One of the challenges consumers, and especially business travelers, face today is finding a common yet practical technique to charge the wide variety of portable devices such as cell phones, wireless headsets, and MP3 players now at our disposal. It's a hassle to carry a number of different (AC/DC) wall adapters, and so is finding a wall plug to access the necessary power. Addressing this challenge requires safe charging methods that allow portable devices with higher energy storage (smart-phones, laptops, etc.) to charge portable devices with lower power requirements. Several industry initiatives worldwide support this trend to standardize the physical interfaces among devices and wall adapters, allowing for more flexibility and universal charging.
Generic battery charging specifications
First let's establish the nomenclature for charging lithium-ion cells. The battery pack consists of the battery cell, protection circuitry (i.e., provides over-voltage, under-voltage, and over-current protection), and connection terminals, all enclosed in a protective pack. One of the most important parameters is the pack voltage, which refers to the voltage at the pack of the battery, and differs from the cell voltage or open circuit voltage, as these are separated by an equivalent series resistor (ESR) that is a function of various chemical and mechanical factors.
The traditional, and most widely-used method for charging lithium-ion and lithium-polymer batteries is the constant-current/constant-voltage (CC-CV) technique. Here, we charge the battery with a constant current, usually at 1C (for a 1000 mA-h battery), where 1C is a charge current of 1 amp, until the pack voltage reaches its float voltage (4.2 volts to within 1 percent accuracy). Once we reach the float voltage, we apply a constant voltage (CV), in which the voltage is now fixed at float voltage and the current is allowed to taper down towards the so-called termination current level.
System design considerations
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Charging Time
The fall-off of current in taper charging is exponential, so there are diminishing returns to setting the termination current too low (Fig. 1). In addition, while a longer charge time allows more charge to enter the battery and thus increase the percentage of available battery capacity, the time to secure the incremental increase in battery capacity becomes significantly greater as the battery approaches full charge. Take, for example, a 150 mA-h battery. If we set the termination current to 75 mA, it will greatly decrease the battery's perceived charge time, but the battery may reach just 85 percent of full capacity. On the other hand, if we set the termination current to 50 mA, the battery will charge to perhaps 90 percent, but the charge time will be significantly greater, and so on for smaller termination currents.
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Figure 1: Termination current tradeoffs
Another issue that arises during the CC charging state is the need for high charge-current levels in order to reduce charge time, which generally makes the use of a linear battery charger impractical for thermal reasons. Consider, for instance, an application where a battery discharges to 3.3 volts as we use a mobile device up to system shutdown, whereupon the charge cycle commences. If the battery is rated at 1000 mA-h battery and we desire a 1C charge rate from a 5-volt input, the power dissipation in a linear battery charger will be (Eq.1):
PDiss,Linear = IBQ (Vin - Vbatt)
where IBQ is the battery charge current. In this case, the dissipation is 1.7 watts, which represents an overall charging efficiency of 66 percent. Clearly, a linear charger is not very practical for solutions that require a charger current greater than 500 mA. For these other applications, the buck switchmode charging topology is superior for many reasons beyond thermal considerations.
From a thermal perspective the buck switch-mode topology is far better because, rather than regulating the output by dissipating extra energy as heat, it regulates by providing energy only when the output requires it and uses energy storage devices to aid in this. Using the same example from above, and assuming the efficiency of the buck regulator is 90 percent, the power dissipation in a buck regulator is (Eq. 2),
or just 360 mW.
Another benefit of the buck switch-mode topology is in the use of current limited sources such as a USB port. The buck topology maximizes the effectiveness of the USB port and any current limiting power source using the concept of current multiplication (Summit's TurboCharge technology). By holding the input current constant and using Eq. 3 (below), we can calculate the battery charge current that a buck regulator would produce for a given efficiency and battery voltage.
So for a maximum input current of 500 mA, a charger with an input voltage of 5 volts and operating at 90 percent efficiency, and a battery voltage of 3.3 volts, the battery charge current would be as high as 681 mA.
—Safety
The sensitivity of the lithium-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 over-voltage 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 over-voltage 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 under-temperature 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.
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Figure 2: Distributed safety system based on IEEE1725 standard [Source: IEEE]
Enabling device-to-device charging implementations
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.
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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 100 mA when connected and not suspended; and once configured, can step up its current output to 500 mA. Even 500 mA 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 500 mA) 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 (Fig. 4).
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Figure 4: Charge time for a 1000 mA-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 cellular phone 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.
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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 inter-compatible 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/
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About the authors
Shadi Hawawini is an applications engineer at Summit Microelectronics (Sunnyvale, Calif.), responsible for customer support and customer design tool development. Mr. Hawawini has authored numerous technical articles and holds a provisional patent in Advanced Battery Charger Algorithms. He earned a BSEE degree from San Jose State University.
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. Mr. Paparrizos has authored numerous articles for industry publications. He holds an MSEE degree from the Technical University of Aachen, Germany (RWTH) and an MBA from the Haas School of Business at UC Berkeley.
