A battery that has an output of 3.7 volts is a rechargeable lithium polymer (LiPo) battery. It can be used as a single 3.7 volt cell or wired in series as part of a battery pack. In many ways it is similar to a lithium ion (Li-Ion) battery, but is generally the preferred battery for use in remote control models, especially aeroplanes. Li-Ion batteries are mainly used to power cell phones and laptop computers.
Insert your battery into your LiPo battery charger. You can only charge 3.7 lithium batteries using a specifically designed charger, otherwise you can damage your battery. More importantly, it could get overcharged and catch fire or even explode.
Check the settings on your battery charger, if it has variable voltage settings. Ensure that your battery charger charges at a rate not higher than 4.2 volts, otherwise your 3.7 volt battery will get damaged.
Turn on your battery charger, once you have confirmed the charge rate is correct. A red light usually illuminates to indicate that your battery is charging.
Let your 3.7 volt battery charge. A LiPo charger has an automatic cut-off when it senses the battery is fully charged. When the battery is fully charged the charging light either turns green or turns off.
Turn off the power to the charger. Remove the battery from the charger. Never leave chargers turned on or keep batteries in a charger.