Battery-charging considerations for high-power portable devices

August 19th, 2015, Published in Articles: EngineerIT

Cell phones are a good example of how functionality and performance have both increased significantly in portable devices over the last few decades. They have become more complex and can do many basic tasks as well as any computer. The extra functionality that has transitioned the smartphone from a phone-call-only device to a multi-purpose portable device makes it more power-hungry than ever before. The internal battery pack is the main source of storing and delivering power to portable-device circuitry.

Battery-charger ICs are responsible for charging the battery pack safely and efficiently. They must also control the power delivery to the system to maintain normal operation while plugged into wall power. The battery pack is required to store a large amount of energy and be charged in a short amount of time without sacrificing weight and volume.

The increased charge and discharge currents, as well as the smaller physical size, make the packs vulnerable to physical and thermal stresses. Therefore, battery chargers are no longer required to perform just as a simple stand- alone charger.

To maintain reasonable charge times and safe charging conditions, a battery-charger IC is required to be flexible because it must guarantee power to the system at all times and provide proper protection for both the battery and system. This article explores single-cell battery-charger solutions and includes a detailed discussion about the performance and constraints of chargers for compact high- power applications.

An overview of single-cell charging solutions

Rechargeable batteries are vital to portable electronic devices such as cell phones and wearable electronics. Charging circuits must be carefully designed and are highly dependent on three factors: battery chemistry, power levels, and system load. Different battery chemistries require different charging methods. An application’s power requirements directly impact the charging system’s cost and size. Finally, the system power requirement must be considered to determine the necessity of a power-path versus a non-power-path system.

Lithium-Ion (Li-Ion) batteries are becoming the chemistry of choice for many portable applications for several reasons. They offers a high capacity-to-size and weight ratio, and they have low self-discharge characteristics. They also have high cell-voltage characteristics, typically 3,6 V, which allows a battery pack to be designed with only one-cell. Despite all these advantages, Li-Ion batteries are fragile to stress. They require many special considerations regarding charge current, regulation voltage, trickle charge levels, temperature monitoring, and so on.

There are two basic types of charging methods: linear and switch-mode charging. Switch-mode charging minimises power dissipation over a wide range of AC-adapter voltages, but consumes more board space and adds complexity. Additionally, switch-mode applications generally are higher cost than an equivalent linear application.

Linear chargers are smaller and great for noise-sensitive equipment. However, they are not as efficient across the entire charge cycle as their switch-mode counterparts. When selecting a charging method, the designer makes the decision by prioritising cost, space, bill-of-material (BOM) count, and efficiency (thermal loading).

The variety of system requirements drive many different battery-charger solutions; from a simple standalone charger to an embedded charger that also provides system power. System requirements include, but are not limited to:

  • The need for dynamic power path management (DPPM) that guarantees system instant-on with discharged or disconnected battery.
  • Low FET RDS(on) for both the battery and system path to guarantee acceptable overall system efficiency and thermal management.
  • High charge current to support high-capacity battery packs and shorten the charge time.
  • Input-voltage dynamic power manage-ment (DPM) that supports the limitation of any adapter and/or USB port.
avnet-august-2015-fig1 avnet-august-2015-fig2 avnet-august-2015-fig3

Fig. 1: Input power
needed for different
charge currents.

Fig. 2: Charge cycle
of a legacy charger
without time-
optimiser technology.

Fig. 3: Time-optimised
charge cycle for
a switch-mode
Li-Ion battery charger.

Compact single-cell charger applications

Power requirements (adapter limitation)

Currently, most smartphone adapters are specified for 5 W to 10 W maximum output power. Fig. 1 shows the input power needed from the USB port or adapter for different charging current levels. For a 1,5 A charge current, the required power increases linearly from 3 W to 5 W as the battery voltage increases from 3 V to full charge. For a 3 A charge rate, up to 12 W is needed from the input during the charge cycle.

In this scenario, depending on the battery’s state of charge, a 5 W or 10 W adapter can crash and the system collapses.

To prevent this from happening, the charger is required to have some kind of protection to reduce the power drawn from the input.

A battery charger such as the bq24250 has dynamic power management (DPM) that monitors input voltage (VIN_DPM). During the normal charging process, if the input power source is not able to support the programmed or default charging current, the input voltage decreases. If the input voltage drops to the VIN_DPM threshold set by the designer, the charge current is reduced. This limits the power drawn from the input supply and prevents further drop of the input voltage.

This feature ensures IC compatibility with adapters that have different current capabilities without any hardware change.

Charge time

As described earlier, charge time depends on the battery capacity and charge rate. The easiest way to decrease charge time is to charge at a faster rate. However, charging a battery with higher than 80% (0,8C) of the battery’s full capacity causes stress on the battery. This decreases its lifetime or possibly damages the pack with catastrophic results. Texas Instruments has developed charge-time optimization of charge cycles to reduce charge time for a given charge rate compared to other solutions.

The charge cycle of Li-Ion batteries is mainly composed of three phases: pre-charger (trickle), fast charge (constant current), and taper (constant voltage). The transition between one phase to another is not ideal for many switch-mode chargers. Fig. 2 highlights the phase transition from constant current to constant voltage in a legacy charger circuit. Both voltage and current do not have a sharp transition. This behaviour causes both time and power loses during the charge cycle.

The Li-Ion battery charger improves this transition using the time-optimiser technology.

Fig. 3 shows a charge cycle of the same battery and under the same charging conditions as in Fig. 2. The charge time. is reduced by more than 15%. The transition is much sharper on the new charger, which spends more time in the fast-charge (CC) phase before transitioning into the taper (CV) phase. This puts more Coulombs into the pack at a faster rate, thereby reducing the charge time without increasing the charge rate.

Fig. 4: DPPM charger application area for different applications.

Fig. 4: DPPM charger application area for different applications.

Board size and BOM cost

For higher charge rates, linear chargers become less attractive. Their reduced efficiency over the charge cycle increases thermal loading on the system. This is especially true in size-constrained boards and high-power applications. These conditions drive the requirement for a fully integrated switch-mode charger. Vendors such as Texas Instrument are pushing the envelope of innovation to meet market demand by reducing the BOM cost and board space without sacrificing device performance. For example, the bq24250 is a highly-integrated single-cell Li-Ion battery charger and system power-path management IC targeted for space-limited, portable applications with high-capacity batteries. Fig. 4 shows a range of devices with actual application area size. For instance, the bq2425x family of chargers can provide a charge current of up to 2 A, an economical BOM, and a 42 mm2 application area.

Thermal performance and efficiency

Reducing the size of the charger area affects the thermal performance of the whole board. Less available area results in less space to dissipate the heat caused by the power dissipated during charging. For a given board area, the only way to reduce thermal loading is to improve charger efficiency during power conversion. Higher efficiency results in lower power dissipation. Thus, less heat is generated from the IC and the board.

avnet-august-2015-fig5 avnet-august-2015-fig6
Fig. 5: Comparison of power
dissipation between a linear
versus switch-mode charger.
Fig. 6: System efficiency of
the bq24250 Li-Ion battery
charger 4,2 V regulation.

When comparing power dissipation between linear and switch-mode chargers in higher power applications, linear chargers becomes less desirable as the power dissipated can be very high – especially for lower battery voltages. This is because the linear chargers use a linear regulator to do the power conversion. On the other hand, switch-mode charging is much more efficient over the entire battery voltage range and results in less power dissipation. Fig. 5 shows a comparison in power dissipation between linear and switch-mode chargers.

Choosing a switch-mode charger over a linear charger is a logical choice to improve charger thermal dissipation on the board. Lowering the RDS(on) of the integrated FET inside the switch charger helps improve charger efficiency at high currents. This is because most power dissipation at higher currents for a switch charger is caused by the FET’s RDS(on). The Li-Ion battery charger has integrated power FETs with low RDS(on). Internal high-side and low-side MOSFETs are rated to only 100 mW each.

This helps reduce power dissipation from the input to system output. The RDS(on) of the FET switch to the battery is only 20 mW. This also helps reduce losses during battery charging and discharging. Fig. 6 provides system efficiency data for the battery charger, which can be as much as 95%.

Battery protection and battery-life extension

A major issue with the high-power portable electronics is battery life cycle. The reduction of battery capacity over time greatly impacts the user’s experience by reducing runtime. The main contributor to improve the battery pack life cycle is to reduce stress during charge and discharge. Li-Ion batteries are very sensitive to stresses caused by overcurrent or overvoltage on the pack.
Battery-charger ICs such as the bq24250 can regulate the battery voltage with ±0,5% accuracy in room temperature. For charge current, this IC provides ±0,75% accuracy for up to 2 A charge current over the 0 to 125°C temperature range. This accuracy allows designers to precisely program the voltage and current level according to the application needs. With these accurate charging parame- ters, batteries can be charged more aggressively without reducing the life cycle. Thus, charge time is reduced while maintaining a safe charging solution.

avnet-august-2015-fig7 avnet-august-2015-fig8
Fig. 7: Charge current accuracy
versus temperature.
Fig. 8: Battery leakage current in SYSOFF mode.

Fig. 7 shows the accuracy of three charge currents over temperatures ranging from 0°C to 125°C. For up to 1,5 A charge currents, accuracy is within 2% of the typical value shown in the datasheet.

System-off mode (SYSOFF)

During presale shipping and storage, the battery needs to be disconnected from the rest of the system to prevent depleting the battery. The charger has a SYSOFF mode that can be set to turn off the battery FET and disconnect the battery from the system. When the SYSOFF mode is used, the leakage current from the bat- tery into the IC is reducedto less than 1 µA (Fig. 8). The designer programs the system to automatically exit SYSOFF mode when the end customer plugs a power supply into the charger.

Application flexibility

In today’s highly competitive market, most players are constantly pursuing lower costs, which potentially can bring higher margins and greater competency.Being able to repurpose the same chip for various products or multiple generations has a direct cost savings for different system designs. It also shortens the application learning curve and avoids unnecessary risk by using a known working solution.

The market is pushing for a family of battery chargers that integrates several features to provide flexibility for different applications. One example is a charger with a wide input-voltage range so it is applicable for a broad range of adapters, which could potentially reduce inventory costs. The flexibility in charge currents can support the higher current for applications like power banks and smartphones as well as low-level charging to applications like Bluetooth headsets.

Many chargers provide two chip-control schemes: I2C communicationand standalone. This allows the tailoring each application as needed. In I2C mode, designers can program various parameters such as VIN_DPM threshold, charge current, input current limit, regulation voltage, and termination level. When operating in standalone mode, where the host control is not desired, designers can use external settings to program the above parameters and utilise external pins to select different levels of inputcurrent limit and to enable/disable the chip.

The BC1.2-compliant, D+/D– USB detection feature offers greater flexibility for more robust USB charging. In the past, USB charging was very straight forward where the device took power directly from the USB port to the battery with little control. In today’s high-power applications, devices are requesting far more power from the USB port, which leads to more complicated standards and protocols being implemented. Furthermore, with the various USB standards normalised in the same USB port connector, the ability to recognise which type of device is connected is a very useful and competitive feature.

There are many options available for charging high-power portable devices. Currently available charging ICs that support power-path management and high charging current with improved efficiency can reduce charge time, thermal stress and solution size.A low-cost BOM and a small size solution bring the device cost down without sacrificing size and capability.


[1]  Texas Instruments. “Reference Design for 2A Single Input I2C/Standalone Switch-Mode Li-Ion Battery Charger,” Reference Design. Available:
[2]  Tahar Allag and James Smith, “Using bq2425x’sDPM Feature for Solar Panel Applications,” Application Report, July 2013. Available:
[3]  Tahar Allag and James Smith, “Selecting the Best bq2425x Single Cell I2C, Standalone Switch-Mode Battery Charger for your Application,” Application Report, June 2013. Available:

Contact Erich Nast,  Avnet South Africa, Tel 011 319-8600,

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