What is it about electricity which causes this? Why can’t a device draw full power until the battery is dead?
In: Physics
The battery is like a water reservoir. You let the water flow out to do work for you. For example, the waterstream could drive a turbine and that rotation can go via a drive-shaft to the wheels of your electric car.
Turning that turbine needs a minimum of flow. If the flow isn’t enough or powerful enough, then the turbine stands still or is turned with low power only.
So before your battery runs out you are seeing this diminished flow struggling to power your device. And then it is not enough flow to do anything, and you declare the battery empty.
Because the voltage that the battery can maintain drops as its state of charge declines. And for many simple devices, as the voltage declines the capability of the device declines.
There are two separate things that may be happening depending on how complex the device is:
1. Simple devices may connect directly to the battery without any voltage regulation. A battery low on charge will decrease in voltage, especially if you try to draw a lot of power (current) from it. This means that any connected motor will run slower or a light will shine dimmer. A typical li-ion battery will be at 4.2V fully charged and at about 3.2V fully discharged. So it’s a pretty big change from full to empty.
2. Complex devices like a phone, laptop or anything with a microcontroller will have a system that deliberately slows it down in order to reduce battery usage and make it last longer.
Imagine a simple electronic device like a bottle of water with a hole at the bottom side.
When the bottle is full the water flows out at high speed, and can turn a water wheel quickly.
When the level in the bottle is lower the water comes out slower and the water wheel turns slower.
That’s exactly what happens in a simple electronic device: the battery when going empty has less ‚pressure‘ or correctly voltage to ‚push‘ the electrons out.
In a more complicated device there‘s electronic components in there, the can step up the voltage but with less electrons flowing.
In the bottle and water wheel example above: you attach gears to the water wheel to make a second wheel spin faster despite the main wheel having slowed down.
However while the second wheel does spin faster, it‘s very easy to stop by just grabbing it, which the water wheel wouldn’t be.
So something like a smartphone or a laptop has electronic components that turn the voltage of the battery that eventually stops down to a constant voltage. The device only turns off once the flow of electrons isn‘t high enough anymore.
Either way: when the battery is nearly empty, even when you step up the voltage you always reduce the flow of electrons.
Because the voltage and the current are directly linked.
At a specific ‚emptiness‘ voltage multiplied with current is constant.
So doubling the voltage halves the current.
And the phone or laptop has a minimum current it needs to light up the display and keep basic functions running.
When either the current or voltage were to drop too low the display wouldn’t be visible anymore and the cpu would crash.
I can not answer your question, but I can say that this can be built around so that they do not slow down. I worked in construction most of my life, and we started using cordless drills and cordless screw guns at some point. For these commercial grade tools, you will get full power until they just completely stop. You may get like a 10-second slow-down feeling, but that is it. It doesn’t apply to all the brands, but most of the good brands I would get full power until shutdown. I am guessing that they have some kind of circuit that simply shuts them down at any noticeable change in charge.
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