Type 18650 3.7V Li-ion Rechargeable Battery

Lithium Ion (Li-ion) batteries have a high energy density with high current output making them ideal for high-power long-term battery use applications. Even after several hundred recharge cycles, they are able to maintain reliable battery supply characteristics. As long as you use them within their rated specifications and take the necessary precautions, you will definitely be happy with these.

Note that 3.7V is the nominal voltage. The actual voltage will however range from about 3.4V (low battery) to 4.2V (fully charged).

Caution: Li-ion batteries can have a very high current output and they should under no circumstances be short-circuited. Improper use can result in fire hazards and explosion. Also ensure that the batteries are not discharged too far below 3.4V as they may become unusable after that or lose their performance.