What is the actual mechanism (or circuitry, I presume) that allows a computer to stop drawing power from the charger?
In: Engineering
The battery can only be charged when the charging voltage is higher than that of the battery itself.
The charging voltage is capped, therefore the charge eventually slows down and comes to zero current when the voltage of the battery is equal of the charging voltage.
Batteries store electrical energy as chemical potential energy. When the battery discharges, a chemical reaction is taking place inside the cell that causes electricity to flow.
Charging a battery involves forcing that same chemical reaction to happen in reverse. Once all of the compounds available to be reacted have done so, the battery is full. Any extra energy will do something other than charge the battery, so the cell might start to heat up, the terminal voltage might rise rapidly, electrolyte might disassociate, or any number of other bad things.
In the old days, you could just apply a fixed voltage to a lead-acid battery and it would gradually stop charging. This wasn’t the optimal way to manage the charging process even for a simple battery chemistry, but it was often ‘good enough’.
With high performance lithium chemistry batteries, there’s essentially a small computer (microcontroller) that monitors the state of the battery at all times. When that Battery Management System determines that the battery is fully charged, it essentially disconnects the battery from the power supply (often called a charger). It will do the same thing if it detects a fault scenario, for example if the battery is too hot to be safely charged.
TLDR; there is dedicated circuitry that monitors modern batteries and disconnects the flow of charging electricity when it determines that the battery is full.
Mos batteries output a different voltage dependent on how unreacted their chemistry is. [here are some li ion voltage curves.](https://images.app.goo.gl/8SL3Z4XXoXYJjxLMA)
By measuring the cell voltage, with some help from keeping track of how much charge has been put into or taken from the battery for the flat spots, the charge state of a battery can be estimated, usually pretty accurately
I know this! I learned it in electrical trade school. Granted that was 10 years ago so this may be rough.
Diodes only allow the flow of electricity one way. You can install “GATED DIODES” now think of this gate as a valve in your kitchen sink. The gates had a voltage sensory to be activated simply. Now say the gate opens or disconnects a 5.9v, when the float battery voltage reaches this level the gate opens and the flow stops.
This is veeeeery rudimentary and simple but yeah, gated diodes.
Diodes are also the reason when you are walking around with your phone plugged in and the charger in hand looking for an outlet the phone doesn’t backfeed 120v into the prongs on the charger
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