Industry News
Key problems in design of NiMH battery charger
2020-09-12
The performance of most portable products depends on the quality of the batteries that supply them. In recent years, with the improvement of battery performance, portable products are becoming smaller, lighter and more powerful. At present, there are three kinds of batteries for portable products: alkaline batteries, NiMH batteries and lithium-ion batteries. Alkaline battery is a non rechargeable battery with very low self discharge rate and low cost. Commonly used in low power consumption products, such as: remote control, electric toys, etc. When the capacity of the load relative to the alkaline battery is too large, such as notebook computers, PDAs and cellular phones, rechargeable batteries should be selected. There are two choices of rechargeable batteries: NiMH batteries and Li + batteries. Compared with Li + battery, NiMH battery has higher self discharge rate, and its energy density is not as high as that of Li + battery, but its cost is far lower than that of Li + battery, so the design requirement of protection circuit is not high. Therefore, it is suitable for cost sensitive products. In addition, the voltage specification of NiMH battery is the same as that of alkaline battery, which can be directly used to replace alkaline battery. However, due to the difference between the current battery capacity of 1700 ~ 2000 MAH battery and the actual application conditions of users, it is not possible to obtain the battery capacity of 1700 ~ 2000mAh in the market. On the other hand, the performance of the battery itself will affect the use effect of the battery to a certain extent.
For battery management design engineers, special attention should be paid to the charger design, because the battery performance is also closely related to the charger used. NiMH battery is suitable for the process of full charge and full discharge. If you use a poor quality charger, you can't recharge the battery completely when charging, then even the best battery can't achieve the desired effect. Therefore, in the design of NiMH battery charger, we need to consider the cost-effective problem, not blindly pursue low price.
The NiMH battery chargers ds2711 and ds2712 recently launched by Dallas Semiconductor Company are suitable for charging 1 ~ 2 AA and AAA NiMH batteries with built-in or external (independent). By monitoring the temperature, voltage and charging time, the best fast charging control is achieved. In addition, it can detect failed or mismatched batteries, such as disposable alkaline batteries. It supports series and parallel topologies, and can detect and control each battery individually.
How to constitute linear charger and pulse charger
By using ds2712, linear charger and pulse charger can be constructed (as shown in Figure 1). Charger designers need to make correct choice of charging structure. The regulator of the linear charger shares the voltage difference between the DC power supply and the battery through a transistor working in the linear region. The power consumption is the product of the charging current and the tube voltage drop. If the regulator is enclosed in a small space with no air circulation, power dissipation will lead to heating and temperature rise.
In Figure 1, a) is a linear charger composed of ds2712, which can charge two NiMH batteries in series. The ds2712 drives the external power transistor fcx718 to reduce the supply voltage to the required battery voltage. The circuit of linear charger is simple and the cost is relatively low, but the heat dissipation should be paid attention to. For example, for a two NiMH battery charger with a charging current of 1a, the end voltage of each NiMH battery is usually 1.6V ~ 1.7V, but it may be as high as 2V, depending on the battery and charging current. Therefore, the DC power supply voltage must be higher than 2 × 2 = 4V. The voltage of single NiMH battery may be reduced to 0.9V after full discharge. According to this calculation, the voltage of the battery pack at this time should be 2 × 0.9 = 1.8V. If the standard + 5V DC power supply is used, the voltage difference of transistor is 5v-1.8v = 3.2V. When the fully discharged battery is charged, the charger will consume 3.2 w of energy, and the battery will consume 1.8 w of power. The charging efficiency is only 36%. Therefore, the linear structure is more suitable for the external charger with better heat dissipation conditions (also known as cradle charger).
Some larger portable devices, including laptops, will place battery chargers inside the system as part of the system. At this point, the efficiency of the charger is crucial, not to be able to transfer the maximum energy, just to reduce the heat. Because the heat produces temperature rise, and the battery working in high temperature environment will shorten its life. Due to the requirement of high efficiency over the entire battery voltage range, the charger should be pulse charger, because its power dissipation is relatively small and independent of input-output pressure difference. Figure 1b) is a pulse charger configured with ds2712. The main disadvantage of pulse charger is the need for inductor and capacitor filter to convert the switch output voltage into DC voltage suitable for battery. Another disadvantage of this regulator is that noise will be generated in the process of switching, which needs to be avoided by proper wiring technology, shielding and filtering measures.
Charge control of NiMH battery
When the NiMH battery is charged, when the battery is fully charged, the terminal voltage rises to the maximum value and then begins to decrease. When the terminal voltage begins to drop, that is, DV / dt becomes negative, the fast charging mode should be terminated. Otherwise, the charging current will electrolyze the water in the battery into hydrogen and oxygen, resulting in the battery scrapping. In order to terminate the fast charging mode before the battery is damaged, the charger needs to detect the terminal voltage of NiMH battery. When the voltage reaches the maximum value (DV / dt = 0 or slightly lower than 0), the fast charging mode is terminated.
In addition to voltage detection, the charger also needs to monitor the battery temperature and charging time as an auxiliary or backup protection scheme. Figure 2 shows the internal charging control process of ds2711 / ds2712. There are three ways to start a full
For battery management design engineers, special attention should be paid to the charger design, because the battery performance is also closely related to the charger used. NiMH battery is suitable for the process of full charge and full discharge. If you use a poor quality charger, you can't recharge the battery completely when charging, then even the best battery can't achieve the desired effect. Therefore, in the design of NiMH battery charger, we need to consider the cost-effective problem, not blindly pursue low price.
The NiMH battery chargers ds2711 and ds2712 recently launched by Dallas Semiconductor Company are suitable for charging 1 ~ 2 AA and AAA NiMH batteries with built-in or external (independent). By monitoring the temperature, voltage and charging time, the best fast charging control is achieved. In addition, it can detect failed or mismatched batteries, such as disposable alkaline batteries. It supports series and parallel topologies, and can detect and control each battery individually.
How to constitute linear charger and pulse charger
By using ds2712, linear charger and pulse charger can be constructed (as shown in Figure 1). Charger designers need to make correct choice of charging structure. The regulator of the linear charger shares the voltage difference between the DC power supply and the battery through a transistor working in the linear region. The power consumption is the product of the charging current and the tube voltage drop. If the regulator is enclosed in a small space with no air circulation, power dissipation will lead to heating and temperature rise.
In Figure 1, a) is a linear charger composed of ds2712, which can charge two NiMH batteries in series. The ds2712 drives the external power transistor fcx718 to reduce the supply voltage to the required battery voltage. The circuit of linear charger is simple and the cost is relatively low, but the heat dissipation should be paid attention to. For example, for a two NiMH battery charger with a charging current of 1a, the end voltage of each NiMH battery is usually 1.6V ~ 1.7V, but it may be as high as 2V, depending on the battery and charging current. Therefore, the DC power supply voltage must be higher than 2 × 2 = 4V. The voltage of single NiMH battery may be reduced to 0.9V after full discharge. According to this calculation, the voltage of the battery pack at this time should be 2 × 0.9 = 1.8V. If the standard + 5V DC power supply is used, the voltage difference of transistor is 5v-1.8v = 3.2V. When the fully discharged battery is charged, the charger will consume 3.2 w of energy, and the battery will consume 1.8 w of power. The charging efficiency is only 36%. Therefore, the linear structure is more suitable for the external charger with better heat dissipation conditions (also known as cradle charger).
Some larger portable devices, including laptops, will place battery chargers inside the system as part of the system. At this point, the efficiency of the charger is crucial, not to be able to transfer the maximum energy, just to reduce the heat. Because the heat produces temperature rise, and the battery working in high temperature environment will shorten its life. Due to the requirement of high efficiency over the entire battery voltage range, the charger should be pulse charger, because its power dissipation is relatively small and independent of input-output pressure difference. Figure 1b) is a pulse charger configured with ds2712. The main disadvantage of pulse charger is the need for inductor and capacitor filter to convert the switch output voltage into DC voltage suitable for battery. Another disadvantage of this regulator is that noise will be generated in the process of switching, which needs to be avoided by proper wiring technology, shielding and filtering measures.
Charge control of NiMH battery
When the NiMH battery is charged, when the battery is fully charged, the terminal voltage rises to the maximum value and then begins to decrease. When the terminal voltage begins to drop, that is, DV / dt becomes negative, the fast charging mode should be terminated. Otherwise, the charging current will electrolyze the water in the battery into hydrogen and oxygen, resulting in the battery scrapping. In order to terminate the fast charging mode before the battery is damaged, the charger needs to detect the terminal voltage of NiMH battery. When the voltage reaches the maximum value (DV / dt = 0 or slightly lower than 0), the fast charging mode is terminated.
In addition to voltage detection, the charger also needs to monitor the battery temperature and charging time as an auxiliary or backup protection scheme. Figure 2 shows the internal charging control process of ds2711 / ds2712. There are three ways to start a full