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.
Charger:
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?
Best Answer
Are you aware that a cellphone charger, although it is called a charger, is actually NOT a charger ?? It is just a power supply. In all phones there is a chip that sits between this adapter and the battery and this chip takes care of the actual charging. Did you know that Lithium based cells can EXPLODE / CATCH FIRE when not charged properly ?
Please get a proper charger circuit like this TP4056 based module it will fit all your requirements, has indicators, you can set the maximum charging current with a resistor. Be safe, just get such a module it costs almost nothing.
Another item to consider if you only charge 18650 cells is this one.