Several options exist for charging battery-powered devices. The most obvious solution is to use a connector for DC power input, in combination with an external AC/DC adapter. The micro-USB connector is a very popular choice for this scenario. However, exposed connectors may not be a viable choice in certain scenarios, such as in the case of automotive/industrial environments or in the case of wearable IoT devices.
Magnetic, spring-loaded connectors are an alternative for traditional connectors that rely on a mechanical force for connector mating. Here, magnets are used to establish the physical mating, whereas spring-loaded pins create the electrical connection. One of the biggest advantages is that the connectors are very robust: when excess force is applied to the cable, the connector simply disconnects. For this reason, it is often found in high-end laptops such as the MacBook Pro and the Surface Pro. The photo to the right shows the same principle in the charging dock of a smart watch. Often, the pinout of the connector is chosen so that the connector is reversible. Sometimes, the connector is treated so that the device becomes water-resistant and/or corrosion-resistant.
Wireless inductive charging technology is another alternative to traditional charging connectors. As the name implies, this technology requires no physical connection between the device and the charger, allowing the device to be made completely waterproof. The technology relies on the principle of magnetic induction: an alternating magnetic field is generated by the transmitter coil of the charger. This field induces a current in the receiver coil of the device, thus transferring energy. The Qi standard is the most popular standard for wireless charging. The standard defines aspects such as identification, power regulation, and communication between charger and device. Off-the-shelf receiver modules exist, simplifying the integration of such technology. There are several aspects to watch out for with wireless inductive charging. To begin, the energy transfer efficiency is highly dependent on the distance and alignment between the sender and receiver coils. Misalignment will reduce efficiency or even block energy transfer. In general, the efficiency of wireless charging is much lower than that of wired charging. Typically, wireless charging is around 50% efficient, meaning that the device receives only half of the power that the charger draws. Efficiency improves with better alignment and smaller distance between the coils.
Finally, in some cases, it makes sense to use a removable battery in combination with an external battery charger. The obvious benefit is that users can immediately replace a drained battery by a charged one. Similarly, old, worn batteries can easily be replaced by new ones. This may be important in scenarios where many charge/drain cycles are expected. Finally, an external charger can charge batteries faster (at higher currents), and can charge and balance multi-cell LiPo batteries. However, removable batteries increase total cost and complexity of the system. An appropriate charger needs to be included, the battery packs need a separate housing, and the device needs a user-accessible battery compartment. Removable multi-cell LiPo batteries are often used in high-energy applications, such as cameras and quadcopter drones.