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Ni-MH battery charging method and precautions

by:dcfpower     2021-03-15

After the Ni-MH battery is fully charged, if the fast charging is not stopped in time, the temperature and internal pressure of the battery will rise rapidly. When the internal pressure is too high, the sealed battery will open the vent hole, so that the electrolyte will escape, causing the viscosity of the electrolyte to increase, the internal resistance of the battery will increase, and the capacity will decrease. Therefore, in order to ensure that the battery is fully charged without being charged, the end of the charge must be controlled, and multiple methods such as timing control, voltage control, and temperature control are generally used.

The first stage

When the constant current is just charged into the discharged Ni-MH battery, the battery's internal resistance produces a voltage drop, so the battery voltage Rise quickly (point A). After that, the battery began to receive charge, and the battery voltage continued to rise at a lower rate. Within this range (between AB), the electrochemical reaction produces oxygen at a certain rate, and oxygen also combines with hydrogen at the same rate. Therefore, the temperature and gas pressure inside the battery are very low.

Second stage

After a certain period of time, bubbles begin to form in the electrolyte. These bubbles gather on the surface of the electrode plate, reducing the effective area of u200bu200bthe electrode plate, so the battery’s internal The impedance increases and the battery voltage starts to rise faster. This is a signal close to full charge.

The third stage

After fully charged, the current charged into the battery is not converted into battery energy storage, but oxygen is generated on the positive plate. Oxygen is produced by the electrolysis of electrolyte. In the electrolyte composed of potassium hydroxide and water, hydroxide ions become oxygen, water and free electrons. The reaction formula is 40H-→O21+2H0+4e-

Although the oxygen generated by the electrolyte can be It quickly recombines in the electrolyte on the surface of the negative plate, but the temperature of the battery still rises significantly. In addition, since the charging current is used to generate oxygen, the pressure in the battery also rises.

Because oxygen is easily decomposed from a large amount of hydroxide ions, the temperature inside the battery rises sharply, which causes the battery voltage to drop. Therefore, the battery voltage curve has a peak.

Charging process of Ni-MH battery

The charging process of the battery can usually be divided into four stages: pre-charge, fast charge, top-up charge, and trickle charge.

Pre-charge: When charging a new battery or battery that has not been used for a long time, fast charging is used at the beginning, which will affect the life of the battery. Therefore, this kind of battery should be charged with a small current first to make it meet certain charging conditions. This stage is called pre-charging.

Quick charge: is to charge with high current to quickly restore battery power. The fast charging rate is generally above 1C, and the fast charging time is determined by the battery capacity and charging rate.

Charging process of Ni-MH battery

Complementary charging: When using some fast charging stop methods, the battery is not fully charged after the fast charging is terminated. In order to ensure that 100% of the power is charged, a supplementary charging process should also be added. The supplementary charging rate generally does not exceed 0.3C.

Trick charge: also known as maintenance charge. According to 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 recharge the battery at a certain charging rate under the maintenance charging state, so that the battery is always in a fully charged state.

Terminal charging method

Timing control

According to the battery capacity and charging current, it is easy to determine the required charging time. This control method is the simplest, but because the initial charge state of the battery is not exactly the same, some batteries are undercharged, and some batteries are overcharged. Therefore, this method is allowed only when the charging rate is less than 0.3C.

Voltage control

Maximum voltage (V): It can be seen from the charging characteristic curve that when the battery voltage reaches the maximum value, the battery is fully charged. During the charging process, when the battery voltage reaches the specified value, the fast charging should be stopped immediately. The disadvantage of this control method is that the maximum voltage of a fully charged battery varies with the ambient temperature and charging rate, and the maximum charging voltage of each single battery in the battery pack is also different. Therefore, it is impossible to accurately judge the battery using this method. Fully charged.

Voltage negative increment (-AV): Since the negative increment of battery voltage has nothing to do with the absolute voltage of the battery pack, and is not affected by factors such as ambient temperature and charging rate, it can be more accurate to judge the battery Fully charged. The disadvantage of this control method is that after a negative increase in the battery voltage, the battery has been overcharged, so the temperature of the battery is relatively high. In addition, after the Ni-MH battery is fully charged, it takes a long time for the battery voltage to show a negative increase, and overcharge is more serious.

Zero Voltage Increment (0AV): In NiMH battery chargers, in order to avoid damaging the battery by waiting for a negative voltage increase for too long, the 0AV control method is usually adopted. The disadvantage of this method is that the battery voltage may change very little within a certain period of time before being fully charged, causing the rapid charging to stop prematurely. For this reason, most of the current Ni-MH battery fast chargers use high-sensitivity -0AV detection. When the battery voltage drops slightly (usually about 10mV), the fast charging is stopped immediately.

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