Laptop Battery Math not adding up.

G

Guest

Guest
Before I start, sorry if this I posted this in the wrong place, I couldn't find any "Battery" catagories.

My school laptop (Early 2014 macbook air) has recently started running out of battery faster than normal. Beause of this im trying to learn about batteries, so I can get a deeper understanding of the cause of the problem.

I ran a few tests, trying to limit my computers use of hardware,
eg. CPU maxed at 2% usage, low strenght backlights, no network signals.

The average time it took my laptop for a "complete" discharge was around 6.6 hours, and the average dischare rate was around -4.780W.

My battery has a full charge capacity (not designed capacity) of 44.750 Wh.

This is where im getting confused...

Wouldn't it make sense that my laptop at a average discharge rate of -4.780W should last around 9 hours?
What is causing this almost 40% difference ?
Would it come from heat?
Throughout the test the laptop barley went above 35C , so im not sure.

Any and all help is appreciated!

 
Solution
G
I've figured out the problem.
Ill post it here in case anyone else is wondering the same.

As batteries age, there impedence grows.

There impedance is a basically a measure of resistance and capacitance.

This article does a great job of outlining all the variables that effect battery resistance: http://www.powerstream.com/internal-resistance.htm

Im my case the battery internal resistance was so high that to 40% of the energy drawn from the battery was used in in the process of extracting the other 60%.
This 40% of all power is then turned into heat.

If I was seeing no internal resistance my battery would infact run for 9 hours.

If anyone in the future is confused just PM me, Ill try to explain in more depth.


G

Guest

Guest
I've figured out the problem.
Ill post it here in case anyone else is wondering the same.

As batteries age, there impedence grows.

There impedance is a basically a measure of resistance and capacitance.

This article does a great job of outlining all the variables that effect battery resistance: http://www.powerstream.com/internal-resistance.htm

Im my case the battery internal resistance was so high that to 40% of the energy drawn from the battery was used in in the process of extracting the other 60%.
This 40% of all power is then turned into heat.

If I was seeing no internal resistance my battery would infact run for 9 hours.

If anyone in the future is confused just PM me, Ill try to explain in more depth.


 
Solution
I thought you have taken that into account. Rechargeable batteries have always been like that. In some it was very pronounced (led-acid, NiCads etc) in newer ones much less so but still perceptible. Those are chemical processes and some loss of chemicals is to be expected.
Some are working on capacitive solutions that do not involve chemical interaction just because of problems like that.