How to use the correct charging to avoid overheating and overcharging problems

Positive electrode reaction when charging NiMH battery: Ni(OH)2+OH-→NiOOH+H2O+e- Negative electrode reaction: M+H20+e-→MH+OH- Overall reaction: M+Ni(OH)2→MH+ NiOOHWhen the NiMH battery is dischargedPositive electrode: NiOOH+H2O+e-→Ni(OH)2+OH- Negative electrode: MH+OH-→M+H2O+e- Overall reaction: MH+NiOOH→M+Ni(OH)2In the above formula, M is a hydrogen storage alloy, and MH is a hydrogen storage alloy in which hydrogen atoms are adsorbed. The most commonly used hydrogen storage alloy is LaNi5.

Nickel-metal hydride battery is over-discharged: nickel hydroxide electrode (positive electrode) 2H2O+2e-H2+2OH- hydrogen absorption electrode (negative electrode) H2+20H-2e→2H20 When overdischarged, the net result of the total battery reaction is zero. The hydrogen appearing on the anode will be newly combined on the negative electrode, which also maintains the stability of the battery system.NiMH standard chargingThe way to fully charge a sealed NiMH battery is to charge it with a nominal constant current (0.1 CA) for a limited time. The timer should be adjusted to stop charging at 150-160% capacity input (15-16 hours) to prevent prolonged overcharging. The applicable temperature range for this charging method is 0 to +45 degrees Celsius. The maximum current is 0.1 CA The overcharge time of the battery should not exceed 1000 hours at room temperature.

NiMH accelerated chargingAnother way to fully charge a NiMH battery in a short period of time is to charge it with a constant current of 0.3 CA for a limited time. The timer should be set to terminate charging after 4 hours, which is equivalent to 120% battery capacity. The applicable temperature range for this charging method is +10 to +45°C.

NiMH fast chargingThis method is used to charge V 450 - V 600 HR NiMH batteries in less time with a constant charge current of 0.5 – 1 CA. It is not enough to use a timer control circuit to terminate fast charging. To maximize battery life, we recommend using dT/dt to control the end of charge. A dT/dt control should be used at a temperature rise rate of 0.7°C/min. When the temperature rises as shown in Fig. 24, the voltage drop can be used to terminate the charging. –△V1) A charge termination device may also be used. The reference value of the –△V termination device shall be 5-10 mV/piece. If none of these disconnect devices work, then an additional TCO2) device is required. When the fast charge termination device cuts off the charging current, the trickle charge of 0.01-0.03CA should be turned on immediately.

NiMH trickle chargingHeavy usage requires the battery to remain fully charged. To compensate for power loss due to self-discharge, it is recommended to use a current of 0.01-0.03 CA for trickle charging. A suitable temperature range for trickle charging is +10°C to +35°C. Trickle charging can be used for subsequent charging after using the above charging method. The difference in trickle charge current and the need for more sensitive full charge detection made the original NiCd charger unsuitable for NiMH batteries. NiMH in NiCd chargers will overheat, but NiCd in NiMH chargers works fine. Modern chargers work with both battery systems.

NiMH battery charging processCharging: When using Quick Charge Stop, the battery is not fully charged after Quick Charge is stopped. In order to ensure 100% charging, a supplement for the charging process should also be added. The charging rate generally does not exceed 0.3c trickle charging: also known as maintenance charging. Depending on the self-discharge characteristics of the battery, the trickle charge rate is generally very low. As long as the battery is connected to the charger and the charger is powered on, the charger will charge the battery at a rate during maintenance charging so that the battery is always fully charged.

Many battery users have complained that the lifespan is shorter than expected, and the fault may be with the charger. Low-cost consumer chargers are prone to incorrect charging. If you want low-cost chargers, you can set the time for the charging status, and take out the battery immediately after it is fully charged.

If the charger temperature is found to be lukewarm, the battery may be full. It's better to remove and charge the batteries as early as possible before each use, rather than leaving them in the charger for eventual use.

Some advice on charging NiMH batteries

1. Use the method of shallow charging and shallow discharging. Do not overcharge and overdischarge, it will seriously shorten the life of NiMH batteries.

2. Nickel-metal hydride batteries and nickel-cadmium batteries have the same memory effect, but they are much smaller than nickel-cadmium batteries. So there is no need to perform a discharge operation every time it is charged (because improper operation will damage the battery), it only needs to be fully charged and discharged once every three months to alleviate the memory effect.

3. Under normal circumstances, the new nickel-metal hydride battery has only a small amount of power. After purchase, it must be charged first and then used. If the battery has a short time from the factory and the power is sufficient, it can be recharged. New nickel-metal hydride batteries generally need to be charged and used 3-4 times before their performance can be brought into full play.

4. Although the memory effect of Ni-MH battery is small, it is best to recharge after each use, and it is fully charged at one time. Do not charge for a while and then recharge, which will shorten the service life of Ni-MH battery.

5. It is best to use the matching charger for charging, because a mismatched charger may cause the battery to overcharge and shorten the battery life.

6. The charging of NiMH batteries should be carried out at an ambient temperature between 0°C and 45°C.

7. It is best to fully charge it before long-term storage and store it in a cool and dry place.

8. The battery cycle life can be recycled more than 500 times under the conditions of correct use. The battery life is reached when the battery life becomes extremely short.

9. The charging time must be strictly controlled according to the time in the manual. If it tells you that it is 8 hours, just charge it for 8 hours. The deviation should not exceed 1 hour. Beyond this range, the performance of the battery’s electric particles will be seriously reduced.

10. During the cycle life of nickel-metal hydride batteries, the electrolyte should be prevented from deteriorating and the hydrogen evolution of the battery should be suppressed.

Article Source: https://www.weijiangpower.com/news/how-to-properly-charge-nimh-batteries-weijiang/

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