How does a battery continue to support the exact same functionality as the charge decreases over time? As in with a phone battery, how does the screen not get slowly dimmer, sounds from the speakers quieter?


Kind of a dumb question but would be interested to know the mechanism. Like does it somehow transform the power to maintain the same amount of voltage as the amperage decreases or some similar technique? It seems like a universal feature of technology 🤷🏼‍♂️

In: Technology

This isn’t a complete answer, but here’s the general idea: voltage decrease as the battery loses charge is fairly miniscule until the last bit of the batteries life. So with phones especially, they can monitor the voltage output and calculate how much “effective” battery is left. So when you’re phone shuts down, it’s because if it stayed on much longer you would see the effects you described. I’m sure battery types and a few other factors go into it, but that’s my general knowledge of it.

lithium batteries do this, old power tools used Ni-cd would lose their power as the battery would lose its charge. now tools keep going until the battery is almost completely depleted.

>does it somehow transform the power to maintain the same amount of voltage as the amperage decreases

Well, that’s mostly correct. The unregulated battery voltage can be from 2.8 to 4.2 V. It is converted up or down to regulated voltages by boost/buck converters. They work by switching the battery current at 100s of kilohertz into inductors which store the energy and release it as needed for a steady voltage. The bit which was wrong is that as the battery voltage drops, the battery current (amperage) actually goes up (assuming the same power demand).

A voltage regulator is what helps set a constant output voltage to a circuit given variable input voltage (batteries).

The cleanest method is with a linear voltage regulator that takes a higher voltage and produces a fixed lower voltage. Clean electricity is necessary for precision circuits like the CPU in your computer. Even if the higher voltage starts to decrease, the output will remain constant up till the input drops too much. The downside of linear regulators is that they drain power to run so they reduce the overall usage capacity.

Other voltage regulators like switching regulators can take a lower voltage to produce a constant voltage than can be higher than the input. These are used in the case of AA batteries where alkaline vs lithium batteries have big differences in their nominal voltage (1.5 V vs. 1.2 V respectively). the regulayors can draw power even if the battery voltage drops a great deal. The overall circuit may have a fixed cutoff to prevent draining the battery too much. The downside of the switching regulators is that the act of boosting the voltage adds electrical noise to the circuit. They cannot be used for sensitive applications.

Let instead of a phone talk about a power bank with USB out that have a voltage of 5V and can provide 1A of current that uses the same type of batteries. The voltage of a new Li-Ion cell have a voltage around 4.2V when it is full and around 3 when it is discharged and you usual say that is have a nominal voltage of 3.7V. You can look how the voltage drop when it is discharge [Li-ion discharge curve](

So the battery in the device always have a huge voltage range even when new. So for simple electronics that operate on regular AA batteries(other battery type but the the voltage also drop) like flashlights the brightness will drop when it is used.

For more complex stuff you have circuit that can convert the voltage to what is needed often a [Boost_converter]( if it should be higher. So in a power bank it will increase the voltage from lest say 3.7V to 5.0V so 1.35x the voltage. The current will drop so the energy is the same. If the converter is 100% efficient (real are not that perhaps 90%) and give out 1A the input current is 1.35 times that. So you have 3.7V and 1.35A 3.7V*1.35A= 5W in and out 5V 1A 5V*1A=5W. So energy is constant but current and voltage changes.

So you convert the battery voltage to what you need but the energy is the same.

That is what happen in a cellphone and it convert the voltage to the appropriate. When the voltage to low in the cell of if a old or a cold battery cant provide enough current what will happen is that the cellphone turn itself off. So as long as the phone is on it will work the same

Lit ion batteries can provide a lot of current so that is usually not a problem. It is not uncommon that you can draw 10A from them and a 2500 mAh phone battery would last of 15 minutes at that load. So new batteries will work find until discharged but all or cold batteris might not provide enough power and the phone will turn itself off.

It is the integrated circuit in the cellphone that need a stable voltage like the chipset and you often have multiple voltage that often is lower then the battery voltage, But you can also increase the voltage if it is needed.

Power tools that uses the same batteries connect them direct to the motor and in them you will notice that they have less power when the battery runs out.