How does a battery charger know when to stop charging?

[ramonegumpert]

Dear Experts,

I am fascinated how a cell phone battery charger knows when the battery is fully charged such that it cuts off the power to prevent further charging.

If its based on sensing a particular voltage has been reached, i wonder how is that achieved?
Or is it achieved by measuring the state of charge?

Is there a simple circuit to detect a certain voltage?

Thank you very much for reading.

Sincerely
Ramone

 

[dlgoff]

Output current depends upon the battery's state. An intelligent charger may monitor the battery's voltage, temperature and/or time under charge to determine the optimum charge current at that instant. Charging is terminated when a combination of the voltage, temperature and/or time indicates that the battery is fully charged.

http://en.wikipedia.org/wiki/Battery_charger#Intelligent"

Here's a schematic of an intelligent charger that uses a microprocessor:
http://www.angelfire.com/electronic/hayles/images/figure3.gif

Regards

 

[gnurf]

Typical cell phone battery (lithium ion) types are charged with a user-defined constant current, Ich, until the battery voltage reaches a preset float voltage (normally 4.2V). During the constant voltage phase the current is allowed to drop to a preset level (e.g., Ich/10) or time, at which point the charge cycle is said to be complete.

The above is usually accomplished with two separate control feedback loops; one for the constant current phase and another for the constant voltage phase.

The voltage sensing can be done in many ways, e.g., by feeding back the battery voltage via a voltage divider into a comparator with a fixed ref. The output from the comparator can then be used to drive a transistor which sits in series with the battery.

In addition to linear chargers, there's also switching types and pulse chargers. There's enough app notes out there on the different types to keep you busy a long time. Good luck.

EDIT: My comment is unrelated to the image above, which was posted while I was writing.

 

[Naty1]

gnurf has a nice explanation..for a complenentary discussion see wikipedia:
http://en.wikipedia.org/wiki/Differential_amplifier