Connecting batteries in series is a method where the positive and negative terminals of multiple 18650 lithium batteries are connected together, creating a higher total voltage. This connection method increases the overall voltage of the battery pack while keeping the capacity unchanged. It's essential to ensure the correct polarity when connecting them in series to avoid short-circuits and damage.

　　Here are the steps for connecting 18650 lithium batteries in series:

　　Ensure the polarity of the batteries: Before proceeding with the series connection, check the positive and negative terminals of each 18650 lithium battery to make sure their polarities are correct. The positive terminal is typically marked with a "+" sign, while the negative terminal usually lacks any symbol.

　　Prepare battery connectors: You'll need appropriate battery connectors to link the batteries in series. The connectors can be soldering connectors, battery holders, battery clips, etc. Choose the right connector to ensure a secure and safe connection.

　　Connect the batteries: Connect the positive terminal of one battery to the negative terminal of the next battery in line. Continue this process until all the batteries are connected in series.

　　Ensure a secure connection: Make sure the battery connectors are firmly attached. If you're using soldering connectors, ensure the solder joints are secure and stable. If using battery holders or clips, ensure the batteries can be securely mounted.

　Battery insulation: After the series connection, ensure the external insulation of the battery pack is adequate to prevent short circuits or safety hazards.

　　Please note that when connecting batteries in series, it's crucial to ensure that all batteries have similar capacities, voltages, and conditions to avoid imbalances between the batteries. Connecting batteries with different capacities or conditions in series may cause some batteries to be over-discharged, leading to battery damage.

　　When connecting batteries in series, exercise caution and prudence, especially for non-professionals. It is recommended to seek professional electronic technical support. Incorrect series connections may lead to battery damage, leakage, fires, and other serious consequences.