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I have recently bought some rechargeable ML2032 cells and also bought a charger for it.

Unfortunately the charger does not work, so I tried to find alternatives, but here the problem starts. According to the spec the cells should not be charged with more than 2 mA, but all the chargers I found including the one I already bought would charge with 20 mA or even more. Does anyone know why? Are the manufacturers not aware of the max. charge current? Could that not get dangerous, although these are small coins cells?

I started to think about building a charger on my own. I found an approach here, but this design uses a constant voltage approach and therefore would take much longer to fully load the coin cell than with a constant current approach, for which theoretically with a 2 mA charge rate and 65 mAh capacity the charge time would still need > 32 hours.

So I searched for as simple as possible a constant current circuit design which I could possibly use for charging. I found a constant current circuit with two NPN transistors here:

schematic

simulate this circuit – Schematic created using CircuitLab

I implemented this circuit on my breadboard and it really works great, as long as the DC voltage is high enough.

According to the document from Maxcell for the ML2032, the charge voltage of the ML2032 must not exceed 3.3 V. According to the simulation the voltage on the positive pole of the ML2032 always equals the voltage of the power source (=5.5 V) minus the voltage drop of the LED (1.595 V), which results in 3.915 V. The voltage on the negative pole of the ML2032 always equals the voltage on the plus pole minus the voltage of the ML2032, so the voltage applied to the coin cell does not exceed the 3.3 V (if the cell is not overcharged).

Does this situation satisfy the requirement that the charging voltage must not exceed 3.3 V? I.e. is the voltage difference applied on the coin cell relevant or is it the voltage on the positive pole relative to ground that matters?

Any suggestions on improving the circuit are welcome. Please note I am no expert.

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    \$\begingroup\$ I just wanted to add this link where, on page 32, they discuss the details you include in your writing. Important for others to see the original source. \$\endgroup\$
    – jonk
    Oct 17, 2022 at 18:39
  • \$\begingroup\$ I'm not convinced this won't overcharge the cell. It is the difference between the two cell terminals that matters, as you suspect, but this could charge the cell well beyond 3.3 V. \$\endgroup\$
    – Hearth
    Oct 17, 2022 at 18:39
  • \$\begingroup\$ Markus, if you want to stay with BJTs and resistors (and maybe diodes), you could just add a BJT to compare the voltage across your ML2032 and, if above a threshold, sink current into the base of Q1 to limit the voltage. In this way, you could use a 12 V supply or a 20 V supply with similar voltage limiting results while at the same time keeping your current-limiter. Is that kind of thing of interest? \$\endgroup\$
    – jonk
    Oct 17, 2022 at 19:47
  • \$\begingroup\$ @jonk yes, thank you, how would you do that? Actually I planned to monitor the charging process by measuring the voltage before and after the coin cell with an Esp32, which could when reaching the max voltage for example turn off the power source \$\endgroup\$
    – Markus Schlatzer
    Oct 18, 2022 at 4:53
  • \$\begingroup\$ @MarkusSchlatzer Oh. Well, if you prefer making ADC measurements with an MCU, instead, then that's also a fine way to go. I was thinking a purely analog approach. No MCU. Mostly, I was thinking that way because of the current-limiter circuit you already show. \$\endgroup\$
    – jonk
    Oct 18, 2022 at 5:02

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