Extended NiMH rechargeable battery life method

Abstract: Nickel-metal hydride batteries are the progeny of nickel-cadmium batteries, and their performance is superior to nickel-cadmium batteries in all aspects. This article will give you a detailed introduction to the long life of Ni-MH rechargeable batteries and how to extend the life of Ni-MH rechargeable batteries, I hope to help you.

[NiMH rechargeable battery] How to extend the life of Ni-MH rechargeable battery

Nickel-metal hydride batteries are the progeny of nickel-cadmium batteries, and their performance is superior to nickel-cadmium batteries in all aspects. Nickel-metal hydride batteries use nickel oxide as the positive electrode, hydrogen storage metal as the negative electrode, and alkali solution (mainly potassium hydroxide) as the electrolyte. The rated voltage is 1.2V, and the maximum voltage at full charge can reach 1.6V~1.8V. The normal discharge termination voltage is 1.0V, and 0.9V can actually be used. The number of repeated charging is more than 500 times, and the self-discharge rate is 20%/month.

The maximum discharge current of a nickel-hydrogen battery can reach 3C (discharge rate C refers to the current value of the full capacity of one hour. For example, the C of a 500mAh battery is 500mA). The energy-to-weight ratio is 60-80Wh/kg, and the energy-to-volume ratio is much higher than that of the nickel-cadmium battery. The maximum capacity of the single-cell 5th battery can reach 2300mAh, which is nearly four times that of the nickel-cadmium battery. Nickel-metal hydride batteries have low cost, no cadmium pollution, and a wide range of temperature applications. Has a good fast charging performance, no memory effect, can be used with the charge. Therefore, in the equipment using universal batteries, the market for nickel-hydrogen batteries is increasing.

However, in practice, it has been found that the number of recharges of nickel-metal hydride batteries is not as high as that of nickel-cadmium batteries, so there is a long life of nickel-metal hydride batteries. In fact, in theory, the life of nickel-metal hydride batteries is slightly longer than that of nickel-cadmium batteries. However, when nickel-metal hydride batteries are overcharged, the adverse effects on batteries are greater than those of nickel-cadmium batteries.

If the battery is not properly charged for a long time, it will damage the battery.

At present, the tens of dollars of nickel-cadmium/nickel-hydrogen battery chargers on the market are too simple, and it is easy to overcharge the nickel-hydrogen battery. The charger with the charging control chip sells for several hundred dollars, and there are not many users.

Therefore, the nickel-hydrogen battery cannot fully perform excellent performance.

Therefore, understanding the charging mechanism of nickel-metal hydride batteries, the correct purchase and use of chargers, is essential for extending the service life.

The so-called correct charging is to make the capacity of the rechargeable battery 100%, without damaging the battery. There are two results in incorrect charging. The capacity cannot be 100%, or the battery is seriously damaged. It is abnormally hot, cracked, or even caught in the process of charging.

The figure below shows the relationship between the charging current and the battery voltage when the Ni-MH rechargeable battery is charged.

It can be seen that in the case of high current charging, after the voltage rises to 100%, the battery voltage does not rise and fall. The charging fC control chip can end the charging by utilizing the characteristic that the battery voltage changes from rising to falling. This criterion for ending charging is called -ΔV detection.

It is necessary to charge the constant current with the detection of -ΔV, because the fluctuation of the current also causes the voltage of the battery to fluctuate. A simple charger is generally charged with a constant voltage current limit, and the current is getting smaller and smaller in the later stage of charging, and the detection of -ΔV becomes difficult. Therefore, when this feature is used to end the charging of the nickel-hydrogen battery, the charging current must be kept constant, and it must be charged with a large current of 1 C or more.

After the rechargeable battery is fully charged, continue charging, the electrical energy becomes the thermal energy of the battery, and the battery begins to heat up.

Another control of the charging IC control chip is to end the charging by the increase of the temperature rise rate, which is called ΔT/Δt detection. When charging is generally 2 ° C / min, charging stops.

When ΔT/Δt is detected, it is also required that the charging current must maintain a constant current. Moreover, when the charging current is large (0.3C or more), the detection is correct. When the charging current is small or the ambient temperature is low and the heat dissipation is good, it will not be detected and overcharge will be formed.

Both of these formal detection methods require high current constant current charging. Constructed with discrete components, the control circuit is complex. It is convenient to use an IC chip, but the temperature sensor must also be externally connected. The key is that the power supply part of the charger is required to be high, and the constant current is large. It is not suitable to use a simple transformer to rectify. It is necessary to use a switching power supply. Therefore, the price of a regular nickel-metal hydride battery charger is higher.

The simplest way to control is to use maximum time control. The maximum charging time is determined by the charging current, the battery capacity, and the charging efficiency. When this time is exceeded, the charging is unconditionally stopped. For example, charging at a rate of 0.1 C, considering the charging efficiency, the charging is terminated at about 12 hours. However, if it is charged in this way, if there is a residual amount in the battery at the beginning, there will be a de facto overcharge.

Therefore, the timing charge current must be less than 0.3C, and most are charged at 0.1C. Since the charging current is small, the heat generated by the overcharge on the battery can be dissipated relatively quickly, which has little effect on the battery.

The maximum time controlled charger has better safety performance and is most suitable for occasions where sufficient space is used. It is not suitable for the electrical appliances that need to be used with the charger, and there will often be no serious overcharging. If the residual amount of the battery can be estimated at the start of charging, a variable time charging method can be used. Manual intervention can also be carried out when necessary, and the charging is stopped when the estimated time or the temperature of the battery cover is significantly higher than the room temperature.

Ok, with these basics, you can choose the right battery and charger. If you prefer to use a larger capacity NiMH battery, such as 1600mAh or more, then it is best to buy a large current, charger with IC controller. This not only greatly shortens the charging time, but also effectively protects the relatively expensive large-capacity rechargeable battery.

The simple charger is easy to use, compact and easy to carry. But it is generally a variety that is common with nickel-cadmium batteries. For constant voltage current limiting charging, the circuit is very simple. Only after half-wave rectification, a string of resistors is charged.

This charging method starts with a large current, and then the current is limited to a small current of 0.1 C, that is, 50 mA. Some have a fast charging file, which is 100mA, which is equivalent to a 0.2C charging of a 500mAh nickel-cadmium battery. The advantage is that even if the user is overcharged, the impact on the battery is not great. This charger is charged with a 500mAh battery, almost one night. Charging after work, it is appropriate to go to work.

It takes 30 to 40 hours to charge a 1600 mAh battery, and it is almost impossible to be sufficient in practical use. Users will also charge as if they were charging with a mobile phone battery. Long-term operation of such nickel-metal hydride batteries in an unsatisfactory state has an impact on life and actual maximum available capacity.

and so. If you don't have a good saddle to buy a nickel-metal hydride battery, you don't have to use a good horse. The capacity is moderate, and it is reasonable to calculate the battery in 8 to 12 hours, and the charger is less than O.3C. Incidentally, large-capacity nickel-metal hydride batteries should be heavy, if the two nickel-metal hydride batteries weigh the same, the nominal capacity may be fake.

The discharge of nickel-metal hydride batteries is also stressful. Since the discharge termination voltage of the nickel-hydrogen battery is normal at 1.0 V, it can also be discharged at 1.0 to 0.9 volts. At this time, it is over-discharge, which is detrimental to battery life. General equipment, such as radios, walkmans, electric knives, etc., do not have a 1.0 volt automatic termination discharge circuit, which is easy to over discharge NiMH batteries. When used on such devices, when it is almost used, it should be charged. It cannot be used to completely recharge. This problem does not occur on devices such as digital cameras and MP3s that have a power management system.

The post-discharge will increase as the internal resistance increases and the battery temperature will rise. The internal resistance of nickel-metal hydride batteries is several times lower than that of lithium-ion batteries, but it is twice the internal resistance of nickel-cadmium batteries. Therefore, the use of Ni-MH batteries in the late stage will be greater than that of nickel-cadmium batteries.

Although nickel-metal hydride batteries initially believed that there was no memory effect, there was still a certain capacity drop compared with the later lithium-ion batteries. If the nickel-metal hydride battery is not used for a long time, it will appear to be dormant.

The capacity is greatly reduced. Therefore, for newly purchased or nickel-metal hydride batteries that have been put on hold for a while, firstly, the turbulent amount of electricity should be sufficient (not to be lacking), and then discharged to 1.0V at a rate of 0.2 C, this is repeated several times to activate.

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