**Introduction to Lithium Battery Protection Board**
A lithium battery protection board is a crucial component used to safeguard series-connected lithium battery packs during charging and discharging. It ensures that the voltage difference between individual cells remains within a specified range (typically ±20mV), enabling cell balancing. This balance significantly improves the efficiency of series charging and enhances the overall performance of the battery pack. Additionally, the protection board continuously monitors each cell for overvoltage, undervoltage, overcurrent, short circuits, and overheating conditions, providing essential safety measures that help extend the lifespan of the battery. Undervoltage protection is especially important as it prevents damage caused by deep discharge, which can permanently harm the cells.
**Homemade Three-String Lithium Battery Protection Board**
This homemade circuit does not include overvoltage protection, so it should not be used for charging purposes. The design was created using a custom layout, but there were some errors in the initial PCB design. After modifying the green solder mask with a knife, one of the IC pins (IC6) was accidentally cut.
The 6-pin IC used is a specialized lithium battery protection IC. The second pin may not be connected, while most commercially available protection boards connect this pin to the negative terminal. Resistors such as 2kΩ and 2.2kΩ were selected randomly without precise calculations. The field-effect transistor (FET) used is a 75NF75 from the battery controller, and any N-channel FET with low on-resistance would work well.
Overcurrent protection was tested using parallel resistors. With six 10Ω resistors in parallel, the overcurrent threshold was approximately 5.5A. Adding seven 10Ω resistors in parallel increased the overcurrent protection level. Several photos of the board are included to show the layout and modifications made during the process.
**How to Connect a 3-String Lithium Battery to the Protection Board**
To properly connect a three-cell lithium battery to the protection board:
- B- is connected to the negative terminal of the first battery.
- 3.6V connects the positive terminal of the first battery to the negative terminal of the second.
- 7.2V connects the positive terminal of the second battery to the negative terminal of the third.
- 10.8V is connected to the positive terminal of the third battery.
- P+ is the output positive terminal, and P- is the output negative terminal.
Lithium batteries are primary cells that use lithium metal or alloys as the anode material and a non-aqueous electrolyte. Rechargeable lithium-ion batteries differ from lithium-polymer batteries in terms of their construction and electrolyte type. Understanding these differences is essential when choosing the right battery for your application.
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