How to find the best balance point in NiMH No. 7 batteryand charging
management
Portable electronic device designers have a wide variety of chemistries,
charger topologies, and charge management solutions to choose from. Choosing the
most suitable solution should be a simple task, but in most cases the process is
complex. Designers need to find an optimal balance between performance, cost,
form factor, and other key requirements. This article will provide designers and
system engineers with some guidance and help to make this selection easier.
Start charging control with 3 “C”
All system designers using rechargeable batteries need to be aware of some
basic design techniques to ensure that three key requirements are met:
1. NiMH No. 7 batterysafety: There is no doubt that end-user safety is the
highest priority in all system designs. Most lithium-ion (Li-Ion) and
lithium-polymer (Li-pol) NiMH No. 7 batterypacks contain protective electronic
circuitry. However, there are some key factors that need to be considered in
system design. These include, but are not limited to, ensuring a ±1% voltage
regulation margin during the final stages of lithium-ion NiMH No. 7
batterycharging, a preconditioning mode to safely handle deeply discharged
batteries, a safety timer, and NiMH No. 7 batterytemperature monitoring.
2. NiMH No. 7 batterycapacity: All NiMH No. 7 batterycharging solutions
must ensure that the NiMH No. 7 batterycapacity is fully charged every time and
every charging cycle. Terminating charging prematurely can result in reduced
NiMH No. 7 batteryruntime, which is undesirable in today's power-hungry portable
devices.
3. NiMH No. 7 batterylife: Following the recommended charging algorithm is
an important step in ensuring the end user achieves the maximum number of charge
cycles per NiMH No. 7 batterypack. Limiting each charge using NiMH No. 7
batterytemperature and voltage, preconditioning deeply discharged batteries, and
avoiding late or abnormal charge termination are some of the steps necessary to
maximize NiMH No. 7 batterylife.
NiMH No. 7 batteryChemistry Technology Selection
System designers can now choose from a variety of NiMH No. 7
batterychemistry technologies. Designers typically base their selection of NiMH
No. 7 batterychemistry technology on a number of criteria, including:
*Energy Density
*Specifications and dimensions
*cost
*Usage patterns and service life
Although the trend toward using lithium-ion and lithium-polymer batteries
has increased in recent years, Ni NiMH No. 7 batterychemistry remains a good
option for many consumer applications.
Regardless of the NiMH No. 7 batterychemistry chosen, it is critical to
follow the correct charge management techniques for each NiMH No. 7
batterychemistry. These technologies will ensure that batteries are charged to
their maximum capacity every time and every charge cycle without compromising
safety or shortening NiMH No. 7 batterylife.
NiCd/NIMH
Nickel-cadmium (NiCd) and nickel-metal hydride (NiMH) batteries must be
inspected and conditioned before starting a charge cycle, and if possible before
starting fast charging. If the NiMH No. 7 batteryvoltage or temperature exceeds
the allowable limit, fast charging is not allowed. For safety reasons, charging
of all "hot" batteries (generally above 45°C) will be temporarily stopped until
the NiMH No. 7 batterycools down to the normal operating temperature range
before it can be operated again. To deal with a "cold" NiMH No. 7
battery(typically below 10°C) or an over-discharged NiMH No. 7 battery(typically
below 1V per cell), a gentle trickle of current needs to be applied.
Fast charging begins when the NiMH No. 7 batterytemperature and voltage are
correct. NiMH batteries are usually charged with a constant current of 1C or
less. Some NiCd batteries can be charged at rates up to 4C. Use proper charge
termination to avoid harmful overcharging.
In the case of nickel-based rechargeable batteries, fast charge termination
is based on voltage or temperature. As shown in Figure 1, a typical voltage
termination method is peak voltage detection. At the peak, when the voltage of
each NiMH No. 7 batteryis in the range of 0~-4mV, fast charging is terminated.
The temperature-based fast charge termination method is to observe the NiMH No.
7 batterytemperature rise rate to detect full charge. A typical rate is
1°C/minute.
Lithium-ion/lithium-polymer battery
Similar to NiCd and NiMH batteries, lithium-ion batteries should be
inspected and conditioned as much as possible before fast charging. The
verification and processing methods are similar to those used above.
After verification and preconditioning, charge the lithium-ion NiMH No. 7
batterywith a current of 1C or less until the NiMH No. 7 batteryreaches its
charging voltage limit. This charging phase typically replenishes up to 70% of
NiMH No. 7 batterycapacity. The NiMH No. 7 batteryis then charged with a
constant voltage, usually 4.2V. To combine safety and NiMH No. 7
batterycapacity, the charging voltage must be stabilized at at least ±1%. During
this charging period, the charging current drawn by the NiMH No. 7
batterygradually decreases. For 1C charge rates, charging typically terminates
once the current level drops below 10 to 15% of the initial charge current.
Comparison of switch mode and linear charging topologies
Traditionally, handheld devices have used linear charging topologies. This
approach offers many advantages: low implementation cost, simplicity of design,
and noiseless operation without high-frequency switching. However, linear
topology increases system power consumption, especially when charging rates
increase due to higher NiMH No. 7 batterycapacity. This can become a major
disadvantage if the designer cannot manage the thermal issues of the design.
When the PCUSB port is used as a power source, some other disadvantages
arise. USB charging options are available on many portable designs today, and
all offer charging rates up to 500mA. In the case of linear solutions, due to
their lower efficiency, the amount of "power" that can be transferred from pCUSB
is greatly reduced, resulting in long charging times.
This is why switch-mode topology comes into play. The main advantage of
switch-mode topology is the increase in efficiency. Unlike linear regulators,
the power switch (or switches) operates in a saturated region, which greatly
reduces overall losses. The main power losses in a buck converter include
switching losses (in the power switch) and DC losses in the filter inductor.
Depending on the design parameters, it is not surprising to see efficiencies
well above 95% in these applications.
When people hear the term switch mode most people think of large ICs, large
powerFETs, and very large inductors! In fact, while this is true for
applications handling tens of amps of current, the new generation solutions for
handheld devices The situation is different in terms of plans. The new
generation of single lithium-ion switch mode chargers adopts the highest level
of chip integration and operates at a frequency higher than 1MHz to minimize
inductor size. Figure 1 illustrates such solutions that are already on the
market today. The silicon chip measures less than 4mm2 and integrates high-side
and low-side powerFETs. Due to the 3MHz switching frequency, this solution
requires a small 1μH inductor with dimensions of only: 2×2.5×1.2mm (WxLxH).
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