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Implement safe battery charging between gadgets

Posted: 20 Aug 2008 ?? ?Print Version ?Bookmark and Share

Keywords:generic battery charging? safe battery charging between gadgets?

So for a maximum input current of 500mA, a charger with an input voltage of 5V and operating at 90 percent efficiency, and a battery voltage of 3.3V, the battery charge current would be as high as 681mA.

SafetyThe sensitivity of the Li-ion battery technology to over-charging, temperature stress, and short-circuit conditions has always been addressed with the addition of battery protection circuitry inside the battery pack (primary protection). The wider adoption of lithium-powered devices during the last year has substantially increased the possible usage scenarios that could lead to unexpected battery failures, and consequently introduced a new, wide range of concerns. This in turn impacts on additional safety requirements, primarily focusing on secondary protection. Many of the new strict requirements have been initiated by hardware companies, others by service providers (like carriers) and some by industry bodies, like the IEEE. As an example, the IEEE1725 safety standard, focusing on cellular phones, was issued last year with an increasing number of companies seeking compliancy with this new standard.

Input overvoltage protection is commonly required by applications to ensure that the portable device will not suffer damage in the case of a faulty car or wall adapter. When the charge control circuit senses an input voltage that is significantly higher than the expected value, the circuitry suspends charging and notifies the system. Battery packs also need to be protected from overvoltage conditions, both via an integrated protection IC and a secondary monitor in the system. Safety-timers are often utilized for monitoring charging duration and suspending the charging process in the case of a defective battery pack. Secondary current-limiting is also critical to protect the battery. In addition, higher-capacity battery packs often integrate negative temperature coefficient (NTC) thermistors, which the system can utilize to monitor over- and undertemperature battery conditions. While system cost is always a key factor when deciding the level of protection required for a given system design, modern battery charging solutions already integrate most of the features necessary to provide complete, secondary protection.

Figure 2: Distributed safety system based on IEEE1725 standard [Source: IEEE].

Device-to-device charging
A device-to-device charging implementation that is gaining a lot of momentum is the use of a notebook USB port as a power source. This method allows users to charge their handheld devices wherever they are, with just the use of a mini USB-to-phone connector cable. The popularity and simplicity of this implementation has been the driver of many industry initiatives, including the USB battery charging specification 1.0 from the USB Implementer's Forum and the Telecommunications Standard from the Chinese Ministry of Information Industry.

Figure 3: Charging a portable device via a notebook USB port.

Using a notebook as the power source for charging offers a high level of flexibility, but its major downside is that the USB port's current capability is limited and thus extends the charging time. A USB2.0 compliant USB port needs to provide 100mA when connected and not suspended; and once configured, can step up its current output to 500mA. Even 500mA is in many cases not adequate, especially when the system is in operation during charging (i.e., the net current for charging is significantly lower than 500mA) and with many new portable devices utilizing higher capacity batteries. One solution to this problem is using battery charging solutions that utilize the TurboCharge technology, allowing the output current (sum of charge and system current) to be higher than the current provided by the USB port (input current), especially when the battery is deeply discharged (Figure 4).

Figure 4: Charge time for a 1,000mA-h battery using a USB500 input-limited source and TurboCharge.

An increasing number of applications are adopting the USB On-The-Go (OTG) standard, which allows two peripheral devices to connect to each other without the need of a host. For example, we can directly connect a printer to a digital camera for direct printing (without going through a PC), or a cellphone can be connected to a portable multimedia player for exchanging music or video files. OTG connectivity requires one of the portable devices to become the host and to provide OTG power (VBUS of 5V and a certain current level) to the peripheral device. The availability of this supply voltage from the host can be used by the peripheral device as a power source for system operation as well as for charging.

As demonstrated in Figure 5, an MP3 player can use this power source as an input to its own battery charging circuitry, and charge its own battery. Such an implementation can be very useful for consumers since very frequently they carry both their cellular phone and their MP3 player with them, but not necessarily a notebook or have access to a wall adapter. They should ensure that the battery of the "host" device is not drained, thus it is recommended that charging is suspended when the battery voltage level of the cellular phone reaches a low level and/or when the current drawn by the peripheral device is too high.

Figure 5: Peripheral device charging via OTG-compatible portable host.

The mass adoption of a variety of portable electronic devices has introduced new targets for ease-of use and safety. New products are required to provide true portability, increased usage time and reduced charging time. New system and battery charging implementations can greatly increase the convenience of charging, particularly during traveling, while at the same time provided standardization can ultimately aid in producing safer and intercompatible products. The high level of integration of modern battery charging solutions can significantly reduce the cost and board space for implementing such solutions, resulting in feature-rich, safe, convenient and small form-factor portable devices.

References
1. USB-IF, On-The-Go Supplement to the USB 2.0 Specification revision 1.3.
2. USB-IF, USB2.0 Specification.
3. USB-IF, Battery Charging Specification v1.0.
4.http://www.summitmicro.com/prod_select/summary/SMB138/SMB138.htm.
5. http://grouper.ieee.org/groups/1725/.

About the authors
Shadi Hawawini
is an applications engineer at Summit Microelectronics, responsible for customer support and customer design tool development. George Paparrizos is marketing director at Summit Microelectronics. Previously, he was a senior product marketing manager at Microchip Technology, specializing in the battery, power and thermal management product lines.


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