^{The starting voltage of light-emitting diodes is generally above 2V. The seventh battery is only 1.5V and requires 2 cells to light up. However, the starting voltage of poor-quality light-emitting diodes is higher, some are higher than 3V, which may require Only three AA batteries can light up the LED for a long time. On the other hand, since the current curve of the LED is non-linear, the current becomes larger when the voltage rises, so it cannot be driven directly at 4.5V, resulting in burnout and Rapid light attenuation. Need to add a resistor to limit the current. This increases the power supply The diversity of the circuit and the process flow. In addition, the 3rd part of the AA battery requires a special battery holder, and the size of the flashlight casing has also increased a lot, which also increases the cost. 3 button batteries (as long as the LED is of good quality, 2 are OK) can be stacked directly without the need for a special battery holder. Button batteries have poor power supply capabilities and high internal resistance, so there is no need to apply a current limiting resistor. Therefore, it is considered from various aspects such as raw material cost and processing technology cost. , button batteries are the optimal choice.}

^{The design of the button battery flashlight greatly saves costs. The AAA battery flashlight is actually designed to save costs and improve usability. It is also more common in the market. One or 3 AA batteries can be used very easily without charging and maintenance. Easily purchased from department stores and can be replaced as needed .In fact, a single AA battery also has a very complex drive circuit design, that is, a boost circuit with only a few components. The simplest one has only a 3-stage tube. An inductor plus several resistors and capacitors can work at a very low cost. Slightly better is a control IC plus an inductor and a few resistors and capacitors.}

^{This kind of flashlight is also very common, but the conversion efficiency is limited due to the cost constraints of the driver circuit, and the effect is limited due to the cost constraints of generally not using good light-emitting diodes. More upscale, some designs only use 4.2V Single 10440 lithium battery, or compatible with regular 1.5V7 batteries and 4.2V 10440 lithium batteries, same size as the 7th cell .This kind of drive circuit is a buck-boost integrated type, which is relatively expensive. Some flashlights can only use one AA battery, or are compatible with ordinary 1.5V AA batteries or 4.2V single 14500 batteries, and are the same size as AA batteries. More from the previous one The first layer is a battery that uses a 4.2V single-cell 18650 lithium battery (a cell often used in laptop batteries in the past), which is a popular battery now.}

^{It is relatively convenient to use. Some flashlights are designed with charging sockets that can be plugged into the charger for charging. However, I don’t think the 18650 torch has its own charging interface, so the battery needs to be taken out for charging. If you go further up, the lithium battery size is 18650 or above. The flashlight. In addition, the light-emitting diode is also very important. Many cheap flashlights use domestic light-emitting diode. The light-emitting diode has problems of luminous efficiency and light color. The blue color is not well rendered and looks cold and white. Since human eyes are very sensitive to blue and green light, they feel very bright. However, if they glow on the road at night, they will glow and make it difficult to see the road surface.}

^{A better light-emitting diode is warm white, that is, yellow. This kind of light-emitting diode does not look too bright, but the road condition will be better. As for the luminous efficiency, it is also very simple, that is, if the light-emitting diode is input with 1W power tube, "how bright light" it can emit. Technically speaking, it is brightness or luminous flux. This is no longer the case. Generally speaking, at the same power, the luminous efficiency is higher. Imported LEDs are more efficient than most Domestic LEDs are all good - at least what I know now is that Crick, Osram, Phillips and other LEDs. In lithium batteries, safety, charger, charging and maintenance issues are also related.}