Resistance indicates the degree to which a circuit element obstructs the transmission of current, in ohms. Internal resistance (inner resistance), also known as internal resistance. The internal resistance of the battery refers to the resistance of the current flowing through the inside of the battery when the battery is working.

Generally divided into AC internal resistance and DC internal resistance. Due to the small internal resistance of the rechargeable battery, the DC internal resistance is measured due to the electrode capacity polarization, resulting in the polarization of the internal resistance, so the true value can not be measured, and the AC internal resistance can be measured to avoid the influence of the polarization of the internal resistance, the true internal value is obtained.

According to the physical formula R=U/I, the test equipment forces the lithium battery to pass a large constant DC current (currently a large current of 40A ~ 80A is generally used) in a short period of time (generally 2-3 seconds), measures the voltage at both ends of the lithium battery at this time, and calculates the current internal resistance of the lithium battery according to the formula.

(R-- resistance U-- voltage I-- current)

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

How to measure the battery internal resistance

1. Battery DC discharge internal resistance measurement method

According to the physical formula R=V/I, the test device forces the battery to pass a large constant DC current for a short period of time (generally 2 ~ 3s), measures the voltage at both ends of the battery at this time, and calculates the current battery internal resistance according to the formula. The accuracy of this measurement method is high, if properly controlled, the measurement accuracy error can be controlled within 0.1%. However, this method has obvious shortcomings: it can only measure large-capacity batteries or batteries, and small-capacity batteries cannot load 40-80A large current within 2 ~ 3s; When the battery passes through a large current, the electrode inside the battery will be polarized, resulting in an internal resistance to polarization. Therefore, the measurement time must be very short, otherwise the error of the measured internal resistance value is very large, otherwise the large current passing through the battery will cause certain damage to the electrodes inside the battery.

2. Battery AC voltage drop internal resistance measurement method

The battery is actually equivalent to an active resistance, so a fixed frequency and a fixed current are applied to the battery (at present, a frequency of 1kHz is generally used, and a small current of 50mA is used), and then the voltage is sampled, and after a series of processing such as rectification and filtering, the internal resistance of the electric vehicle battery is calculated through the op amp circuit. The battery measurement time of AC voltage drop internal resistance measurement method is very short, generally about 100ms, almost as soon as the measurement switch is measured, the accuracy of this measurement method is also good, the measurement accuracy error is generally 1% ~ 2%. This AC voltage drop internal resistance measurement method can measure almost all batteries, including small capacity batteries. However, the measurement accuracy of AC voltage drop measurement method is likely to be affected by ripple current, and harmonic current interference is possible, which is a test for the anti-interference ability of measuring instruments.Measured in this way, the battery itself will not have much damage.

Lithium battery internal resistance calculation method

According to the physical formula R=U/I, the test equipment makes the lithium battery in a short time (generally 2-3 seconds) to force through a large stable DC current (now generally use 40A ~ 80A large current), measure the voltage at both ends of the lithium battery at this time, and calculate the internal resistance of the lithium battery according to the formula.