How do Laptop Batteries maintain a constant voltage throughout their usage period?

[sad_guy]

well if you have a flashlight with batteries, as you go on using it, the EMF of the batteries goes down and the light becomes dimmer, right?

Going by this logic, as we use our laptops on battery power, our screen should become dimmer and dimmer as the battery charge goes down. And the display should start flickering and we should get BSODs, as happens with our desktops when our PSU isn't enough for our components(maybe i'm wrong here, this happens when PSU lacks power instead of voltage, but whatever you guys get the idea).

Then why doesn't this happen?

 

[Cerb]

well if you have a flashlight with batteries, as you go on using it, the EMF of the batteries goes down and the light becomes dimmer, right?

No. I have several that will stay the same brightness from a fresh cell to a near dead cell (in practice, this is actually terrible behavior, very common among Chinese brands and U.S. outdoors brands that resell them). I actually paid more for my last one to not have that behavior (I may become a Malkoff addict, because of it, too...so basic, but they grow on you), instead staying in regulation for several hours, then dropping out and dimming for several more. Only unregulated flashlights will get dimmer due to the battery voltage getting lower.

Then why doesn't this happen?

http://en.wikipedia.org/wiki/Switched-mode_power_supply

The battery voltage goes down. The 12V, 5V, and 3.3V lines that it is converted to, do not.

 

[sm625]

There is a power supply circuit that takes the battery voltage (13V, 15V, whatever it is) and it converts it into all the voltages the motherboard needs. So the power supply has some margin to work with, and it can measure that margin and that is how it knows how dead your battery is. When the margin is gone, it says you're at 0% and your notebook will shut down. But even at 0% there is still plenty of juice left.

Even when an electric shaver or a flashlight dies there is still juice left in the battery, its just that the voltage is too low to drive the motor or overcome the diode junction threshold.

 

[SecurityTheatre]

Switching power supply is one answer.

It's also worth pointing out that Alkaline batteries (standard non-rechargable type) have a very linear pattern to their discharge voltage. They gradually decrease from about 1.6v to 1v over their life.

Batteries with Lithium Ion or Lithium Polymer cells are much better at holding their voltage steady for much of their life, but then the voltages collapses quickly when it is nearly discharged.

 

[mindless1]

Even when a Li-Ion cell powered electric shaver or a flashlight dies there is still juice left in the battery, its just that the voltage is too low to drive the motor or overcome the diode junction threshold.

Fixed. There are NiCd and NiMH shavers and incan flashlights that totally drain the battery down to 0, without any diode present and the typical brushed motor or incan bulb will completely drain a battery even if it has insufficient current to spin or light up, but any Li-Ion powered product has to have a low voltage cutoff either on the battery or otherwise in device itself to be safe for consumers.

 

[Cogman]

Answer: it doesn't. Laptop batteries drop their voltage like everything else.

How they manage to do it is from something like a voltage regulator. So long as the voltage on the battery is high enough, the output voltage of the regulator will stay (fairly) constant. This is part of the reason why most laptop batteries run around 18V while most motherboards have a maximum voltage requirement of 12V.