What are the internal resistance measurement methods for lithium batteries?

According to the physical formula R=U/I, the test equipment allows the lithium battery to forcibly pass a large constant DC current (usually a large current of 40A to 80A) in a short time (generally 2 to 3 seconds). Measure the voltage at both ends of the lithium battery and calculate the current internal resistance according to the formula.

The accuracy of this measurement method is high, and if it is properly controlled, the measurement accuracy error can be controlled within 0.1%.

But this method has obvious shortcomings:

(1) Only large-capacity lithium batteries or lithium-storage batteries can be measured, and small-capacity lithium batteries cannot load large currents of 40A to 80A in 2 to 3 seconds;

(2) When the lithium battery passes a large current, the electrode inside the lithium battery will be polarized, resulting in a polarization internal resistance. Therefore, the measurement time must be very short. Otherwise the measured internal resistance value is very large;

(3) A large current has certain damage to the electrodes inside the lithium battery through the lithium battery.

AC voltage drop internal resistance measurement

Because the lithium battery is actually equivalent to an active resistor, we apply a fixed frequency and a fixed current to the lithium battery (currently using a 1kHz frequency, 50mA low current), then sampling its voltage, after rectification, filtering, etc. After the series processing, the internal resistance value of the lithium battery is calculated through the operational amplifier circuit. The lithium battery measurement time of the AC voltage drop internal resistance measurement method is extremely short, generally about 100 milliseconds.

The accuracy of this measurement method is also good, and the measurement accuracy error is generally between 1% and 2%.

The advantages and disadvantages of this method:

(1) Almost all lithium batteries, including small-capacity lithium batteries, can be measured using the AC voltage drop internal resistance measurement method. This method is generally used for the internal resistance measurement of notebook lithium battery cells.

(2) The measurement accuracy of the AC voltage drop measurement method is likely to be affected by the ripple current, and there is also the possibility of harmonic current interference. This is a test of the anti-interference ability in the measuring instrument circuit.

(3) Measurement by this method does not cause much damage to the lithium battery itself.

(4) The measurement accuracy of the AC voltage drop measurement method is not as good as the DC discharge internal resistance measurement method.

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