Lithium battery capacity measurement circuit

I have some old lithium batteries. Some are used in obsolete mobile phones. Some are removed from laptop battery packs. They have been used for some time and the capacity has decreased. I don't know how much capacity is left, so I plan to make a simple circuit to measure it. After repeated experiments, I designed a measurement circuit that meets the requirements. It does not need to be connected to a separate power supply. The circuit is powered by the lithium battery itself. It is convenient to use. Because I only need to know the approximate capacity and do not need to draw a discharge curve, I use a small quartz watch to time it. It is cheap and easy to get. The shell is modified from a discarded mobile phone battery universal charger, and the original parts inside are used as much as possible. It is relatively easy to make.

Figure 1 is the simplest battery capacity measurement circuit. It is suitable for lithium batteries with discharge protection boards. The constant current circuit composed of Q1, Q2, R1, and R2 discharges the battery, and a 1.5V voltage is obtained across D1 and D2. The small quartz watch is powered for timing. The disadvantage of this circuit is that the accuracy is not high. In the later stage of discharge, the actual current is much less than 100mA, but the small quartz watch is still timing. The measured capacity is too large.

Figure 2 adds a voltage detection circuit based on TL431 on the basis of Figure 1. After the battery is discharged to the set voltage, the discharge current is cut off, which is more suitable for batteries without a discharge protection board. At the same time, it prevents small current discharge to obtain an accurate capacity value. When SW2 is disconnected. Two 3.3Ω resistors are connected in series. The discharge current is about 100mA, which is multiplied by the running time of the quartz watch to get the battery capacity (mAh). When the switch Sw2 is closed, the discharge current doubles to 200mA, which can save half of the test time and is used for measuring batteries with larger capacity. If you only measure small-capacity lithium batteries, SW2 can be omitted.

Whether the constant current circuit works or not is determined by the voltage division of Ic1 and R7 and R8. The cut-off working voltage of this circuit is designed to be 3.3V. Of course, it can also be changed to other voltages (such as cut-off at 3V) by adjusting R7 and R8. The cut-off voltage is set at 3.3V because it was found in the experiment that when the lithium battery voltage drops to 3.3V, the discharge current has dropped to less than 100mA, and the actual remaining power is not much. Too low a cut-off voltage is not good for the battery. R6 provides a positive feedback, which can speed up the reversal of the circuit and generate a hysteresis of about 0.3V. When the battery voltage drops to 3.3V, the circuit will immediately rise to about 3.5V after being cut off. This resistor can prevent the circuit from operating frequently.

LED3 and IC2 form a discharge indication circuit. During the discharge process, LED3 flashes at a frequency of 2Hz. IC2 has two package types: TO92 and soft package. Its pin arrangement is shown in Figure 2. LED1 is a reverse battery indication, and LED2 is a battery connection indication. It remains on after the discharge is completed, reminding you to remove the battery as soon as possible.

Q1 can use the 8550 in the original charger, or a PNP transistor with Icm>300mA such as 9012. Q2 can use a low-power PNP silicon tube such as 9015 and A1015. It is better to have a larger magnification, generally greater than 150. D1 and D2 use common 1N4000 series rectifier diodes, and the forward voltage drop of the two is exactly 1.5V.

R1, R2, R3 use 1/8W metal film resistors, and other resistors can be 1/8W or 1/16W. IC1, IC2, light-emitting diode and battery polarity conversion switch Sw1 use the original charger. Some universal chargers may not have flashing integrated circuits. A resistor of tens of ohms can be connected in series with LED3 instead of IC2. SW2 is a small horizontal toggle switch.