At present, the portable device designers present many challenges to the system power supply problems, using batteries as primary power is more and more popular, so system designers need to design highly sophisticated systems, fully exploiting all potential battery. Each application is different, but one thing is the same: full use of the capacity of the battery as much as possible, the objectives and how best to directly related to charge the rechargeable battery. In order to design appropriate and reliable battery charging system, battery needs to charge characteristics and requires an in-depth understanding of the application itself, each method has its advantages and disadvantages, specific applications and their requirements are the key factors determining which method is best for.
Charging system is often not in the design is too attention, especially in cost-sensitive applications. However, the charge system for battery life and the quality of reliability is critical. This article discusses lithium-ion battery charging basics, and Nikon MH-65 Battery Charger discussed specific linear and charging solutions based on single-chip microcomputer switch solutions to Microchip's linear charge management controller and MCP73843 and MCP73861 and MCP1630 PIC16F684 microcontroller pulse width modulator (PWM) discussion as an example.
Lithium-ion rechargeable
Charge or discharge rate, usually based on the battery capacity. This speed is called the c-rate. C discharge current rate equal to the charge or under certain conditions, defined as follows:
I=M×Cn
Where:
I= charging or discharging current, a,
M=C multiples or fractions
C= value of the rated capacity, Ah
N = number of hours (corresponding to c).
At 1 time times c rate discharge batteries of nominal rated capacity will be released within one hour. For example, if the nominal capacity is 1000mAhr, then the discharge rate of 1C corresponds to the discharge current of 1000mA, C/10 rate of discharge current of 100mA.
Usually when the battery capacity is n=5 of the trademark, or 5-hour discharge capacity. For example, the above 200mA battery constant current discharging can provide 5 hours of work time. Theoretically the 1000mA battery constant current discharging can provide 1 hour of work time. But for all practical purposes because the batteries discharge time will reduce, in which case the work time will be less than 1 hour.
How to properly charge the lithium ion battery? Most lithium-ion batteries for charging process can be divided into four stages: trickle charging, constant current and constant voltage charging charging and charging termination. Refer to figure 1.
Stage 1: trickle charge – trickle charge to fully discharge the battery before priming the unit (recovery charge). When the battery voltage falls below 3V, using maximum of 0.1C is constant current to charge the battery.
Stage 2: constant current charge-when the battery voltage up to trickle charge when above a threshold, increase charge current constant current charging. Constant current charging current of between 0.2C nm. Constant current charging current does not require very precise, constant current, too. In the design of linear charger, current rise in current as the battery voltage rises, to minimize the transmission on the transistor heat sink problem.
Constant current charging and not greater than 1 c shorten the full charging cycle time, so this approach is not desirable. When high current charge, because the electrode reactions, as well as on the internal impedance of the battery voltage to Panasonic Lumix DMC-FZ28 Battery Charger rise, the battery voltage will rise more quickly. Constant current charging period will be shorter, but because of the constant pressure below the charge time will increase, therefore does not reduce the total charge cycles of time.
Stage 3: constant voltage charge-when the battery voltage up to 4.2V, the constant-current charging the end, begin constant voltage charge. For best performance, constant voltage tolerance should be superior to + 1%.
Stage 4: charge termination--and Ni-MH batteries are different, not recommended for continuous trickle charging lithium-ion batteries. Continuous trickle charging can lead to lithium metal plate plating effect occurs. This will make the battery is not stable, and could lead to automatic rapid disintegration of all of a sudden.
There are two kinds of charge termination method: use timer or using minimum charging current judgment (or a combination). Minimum charging current of the current monitoring and constant pressure by the charge, and in reducing the charge current to charge termination when the 0.02C nm range. Second method from the constant pressure at the beginning of the charge time, stop the charging process after charging for two hours.
This four stage charging process to complete to fully discharge the battery charging takes about 2.5 to 3 hours. Advanced charger has also adopted additional security measures. For example, if the battery temperature exceeds the specified window (usually 0 ° c to 45 ° c), then the charge will be suspended.
Lithium-ion rechargeable--system considerations
To quickly and reliably through the charging process requires a high performance charging system. To achieve reliable and cost-efficient solutions, design should take into account the following parameters:
Input source
Many applications use very cheap in-wall power adapter as input. The output voltage depends largely on the AC input voltage and load current outflow from the wall adapter.
In the United States on a standard wall outlet AC bus input voltage range of 90VRMS nm. Assuming the input voltage 120VRMS, tolerance to +10%, – 25%. Voltage of the battery charger must be provided with adequate measures, which is not influenced by the input voltage. Input voltage charger and AC bus voltage and charging current is proportional to:
VO=2VIN×a-1O(REQ+RPTC)-2×VFD
REQ is a reflection of primary winding resistance and resistance of the secondary winding (RP/a2). RPTC is the resistance of the PTC, VFD is the forward voltage drop of the bridge rectifier. In addition loss of transformer cores will also slightly lower output voltage.
Car charging adapter application will encounter similar problems. Car adapter of the typical range of output voltage from 9V to 18V.
Constant current charging rate and accuracy
Specific applications of topology selection may be decided by the charging current. Many large constant-current charging or battery charging application switch charging solutions to be implemented to achieve higher efficiency and to avoid excessive heat. Due to size and cost considerations, low end and mid-range fast charge applications tend to JVC BN-VF707U Battery Charger use linear solutions, however the linear solution is more energy in the form of heat loss. Charging system for linear, constant-current charge tolerance has become extremely important. If voltage tolerance is too large to transfer transistors and other components require a greater volume, increasing the size and cost. In addition, if the constant-current charging current is too small, the entire charging period will be extended.
Stability of the output voltage accuracy
In order to take full advantage of battery capacity, output voltage stabilizing precision is key. Small drop in output voltage accuracy will result in significantly reduced the capacity of the battery. For security and reliability considerations, however, the output voltage is not free to set too high. Figure 2 shows the importance of stable output voltage accuracy.
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