wanted to know why does battery indicator is not linearly related with the amount of time i.e. If we see the screenshot which was taken at 4.55 PM , it said battery percentage was 27% or 47 min remaining however, at 5.00 PM the battery percentage was showing 25% but the time increased to 1 hr 3 mins. I don't understand this behavior ? I was not using any program at both instances of time and my system idle process was 99%. Ideally if battery capacity decreased to 25% the time should also been reduced.Why windows behave in this way? or am i running with a poor battery? (which i doubt as i have just got new 6 months back).

OS- windows 7 - 64 bit

Model - Dell inspiron

Thanks very much for answers in advance.

Attached is screenshot for everyone's reference.

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enter image description here

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The time remaining is an estimate based on past usage rates. Maybe the first estimate was taken while a background task was doing something. Maybe your screen auto-dimmed. Maybe your network connection went idle. Maybe the system bus turned off an inactive USB device. There are too many variables to know. The battery itself may also have firmware that poorly estimates the amount of battery left (they usually do this by detecting minute decreases in the voltage output; not enough to damage the device, but enough to detect a decrease). Temperature can also affect (dis)charge rates. –  allquixotic Mar 18 at 14:05

2 Answers 2

up vote 3 down vote accepted

The short answer: predicting the future is hard.

The long answer:

Batteries vary in their sensor capabilities. Some are more accurate in terms of remaining power, and some give a general idea of remaining power based on power output decrease.

Here is what Microsoft has to say about it.

Note that as your activity changes, the estimate of time remaining will change as well:

What you use the laptop for. Because some activities drain the battery faster than others (for example, watching a DVD consumes more power than reading and writing e‑mail), alternating between activities that have significantly different power requirements changes the rate at which your laptop uses battery power. This can vary the estimate of how much battery charge remains.

The specific algorithm used to determine how much time remains is addressed in this SO question: http://stackoverflow.com/questions/21612032/how-does-windows-calculate-remaining-time-on-battery

Windows follows the ACPI specification, and given the specification gives a method of calculating remaining battery time, I'd assume this would be how they'd do it.

Edit: Found a somewhat confirming source.

I'm referring specifically to chapter 3.9.3 "Battery Gas Gauge".

Remaining Battery Percentage[%] = Battery Remaining Capacity [mAh/mWh] / Last Full Charged Capacity [mAh/mWh]* 100

if you need that in hours:

Remaining Battery Life [h]= Battery Remaining Capacity [mAh/mWh] / Battery Present Drain Rate [mA/mW]

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The information provided tries to give the user clues based on the data it collects. It observes either the current consumption, or the voltage to base the assumed capacity left of the battery. It calculates the ammount of time that it would last with the consumption of that power it observes.

Because the way the operating system works it does quite a bit of work as it wakes up, superfetching, cacheing stuff, and even other time based things going on. You do not have to change your activity for the system to change its activity and therin consumption by quite a bit.

It looks like your battery here is doing ok.
It is just the simplicity of the alogrythms (on not just your device) To be able to determine the actual timing. It does not put into the scenario its own variation in usage. Add in the unpredictibility of the the human to change the use of the computer in an instant, and they are always bad guesses.

Add the human brain back into the mix, and you could still use the guaging, and maybey some monitoring of the cpu & gpu and screen light and all, to guess better yourself.

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