Charge Mobiles in a Dash
Despite the monumental development in mobile phone batteries, charging it at least once a day is still not a bane we have been able to overcome. The need for quick charging is forcing manufacturers to offer fast charging support. Quick charging technology has existed for several generations of smartphones already and manufacturers are adopting them with gusto.
The first patent US3906166 for a mobile cellular phone filed by Dr. Martin Cooper was granted in 1975. However, they included bulky batteries with a shorter working life, which made them unpopular among users. NiCd batteries came into use in the 1980’s, but their low energy to mass ratio made them bulky too. In addition, they were toxic in nature and had ‘Memory Effect’ — an effect that reduces battery capacity when charged repeatedly after partial discharge.
NiCd batteries were succeeded by Nickel Metal Hydride batteries in the 1990’s. These fared better than NiCds but still had memory effect issues. Finally, Li-ion batteries came into existence in the later part of the 1990’s. These batteries were 30% less in weight than NiMH batteries, had no memory effect issues and were non-toxic. Even today[RPK(1] , Li-ion batteries are considered close to an ideal solution for cell phones as they are light in weight and can be moulded into different sizes and shapes.
Research for fast charging techniques started even before Li-ion batteries came into existence. Current fast charging techniques, including characteristics of Li-ion batteries, are improvements over previous research.
Li-ion batteries reach a peak voltage (i.e. about 70%) quickly but the speed slows down to reach the saturation charge. This became the basis of deeper research to build fast charging techniques. Dash Charging is one such technique that is employed in the smartphones of OnePlus. The technology charges the device up to 65% in 30 minutes. There are other techniques as well in the market such as Qualcomm’s Quickcharge and Samsung’s Adaptive fast charge. However, these techniques use high voltage for quick charging instead of Dash Charging’s use of high current and Oppo’s Voltage Open Loop Multi-step Constant-Current Charging (VOOC). In fact, Dash Charging is licensed from OPPO’s VOOC.
How does it work?
OnePlus Dash Charger is equipped with a thicker cable to manage high currents. The charger includes an MCU that protects the adapter from voltage overload and supports 5V/4A charging speed.
Oppo’s patent application US20180090977 illustrates a circuitry of a fast charging power adapter that uses high current. A bridge rectifier in the circuit rectifies AC input power. A transformer connected to the output terminal of the bridge rectifier acts as a step-down voltage converter. It is further connected to a diode that acts as a second rectifier.
To determine whether the device needs to be charged in normal mode or fast charging mode, the power adapter initially sends an inquiry signal to determine whether current at the battery is above a threshold for a predetermined period of time. If yes, then fast charging mode is enabled. A control circuit enables the fast charging mode by opening a switch that is connected in series to a capacitor. In the next stage, the control circuit checks whether the adapter Vout matches the current battery voltage. If not, then the control circuit adjusts the duty cycle of a PWM signal of the adapter Vout until it matches.
Now, the power adapter sends another inquiry signal to obtain maximum charging current supported by the battery. After obtaining the value, the control circuit adjusts the adapter output current to match that value. This adjustment is also conducted by adjusting the duty cycle of a PWM signal. The control circuit continuously monitors the sampled voltage, current output levels and present voltage, current levels of the battery.
Fast charging is primarily achieved by using a switch connected in a series to a capacitor. During the fast charging mode, the control circuit opens the switch. Hence, time constant decreases due to decrease in capacitance and thereby decreasing the time for charging the device. In normal charging mode, the switch is closed and the capacitor is included in the circuit, thereby increasing the capacitance and time constant.
Heat dissipating elements are present in the power adapter itself to ensure the battery isn’t overheated. This helps the device charge at a constant rate even during high load situations such as while playing games and streaming videos. However, the adapter gets heated up because of heat dissipation from these elements.
Recent developments in fast charging techniques include Oppo’s Super VOOC Flash Charge technology that charges the device fully in 15 minutes. Oppo had unveiled this technology at the MWC in the year 2016, which was deployed in a few of its models. Meizu Super mCharge has joined the race too to charge a device fully in 20 minutes.
In the second part of this series, our blogger will discuss and compare different popular fast charging techniques that are present in the market.
