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18650 Lithium Battery Pack Design
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18650 Lithium Battery Pack Design

18650 is the pioneer of lithium-ion batteries, which was set as a standardized model by a Japanese company to save costs. The theoretical lifespan of 18650 battery is 1000 recharge cycles. Due to its high capacity per unit volume, it is mostly used for laptop batteries. In addition, due to its excellent stability in operation, 18650 is widely used in various electronic fields, such as high-end flashlights, portable power supplies, wireless data transmitters, electrically heated clothing and shoes, portable instruments and meters, portable lighting devices, portable printers, industrial instruments, medical instruments, etc.

Meaning of 18650

The number 18 represents the diameter of 18mm, 65 represents the length of 65mm, and 0 represents the cylindrical battery.


Performance parameters of lithium-ion batteries

The nominal voltage of lithium-ion batteries is usually 3.7V, and the full charge voltage is 4.2V. The charging control of lithium-ion batteries is divided into two stages: the first stage is constant current charging, and the charger will charge at a constant current when the battery voltage is below 4.2V. The second stage is constant voltage charging stage. When the battery voltage reaches 4.2V, the charger will fix the voltage at 4.2V. The charging current will gradually decrease. When the current decreases to a certain value (generally 1/10 of the set current), the charging circuit will be cut off and the charging is completed. The charging completion indicator light will be on.


Bracket design of lithium-ion batteries


Structural design:

(1) The outer diameter of the cell is designed as 18.4mm, and the hole of the bracket is designed as 18.6mm at the root, which is about 18.4-18.5mm in shrinkage;

(2) The minimum distance between the cells is designed as 19.0mm;

(3) The minimum distance between the edge of the cell and the shell is 1.5mm.


Material selection:

The bracket needs to consider fire prevention. It is recommended to use flame-retardant ABS or PC+ABS. Flame-retardant HIPS has also been tried, but the flowability of HIPS is worse than that of ABS, and the thin wall in the middle is easily unfilled, with a defective rate of more than 50%. Moreover, the HIPS material is relatively brittle and the thin wall is easy to crack, so it is not recommended to use the HIPS material.

Grouping of lithium-ion batteries in production

Before producing lithium-ion batteries, they need to be selected and grouped according to the internal resistance parameters. Those with similar parameters should be grouped together and placed in one battery pack.

Lithium-ion batteries marked with 3.7V or 4.2V are the same. It is only marked differently by the manufacturer. 3.7V refers to the platform voltage during discharge (typical voltage), while 4.2V refers to the voltage when fully charged. The voltage has nothing to do with the capacity. The mainstream capacity of 18650 batteries ranges from 1800mAh to 3500mAh (the capacity of 18650 power batteries is mostly between 2200-3000mAh), and even the mainstream capacity with labels of 3500 or 4000mAh and above are available.

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