0

First of all, I would like to charge my battery with a constant current. When my battery is charged to 4.2 V, I want to stop the constant-current charging and switch to CV mode.

My problem is the following: When I am in constant current, we can observe on the graphs that my current drops. Moreover, I can't set my current to the desired value. I would like to set it to 1 mA. For that, I change the value of R5, but I never manage to reach 1 mA.

enter image description here

enter image description here

ocrdu
  • 8,705
  • 21
  • 30
  • 42
Clemfraiche
  • 3
  • 2
  • 2
    Before we work at trying to get this circuit working the way you intend -- simple CC/CV charging of lithium-ion batteries is not good enough for safe charging. You need a CC/CV stage, yes, but then you _must_ cut the charge cycle during the CV portion when the charge current falls below a threshold dictated by the battery characteristics. Just holding a Lithium-Ion battery at a constant voltage forever will overcharge it, possibly causing a fire. – TimWescott Mar 13 '23 at 23:01

1 Answers1

0

There are two obvious errors in your circuit:-

  1. You haven't picked a MOSFET, so you got the default which is very weak. In your circuit its minimum Drain-Source resistance is ~10 kΩ.

  2. The Collector of Q1 is connected directly to the output of U1. The LTC6082 can sink ~30 mA, which overrides the transistor and prevents it from pulling the voltage up to limit current.

You should pick a MOSFET with appropriate characteristics, eg. BSS84.

The Collector of Q1 should go to the Gate of M1, then R7 will limit sink current from the op amp and Q1 will be able to pull the Gate voltage up.

With these changes I get 480 μA constant charging current up to 4.2 V, then current tapers off to 0 μA while the voltage stays at 4.2 V (battery simulated with a 1000 μF capacitor and 1 kΩ resistor in series).

Bruce Abbott
  • 55,540
  • 1
  • 47
  • 89