Before you begin soldering, note the following safety precautions:

Never solder on devices that are powered on or plugged in. Unplug, turn off, and remove power sources before soldering.

Don't solder directly to hard-shell lithium-ion batteries (such as 18650 cells). The heat from the soldering iron will damage the battery internals. Use a battery spot welder instead.

Be extremely careful if you're soldering/desoldering lithium-polymer battery wires! You can easily short the battery with solder or your tools, resulting in battery damage and a fire hazard. Follow these precautions:

Drain the battery below 25%. This reduces the battery's ability to start a fire if it was short-circuited.

Work with one wire at a time to avoid short circuits. Use tweezers and hold each wire to control its movement and ensure it doesn't short. If you're desoldering a battery from a circuit board, use flush cutters to cut each wire one-at-a-time to isolate the battery before you desolder the wires.

Whenever possible, create an indirect path by soldering connectors onto the battery and the circuit board. This reduces the chance of an unintentional short.