**Introduction to Lithium Battery Protection Board**
A lithium battery protection board is a crucial component designed 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 enhances the efficiency of series charging, ensuring all cells are charged uniformly. Additionally, the protection board continuously monitors critical conditions such as overvoltage, undervoltage, overcurrent, short circuits, and overheating in each cell. By detecting these issues, it prevents damage and extends the overall lifespan of the battery pack. Undervoltage protection is especially important, as it prevents excessive discharge that could harm individual 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 with some trial and error, and there were a few mistakes made during the process. After modifying the green solder mask, a small mistake occurred when cutting the board, accidentally damaging pin 6 of IC6.
The 6-pin IC used is a dedicated lithium battery protection IC. In some cases, the second pin may not be connected, while commercially available protection boards typically connect this pin to the negative terminal. The resistors used in the circuit, such as 2kΩ and 2.2kΩ, were chosen randomly without precise calculation.
For the MOSFETs, I used the 75NF75 model found inside the battery controller. Any N-channel MOSFET can be used, but the lower the on-resistance, the better for efficiency. Overcurrent protection was tested using parallel resistors: six 10Ω resistors provided around 5.5A of current, and seven 10Ω resistors offered higher overcurrent protection.
Several images of the board are included to show its layout and modifications.
**How to Connect a 3-String Lithium Battery to the Protection Board**
To properly connect a three-cell lithium battery to the protection board, the connections should be made as follows: B- is connected to the negative terminal of the first cell, 3.6V connects the positive terminal of the first cell to the negative terminal of the second, 7.2V connects the positive of the second to the negative of the third, and 10.8V connects the positive of the third cell. P+ is the output positive terminal, and P- is the output negative terminal.
It's important to note that a lithium battery is a primary battery that uses lithium metal or alloys as the anode and a non-aqueous electrolyte. This is different from a lithium-ion battery, which is rechargeable, and a lithium polymer battery, which uses a polymer electrolyte instead of a liquid one. Understanding these differences helps in choosing the right battery for specific applications.
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