Battery internal resistance meter

I have a few 18650 batteries in my hand, but I don't know whether they are good or not, so I made a battery internal resistance meter. However, I am not good at measuring accurately, so I can only measure roughly. The range is 1 milliohm to 3 kilohm, and the balance point is 3 ohms. The farther it deviates, the less accurate it is. I don't know how to write the calibration program.
However, it still has some advantages, that is, the protection circuit is relatively complete, which can support the measurement of batteries of several hundred volts, but the capacitor has no discharge resistor, so you should pay attention to discharge when measuring high-voltage batteries.

 

Except for the COG12864 LCD of ST7567, all other remote devices are bought from Lichuang Mall.

 

  Principle: The microcontroller outputs one SPWM, which is converted into a 1KHZ sine wave through RC filtering, and enters six parallel op amps to expand the current. Why parallel? Because I don't know how to make a triode power amplifier circuit. . The transformer is connected behind the op amp (the common-mode inductance of the green magnetic ring is 10mH each, and the DC resistance is 200 milliohms). The other end of the transformer is connected in parallel with three 10-ohm wire-wound resistors as current sampling resistors and connected in series with the battery. Two CBB 4.7uF 630V capacitors are connected in parallel as DC blocking capacitors. The 74HC4052 analog switch is used to collect data between the sampling resistor and the battery for one cycle and then switches. The collected voltage enters the 1x, 10x, 100x, and 1000x adjustable amplifier behind it. The amplifier is combined to achieve 3.3 milliohms, 33 milliohms, 333 milliohms, 3.33 ohms, 33.3 ohms, 333 ohms, and 3333 ohms. The amplified signal enters the microcontroller and is multiplied and added with the 1KHZ COS and SIN tables to obtain the real and imaginary parts. The voltage on the battery is divided by the voltage on the current sampling resistor and then multiplied by the sampling resistor value to get the internal resistance of the battery.
 
 
Test video