Production of NiMH Battery Charger

The homemade charger introduced in this article uses LM324's four operational amplifiers as comparators, TL431 to set the voltage reference, and S8550 as the adjustment tube to step down the input voltage and charge the battery. The principle circuit is shown in the figure below. Its characteristics are simple circuit, reliable operation, no need for adjustment, and easy purchase of components. It is introduced in several parts below.

NiMH battery charger schematic

1. Formation of reference voltage Vref

The external power supply is filtered by capacitor C1 after passing through socket X and diode VD1. VD1 plays a protective role to prevent damage to TL431 when the polarity of the external power supply is reversed. R3, R4, R5 and TL431 form the reference voltage Vref. According to the parameters in the figure, Vref = 2.5×(100+820)/820 = 2.80(v). This data is mainly designed for NiMH rechargeable batteries (the voltage of a single NiMH rechargeable battery is about 1.40V after it is fully charged).

2. High current charging

(1) Working principle

Connect the power supply, the power indicator LED (VD2) lights up. Install the battery (refer to the picture, it is actually led to the battery box with a wire, and the battery is installed in the battery box). When the battery voltage is lower than Vref, IC1-1 outputs a low level, VT1 is turned on, and outputs a large current to charge the battery. At this time, VT1 is in the amplification state - this is because the sum of the battery voltage and the -VD4 voltage drop is about 3.2V (assuming that the battery voltage is about 2.5V when charging starts), and the voltage after VD1 is about 5.OV, so the emitter-collector voltage difference of VT1 is much greater than 0.2V. When the charging current is 300mA, VT1 heats up more seriously, so it is best to use S8550 with PT=625mW, or appropriately increase the base resistance to reduce the charging current (Note: Due to the low-level driving ability of LM324, the low-level output of IC1-2 and IC1-4 is not 0V, but about 0.8V).

(2) Charging Instructions

First, let's look at the working conditions of IC1-3: its in-phase terminal 10 is connected to Vref through R13, R14 is connected to form positive feedback, and the inverting terminal 9 is connected to an external capacitor, and there is a negative feedback path, so it actually constitutes a hysteresis comparator. At the beginning, there is no voltage on the upper end of C2, so IC1-3 outputs a high level. This high level has two discharge paths, one is fed back to the 10th foot through R14, and the other is to charge the capacitor C2 through the resistor R15. When the charged voltage is higher than the voltage V+ of the 10th foot, the comparator flips and outputs a low level; at the same time, due to the feedback of R14, the voltage of the 10th foot immediately jumps down to V-. At this time, the capacitor C2 is discharged through the resistor R15. When the discharged voltage is less than the voltage V- of the 10th foot, the comparator flips again and outputs a high level. Due to the feedback of R14, the voltage of the 10th foot immediately jumps up to V+. After that, the circuit has been repeating the above process. Therefore, the output of IC1-3 is a square wave signal with a fixed frequency.

Next, let's look at the working condition of IC1-4: the battery voltage is divided by R2 and R16 and connected to the 12th pin of IC1-4. Because R2<<R16, the voltage at the 12th pin of IC1-4 is basically slightly lower than the battery voltage, and it is obviously lower than the voltage at its 13th pin. Therefore, IC1-4 outputs a stable low level. Combined with the above discussion, we can see that the square wave voltage with a fixed frequency is added to one end of the R12 and VD3 path, and the other end is a stable low level. Therefore, the light-emitting diode VD3 will light up periodically, giving people a flashing feeling.

Finally, let's look at the working condition of IC1-1: when IC1-2 outputs a low level, it is obvious that the 3rd pin of IC1-1 is at a low level, and its 2nd pin is connected to Vref through R1, so IC1-1 also outputs a low level. Combined with the above discussion, we can see that the voltage difference between R11 and VD5 is zero, so VD5 (saturation indicator) cannot light up!

In addition, since IC1-1 outputs a low level, no matter how the voltage at pin 9 of IC1-3 changes (the capacitor charges and discharges at this pin to form a triangular wave voltage), it will not be affected by the output of IC1-1 - because the voltage at pin 9 of IC1-3 (either as high as V+ or as low as V-) is always higher than the output of IC1-1, VD6 is reverse biased and cut off! Therefore, in this state, the working conditions of the three indicator lights are as follows: VD2 lights up, indicating that the power supply is normal; VD3 flashes, indicating that the battery is charging normally; VD5 is not lit.

3. Low current charging

After charging for a period of time, when the battery voltage slowly rises to close to Vref, the output voltage of IC1-2 slowly rises, so the current flowing through R7 slowly decreases, that is, the current flowing through the base of VT1 slowly decreases, so the current output by VT1 will also slowly decrease, but the battery voltage will continue to rise slowly. When the battery voltage is almost equal to Vref, IC1-2 will output a higher voltage. At this time, the voltage of pin 3 of IC1-1 is higher than 2.8OV (the input voltage of the inverting pin 2), and the comparator flips and outputs a high level. This voltage has two functions: on the one hand, it will make VD5 forward biased and turned on (at this time, the output of IC1-4 is still low), indicating that the charge is saturated; on the other hand, VD6 is also forward biased, and R17 is very small, which actually forces the upper end of C2 to be high, so the voltage of pin 9 of IC1-3 is higher than the voltage of pin 10, IC1-3 is forced to output a low level, and VD3 is extinguished due to no positive bias.

Although, from the external appearance, the charging light goes out and the saturation light lights up, and the conversion is completed instantly at a certain moment, but in fact the charging process is a gradual transition: when the battery voltage is much lower than Vref, the high current charging continues, and when the battery voltage is close to Vref, the charging current slowly decreases until the charging gradually approaches zero - even when the saturation light is on, the low current charging continues! So in this state, the working conditions of the three indicator lights are: VD2 lights up, indicating that the power supply is normal; VD3 is not lit; VD5 lights up (saturation indication, low current charging).

4. Application of IC1-4

From the analysis of 2 and 3 above, it can be seen that no matter whether the circuit is charging with a large current or a small current, the output of IC1-4 is always "low level", as if it has no effect. It is better to directly connect the negative poles of VD3 and VD5 to the "ground". When the design was just started, it was indeed not considered to use IC1-4 and directly connect the negative poles of VD3 and VD5 to the ground. However, when it was powered on after it was made, a problem was found: when the battery was not installed, the saturation indicator light VD5 was on - obviously not appropriate! Because, when there is no battery installed, VT1 is in a slightly conductive state, the voltage of the 5th pin of IC 1-2 is higher than, IC1-2 outputs a high level, so IC1-2 also outputs a high level, and VD5 is on.

If IC1-4 is connected in the schematic diagram, VT1 is in a slightly conductive state when no battery is installed, and the voltage of pins 1 and 2 of IC1-4 will also be higher than , so IC1-4 outputs a high level, so VD5 cannot light up.

需要说明一点，外接输入电压不能太高，也不能太低。输入电压太高，大电流充电时调整管发热严重；另一方面，IC1-2输出高电平的时间会因为电源电压较高而提前超过Vref(设定值)，这样就会给我们一个错觉，电池很快就充满了!实际上并非如此。输入电压太低也不好，同上面的分析一样，IC1-2输出高电平的时间会因为电源电压较低而迟后，更有甚者，也可能永远达不到充电指示灯一直闪烁，但大电流充电过程早已结束。所以，外接电压太高或太低，充电和饱和指示的状态是不准确的。