They are very safe if handled and charged properly. Lithium Ion batteries (and the Lithium-Polymer batteries the model aircraft guys use: cheap, light and powerful) are great. There is no problem when using a SMART Charging system, and discharging as well. You can simulate a smart charger by keeping a close watch on the voltage as you are charging, and in whatever device you use them in, you must disconnect the battery once the pack is discharged to about 3.0 volts per cell. Then you can charge to about 4.1 volts per cell, then stop! You can't run these batteries down to a low voltage, they will be ruined. If you try to charge a sick Lithium -Ion cell, that is when the smoke comes. Overcharge it? More smoke!