What are the charging safety requirements for the RPM 40?

Do not use a battery charger higher than 5V, and always use the original charging cable with a reputable charger. Do not charge the device in hot, cold, or damp environments or under direct sunlight, as a short circuit may occur. The manual also advises unplugging the USB cable within 2 hours after the battery is fully charged to extend battery life. (Page 9)