1. Introduction to NiMH batteries
NiMH batteries are a kind of green battery developed since the 1990s. NiMH batteries are alkaline storage batteries, the cathode active material is mainly composed of nickel, and the anode active material is mainly composed of hydrogen storage alloy. NiMH batteries can be used in hybrid electric vehicles, electric buses, electric bicycles, electric tools application fields and other application fields.
2. Composition of NiMH batteries
NiMH batteries are mainly composed of cathode, anode, electrolyte, separator, etc. The cathode active material is nickel hydroxide, the anode active material is a hydrogen storage alloy, the electrolyte is a solution containing 30% potassium hydroxide, and there is a separator between the cathode and anode plates of NiMH batteries.
3. Pros and cons of NiMH batteries
- Pros: low price, strong versatility, large current, environmental protection and stability.
- Cons: Heavy weight, short battery life.
The capacity is higher under the same volume. Taking the common AA battery as an example, the nominal capacity of NiMH batteries can reach 2900mAh, while that of NiCd batteries is only 1100mAh. NiMH batteries have a larger output current than carbon-zinc batteries or alkaline batteries, and are relatively more suitable for high-power consumption products. Some special models of power types even have higher output current than ordinary nickel-cadmium batteries.
The cycle life is long, and it can be used more than 500 times under correct use conditions. Poor high temperature resistance, try not to let the battery temperature higher than 45 degrees. Otherwise, the lifespan will decrease rapidly and the internal resistance of the battery will increase. Overcharging has a great impact on battery life and is dangerous, so stop charging when the battery is fully charged.
The working voltage of both NiMH and NiCd batteries is 1.2V, but comparing nicd vs nimh battery, the volume specific energy of NiMH is higher than that of NiCd batteries. Due to the excellent high-rate discharge performance of nickel-cadmium batteries, NiMH batteries cannot replace nickel-cadmium batteries in many power tools. Therefore, in the ROSH standard, nickel-cadmium batteries are temporarily allowed to be used in the field of power tools.
4. Charge and discharge characteristics of NiMH batteries
When NiMH batteries are charged normally, the change process of their charging characteristics can be roughly divided into three stages: the stage of rapid voltage rise, the stage of steady voltage rise, and the stage of rapid voltage rise again. When the charge exceeds the rated capacity of the battery, NiMH batteries enter the overcharge stage, and the voltage drops slightly.
The lower the charging voltage level of NiMH batteries, the smaller the polarization of the battery, the higher the charging efficiency of the battery, and the longer the service life of the battery may be.
The larger the discharge current of NiMH batteries, the lower the ambient temperature, and the lower the battery discharge voltage and discharge efficiency. Long-term high-current discharge has a negative impact on the service life of NiMH batteries. The end-of-discharge voltage of NiMH batteries is generally between 0.9 and 1.1V.
5. Capacity characteristics of NiMH batteries
Charging current, storage time, discharging current, discharge termination voltage, etc. all have an impact on the capacity characteristics (discharge capacity characteristics) of NiMH batteries.
Effect of charging current on discharge capacity
The larger the charging current of NiMH batteries is, the degree of electrode polarization increases, which will increase the oxygen evolution in NiMH batteries, and eventually lead to the decrease of charging efficiency and discharge capacity of NiMH batteries.
Effect of resting time on discharge capacity
The reason why the rest time has an effect on the discharge capacity of NiMH batteries is that NiMH batteries have self-discharge problems. The discharge capacity of NiMH batteries decreases with the extension of the storage time, and the discharge capacity of NiMH batteries decreases rapidly at the initial stage of storage.
Effect of discharge current on discharge capacity
The discharge current of NiMH batteries increases, the degree of electrode polarization increases, and the internal resistance of electrochemical polarization increases. The increase of internal resistance of the battery has an adverse effect on the discharge capacity of the battery. Therefore, the discharge current increases and the discharge capacity of NiMH batteries decreases.
Effect of discharge termination voltage on discharge capacity
The discharge capacity increases with the decrease of the end-of-discharge voltage.
6. Self-discharge rate and storage performance of NiMH batteries
● Self-discharge rate
The capacity decay rate of NiMH batteries is faster when they are in the idle state. At 20°C, the monthly self-discharge rate of NiMH batteries is between 20% and 25%. Factors affecting the self-discharge rate of NiMH batteries mainly include the temperature and humidity conditions of battery storage.
Effect of temperature on self-discharge rate of NiMH batteries
The temperature can increase the reaction rate of the active material of the cathode and anode materials in NiMH batteries, and can also increase the ion transmission rate of the electrolyte, thereby increasing the self-discharge reaction rate. If the temperature is too high, the chemical balance inside the NiMH batteries may be destroyed, and an irreversible reaction will occur, thereby reducing the overall performance of the battery.
Effect of humidity on the self-discharge rate of NiMH batteries
The principle of the influence of humidity on the self-discharge rate of NiMH batteries is similar to the principle of the influence of temperature on the self-discharge rate of NiMH batteries. High ambient humidity will increase the self-discharge reaction rate of NiMH batteries.
In summary, NiMH batteries are placed in a low-temperature and low-humidity environment, and the self-discharge rate of the battery is low, which is conducive to the storage of the battery. However, if the temperature is too low, it may also cause irreversible changes in the electrode materials of NiMH batteries, reducing the overall performance of the battery.
● Storage performance
Storage performance of NiMH batteries
Battery storage performance refers to the change of the main performance parameters of the battery after storage for a certain period of time under certain conditions. Storage performance specifically includes: capacity decline, appearance change, and whether there is liquid seepage.
The decline in the capacity of NiMH batteries is mainly due to the self-discharge phenomenon of the electrodes, and the high self-discharge rate is not conducive to the storage of NiMH batteries. Therefore, in general, NiMH batteries follow the principle of "charge and use immediately" and are not suitable for long-term storage.
Electrolyte corrosion and leakage may occur in the cathode and anode, separator, and auxiliary materials of NiMH batteries, which have a great impact on battery performance. According to online information, the electrolyte of NiMH batteries is non-toxic, but the electrolyte contains 30% potassium hydroxide, which is corrosive. If it splashes on the skin, it can be washed immediately with plenty of water.
Storage conditions for NiMH batteries
The storage area of NiMH batteries should be kept clean, cool, and ventilated; the temperature should be between 10 and 25°C, and the maximum temperature should not exceed 30°C; the relative humidity should not exceed 65%. In addition to storage temperature and humidity requirements, you should also pay attention to the following when storing NiMH batteries:
- NiMH batteries that have been placed for a long time should be stored in a state of charge, that is, 50~100% of the battery can be pre-charged and stored.
- During the storage of NiMH batteries, it should be charged at least once every 3 months to restore the battery capacity to the saturated state.
7. Lithium batteries vs NiMH batteries, which one is better
Compared with lithium batteries, NiMH batteries are better in terms of safety. The main reason is that the specific heat capacity and energy density of NiMH batteries are relatively low, and the melting point is as high as 400°C. When subjected to collisions, extrusions, punctures, short circuits, etc., the battery temperature will not rise sharply to cause spontaneous combustion.
In contrast, lithium batteries have high energy density due to the strong activity of lithium ions. At the same time, the raw materials of some types of lithium batteries are flammable. Once the short-circuit temperature of the battery rises due to various factors, the internal electrolyte will undergo violent chemical reactions. This in turn may lead to spontaneous combustion of the battery, and it is not uncommon for new energy vehicles equipped with lithium batteries to spontaneously ignite in recent years.
The life of NiMH batteries will be longer, because NiMH batteries have a certain memory effect, so when major automobile manufacturers use NiMH batteries, they will give them a battery management strategy of "shallow charging and shallow discharging".
The current mainstream lifepo4 battery, although its service life can reach 4000 charge and discharge cycles, is still far behind NiMH batteries. As for the ternary lithium battery, the theoretical lifespan is 2000 charge and discharge cycles, but research has found that when the charge and discharge cycle of the ternary lithium battery reaches 900 times, the capacity of the battery will decay by about 50%. So in terms of life, NiMH batteries are better than lithium batteries.