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?

  • \$\begingroup\$ Why not use $1.5 chargers that already have everything included (LEDs, auto-disconnect at 4.2V, charge from low state, etc)? optimusdigital.ro/ro/electronica-de-putere-incarcatoare/… \$\endgroup\$
    – Overmind
    Jan 30, 2018 at 12:00
  • \$\begingroup\$ 3 reasons: 1. the charge rate of such modules is fixed (not variable for charging very low charged cells), 2. the charge rate is higher than I prefer (often 1A or higher), & 3. the charge Vmax is often 4.2+V & not always very stable (which is higher than I prefer because it reduces the life span of li-ion cells). \$\endgroup\$
    – zeffur
    Feb 3, 2018 at 0:58

1 Answer 1


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.

  • \$\begingroup\$ Yes, actually, I am quite aware of all of the facts that you mentioned and, among many other chargers, I also have a couple TP4056 modules that are similar to the one that you referenced--but really, that is not the point of the question that I posted. The point of my question is to build a circuit that functions as I described in my original post. Can you help with that endeavor? \$\endgroup\$
    – zeffur
    Oct 18, 2015 at 16:12
  • \$\begingroup\$ I'd like something that is easy to mount in a bread board. I imagine the final circuit will be based on a voltage divider network, some op amps & probably some mosfets--similar to the circuit inside this device--but actually rated a bit higher for 3.7V li-ion cells--not lower rated li-poly cells. \$\endgroup\$
    – zeffur
    Oct 18, 2015 at 17:01
  • \$\begingroup\$ What would be the advantage of what you want instead of using a cheap IC that is made for the job ? I cannot think of a reason other than educational purposes. And even then I would use a circuit simulator. \$\endgroup\$ Oct 18, 2015 at 18:42
  • \$\begingroup\$ Yes, for educational purposes, but also, I want a useful device that will allow me to adjust the max charge current (.1A to 3A) & adjust the max charge voltage (4.0V to 4.25V). The cheapo Asian boards that I have aren't capable of such adjustments & they default to a 1A charge rate--which is more aggressive than I want--especially for some cells. Ideally, at some point, I'd like to be able to connect a higher V input to bulk charge multiple cells in series or in parallel & finish charge them individually using the adjustable single cell charge mode CC/CV (.1A to 3A) & (4.0V to 4.25V). \$\endgroup\$
    – zeffur
    Oct 18, 2015 at 20:52

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