Introduction: Modifying Flat-Topped Li-ion Cells to Fit Into Battery Holders
Many 18650 cells have surfaces on both sides, making it suitable for solder tabs but they might not fit into certain battery compartments such as those found in flashlights. To modify the flat top, simply melt a blob of solder onto the positive terminal. Do it quickly as you want to heat only the terminal.
Another bonus of this trick is that it makes it easier to identify the polarity in the dark.