A rechargeable battery, USB port is an economical and practical power. Fully compatible with USB 2.0 specifications, USB port connected to the load must be able to perform two-way communication with the host. Load power management requirements must be met, including low power consumption, and easy means of determining when a host ports draw large current. Although some compatible systems to suit most USB hosts, but sometimes unexpected results. USB requirements and complexity of the load between the load, to standardize the expectation of only a good understanding of the compatibility in order to achieve a better balance. USB car charger from interface to switch to voltage conversion, as well as the current power demand match electricity demand. Sometimes you may encounter USB you can buy you can't charge, so there may be problems of matching. For example: If you are using a DC voltage of 5V USB car charger. If the voltage is correct, and this is your electrical power may be large. USB car charger power supply does not match may not charge or charge less for electricity. Then, it should confirm that the charging current, and small game.
USB based on these factors: it is the first car DC 12V or 24V DC power into DC 5V; then, in a unified interface USB 5V DC output corresponding to the charge. Different depending on the power supply, USB car charger power is different, different from mainstream USB car charger, big and small.