Battery life worse at end of battery?

[adamman41]

Does anyone else find that their battery does best at like 60%-70% charged and when it gets below 40 it moves a lot quicker? Also I've done 3 full drain and full recharges so far and my battery has improved sloightly each time, do you reccomend I do it again or is that just bad for the battery?

 

[bthoward]

That is fairly common... The device requires a certain amount of "power". Power is measured in Watts. The power a device uses is represented by the equation P = I * V where I is the current drawn and V is the voltage.

So lets say that the battery is running at 3.5V (its a good round number and its within the range that this battery can run). Lets say that the device requires 2 Watts (this power requirement can vary widely depending on what you're doing with the phone).

So you know that its 2Watts = I * 3.5V Thus I = about .57 or just over a half of an amp. Now as the battery loses charge the voltage sags. Lets say that the voltage sags to 3.0V. Now the current has jumped up to about .67Amps. This is a 100mA (or .1Amp) increase being drawn from the battery.

The Droid battery is rated at 1400mAh (milliamp hours) also known as 1.4Ah (amp hours). This means that the battery can provide 1.4 amps of current for 1 hour before it dies.

(now we get to see that 2Watts was probably a high guestimate from before). Because if the battery is having .57A drawn from it continuously you're going to get about 2.5 hours of battery life. If you were to draw .67 amps from the battery continuously you'd only get 2 hours of battery life. So now that we've calculated it in two simple points with simple algebra can you now somewhat see what is happening?

The power remains constant and as the battery dies the voltage goes down. Thus when the voltage goes down the current goes up. The higher the current the faster the battery drains. In order to really do this you either need to take a lot more measurements and do some averaging or just dip back into calculus a touch. (but not too deep at all)....

Anyway I'm certain more than you wanted to know but hopefully it was at least interesting... If you want to learn more this is one of the most basic of electrical engineering concepts and is related to "Ohm's Law" (a good search term for learning more).

 

[estobaughn]

true dat

 

[kurtbacker]

I've gotten 14 hours out of a single charge on my Droid.

 

[esocid]

I've gotten 14 hours out of a single charge on my Droid.

That is completely irrelevant. What's the rate of discharge? Is it constant from start to finish?

I've also noticed that after about 30% it drains a little quicker than when it's full. Not sure if it's something I've changed, but since 2.01 I've noticed it. I also stopped using a task manager.