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How To Design 7.4v Lithium Battery Charging Circuit Diagram?

Shenzhen Sinoli Electronic Co.,Ltd | Updated: Dec 23, 2018

How to design 7.4v lithium battery charging circuit diagram?


7.4V is the nominal voltage, the lowest voltage is 6V, and the highest voltage is 8.4V. The interior consists of two lithium-ion batteries. The maximum voltage of a single lithium-ion battery is 4.2V and the lowest nominal voltage is 3.5V. If the voltage is lower than 2.75V, the battery will be discarded and the capacity will be greatly reduced. The battery will be charged until more than 3v, and the battery line will resume working and be detectable. At the same time, a single lithium-ion battery can not charge more than 4.2V, otherwise it will be dangerous.


Therefore, a charger should be designed to ensure that a single lithium-ion battery does not exceed 4.2V, charging input, 12V power supply.


When the battery voltage is 8.3V (inflection point), the voltage at each point is marked on the graph, and the voltage at each point of charging cut-off (8.4V) is also marked in parentheses.


Selection of circuit components for 7.4V lithium ion battery:


(1) LM317, three-terminal adjustable series voltage regulator, plastic seal, LM317T, commonly used. According to the different current, the corresponding radiator should be selected.


(2) TL431, a three-terminal adjustable parallel regulator, is commonly used in the same shape as a small transistor.


(3) RL is an external charged battery.


The current sampling resistance R 1 is calculated by R 1=1.23/charging current. For example, if the charging current is 0.3A, the resistance should be 4.1 Euros. This resistor should generally choose a higher power, such as 1A should be 2W.


(4) The adjustable resistance R4 can choose precise blue multi-loops, which are larger than the rated value. For example, the adjustable resistance R4 can choose 25K multi-loops. If it is too expensive or difficult to find multiple loops, a common adjustable resistor can also be connected in series with a fixed resistor. For example, 23.2k can be fixed at 22K plus an adjustable 2.2k-3.9k for fine adjustment.


The requirement of resistance R2 is not very high. It can be obtained by series-parallel connection. For example, 8.8 Euros can choose 10 Euros in parallel with 75 Euros (or 50 Euros-91 Euros).