The battery internal resistance refers to the resistance caused by current flowing through the battery when the battery is working, including Ohmic resistance and Polarization resistance (Rp).
Ohmic resistance is mainly composed of the electrode material resistance, electrolyte resistance, diaphragm resistance and contact resistance of each parts, and is related to the size, structure, process of the battery.
Polarization resistance (Rp) is the internal resistance caused by the polarization of the cathode and anode during the electrochemical reaction. The “polarization” is that during the electrochemical reaction in the battery, the speed of the electron flow is faster than that of the electrode reaction. At the anode, the electrons generated by ionization reaction is not fast enough to make up for the electron flow rate. While the electron consumption rate of the cathode reaction is lower than the electron inflow rate, causing positive shift of the anode potential and negative shift of the cathode potential, which reduces the potential difference and the electrochemical reaction rate.
Different types of batteries have different internal resistance. The same types of batteries have different internal resistance due to the inconsistency of internal chemical characteristics. Internal resistance is an importance technical indicator to measure battery performance. Under normal circumstances, battery with small internal resistance has strong discharge ability, and battery with large internal resistance has weak discharge ability. Meanwhile, the large internal resistance of the battery generates a large amount of Joule heat and increases the battery temperature, which reduces the battery discharge voltage, shortens the discharge time and seriously affects the battery performance and lifetime.
Ways to reduce the internal resistance of the battery
- Add electrode active material
The electrode material can be doped with active atoms to enhance the key structure, optimize the cell volume and reduce the charge transfer resistance of the cathode material. The polarization of anode active material increases greatly in the condition of high rate discharge, which is the main reason of serious polarization. Reducing the radius of the anode particles can effectively reduce the polarization of the anode active material.
- Cover functional coating
Surface treatment of battery conductive substrates with functional coatings is a breakthrough technological innovation. Covering the surface of aluminum foil/copper foil with conductive carbon coating, it can provide excellent static electrical conductivity and collect the micro current of the active material, which greatly reduces the contact resistance between the anode/cathode material and the current collectors, and can improve the adhesion between them. It also can reduce the amount of binder used, so that significantly improving the overall performance of the battery.
- Use ceramic diaphragm and PVDF diaphragm
The ion conduction inside the battery depends on the diffusion of Li+ in the electrolyte through the pores of the diaphragm. The liquid absorption and wetting ability of the diaphragm is the key to forming a good ion flow channel. When the diaphragm has a higher liquid absorption rate and a porous structure, it can improve the conductivity, reduce the battery resistance, and improve the C rate performance of the battery. Compared with the common base diaphragm, ceramic diaphragm and PVDF diaphragm can not only greatly improve the high-temperature shrinkage resistance of the diaphragm, but also enhance the liquid absorption and wetting ability of the diaphragm.
The internal resistance of a rechargeable battery is relatively small when it leaves the factory. After long-term use, due to the exhaustion of the electrolyte and the decrease in the activity of the chemical substances, the internal resistance will gradually increase until the internal resistance is so large that the internal power cannot be released normally, and the battery will fail at this time. A majority of aging batteries are unusable due to excessive internal resistance. Therefore, we should pay more attention to the discharged capacity of the battery rather than the charged capacity.
The internal resistance is not a fixed value
When the battery is in different charge state, its internal resistance value is different; when the battery is in different service life state, its internal resistance value is also different. From a technical point of view, we generally consider the resistance of the battery into two states: internal resistance in the charged state and internal resistance in the discharged state. The internal resistance of the charged state refers to the measured internal resistance of the battery when the battery is fully charged. The internal resistance of the discharged state refers to the internal resistance of the battery measured after the battery is fully discharged (when discharged to the standard cut-off voltage).
Normally, the internal resistance in the discharged state is unstable, and the measured result is much higher than the normal value. Because the internal resistance in the charged state is relatively stable, the measurement of this value has practical significance. Therefore, in the battery measurement process, we all take the internal resistance of the charged state as the measurement standard.
Internal resistance measurement
The internal resistance of the battery is very small, we usually define it in units of microohm or milliohm. In general measurement occasions, we require that the battery internal resistance measurement accuracy must be controlled within ±5%. Such a small resistance and such a precise requirement must be measured with special instruments. RacePow will carry out accurate internal resistance tests on each battery before the battery leaves the factory.
When buying batteries, we can also use a Battery Internal Resistance Test Meter (as shown below) to measure the internal resistance of the battery.
Summarize
In fact, many aging batteries still have a lot of power inside, but the internal resistance is too large to discharge the power. Once the internal resistance of the battery increases, it is difficult to artificially reduce the internal resistance. Therefore, for the aging battery, even if we think of many ways to “activate” it, such as high current impacting, low current floating charging, and putting in the refrigerator, most of them are of no avail. After learning the above knowledge, we can basically know that when choosing a battery, we should choose a battery with a smaller internal resistance as much as possible. It is also very important to know that the internal resistance of batteries increases when they are not used for a long time. It is recommended that you use the batteries regularly to keep their internal chemical substances active.
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