I've recently been using an old cell phone charger as a single cell charger to manually charge 18650 li-ion cells. Below is some info about the charger & charging that I have measured.
Voc = 5.25V (Voltage open circuit (i.e. charger terminal V with no cell in the charger)
Imax = .67A (max current output of the charger -- no cell in the charger - tested with Ammeter on charger output terminal contacts)
Typical 3.7V Li-Ion Charging Conditions:
Voc Vic Aic
3V 5.25V 0A (bad cell - open cell)
3V 0V .67A (bad cell - shorted cell)
3V 3.09V .53A (530 mA) (typical good cell - low state of charge)
4.19V 4.2V .05A (50mA) (typical good cell - near full charge - charging manually stopped)
Voc = Voltage open circuit (i.e. the voltage reading across the terminals of a 18650 Li-ion cell -- not in the charging circuit.)
Vic = Voltage in circuit (i.e. the voltmeter reading at the charger contacts when the cell is in the charger & normal charging is on.
Aic = Amperage in circuit (i.e. the Ammeter is in circuit to measure the current.)
I have installed a spdt switch & a resistor to manually limit charging current to .1A for cells that have an initial low Voc (i.e. < 3V) to reduce the charge rate until they are at 3V. Then I flip the switch & charge them at the normal charge rate of .53A.
What this charger lacks is some way to set a CV of 4.1V to 4.2V and automatically shut off charging when the charge current = 50mA. What is the easiest way for me to modify my charging circuit to allow for an adjustable high V limit of between 4.1V & 4.2V & use some kind of current sense resistor to trigger an audible alarm?