Rechargeable battery and stand-alone fast charger design

Single chip microcomputer STM32L151CCU6

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Today, digital products are updated so fast, rechargeable batteries have become a standard power source for today's electronic products, especially portable devices: notebook computers, handheld navigation devices, mobile phones and so on. These products require more and more power, and the capacity of rechargeable batteries is far from meeting their needs. The main reason for the increase in power consumption is the enhancement of device functions. The increase in power consumption of portable devices makes rechargeable batteries cheaper and more environmentally friendly than standard batteries. This article will introduce a design for a rechargeable battery and a stand-alone fast charger.

Stand-alone NiMH battery fast charger The temperature and terminal voltage of a rechargeable Ni-MH battery gradually increase with the battery charge, and begin to drop after the battery is fully charged. Therefore, the main task of the NiMH battery charger is to detect this sudden change point and interrupt the charging, or switch from fast charging to trickle charging. In addition, continuous monitoring of temperature and voltage during charging provides system safety. The DS2711/DS2712 charger has the above features. In addition, they can work stand-alone and do not require a microcontroller or microprocessor to monitor. Designed for single-cell AA or AAA rechargeable batteries, this series is also suitable for two cells in series or in parallel. The DS2711 uses a linear control structure and the DS2712 uses a switch control structure. In order to maximize working hours and save battery power, these chargers have four charging modes: pre-charge, fast charge, float charge and trickle charge. In the floating mode, the charging rate is switched to a lower rate after the battery is fully charged.
In addition to the monitoring function, the DS2711/DS2712 charger also has an internal timer that sets the maximum charging time via an external resistor connected to the TMR pin to set the fast charging time to 0.5 to 10 hours. The float time has been set to half of the maximum charge time (0.25 to 5 hours). In the fast charging mode, if the maximum charging time is exceeded, the charger will switch from the fast charging mode to the floating charging mode while resetting the timer. The timer begins to time the float process, and if the predetermined float time is reached, the charger will switch from the float mode to the trickle mode.

VP1 and VP2 are used to monitor the voltage, and THM1 and THM2 are used together with the thermistor to monitor the temperature of the battery. TMR (timer) and RSNS (current-sense resistor) are used to set the charging time and charging current. Another feature of the DS2711/DS2712 is the ability to detect battery charging faults and alkaline primary batteries. If this happens, the charger will shut itself down.

The stand-alone lithium-ion battery fast charger is simpler than the nickel-hydrogen battery because it does not need to detect the voltage change rate (dV/dt). At the same time, because the lithium-ion battery is very sensitive to overcharging, the charger needs a precise 4.2V±50mV power supply to ensure constant power charging. As for nickel-metal hydride batteries, chargers require not only voltage monitoring, but also other monitoring functions (temperature, timing, etc.).
The stand-alone Li-Ion battery charger MAX8601 includes a so-called Vbatt controllable voltage source that provides 4.2V ± 0.021V at +25°C or 4.2V ± 0.034V at 40°C. When charging a Li-Ion battery through a VBATT connection, the charger can maintain a constant output power (Figure 5). The external resistor (connected to the SETI pin) and the external capacitor (connected to the CT pin) can set the charging current and internal timing. The charger also monitors the temperature of the battery through a negative temperature coefficient resistor.

The main advantage of the MAX8601 charger is the ability to charge the battery through an external adapter or USB port. The USB port can supply 100mA, 500mA according to the setting of the USEL pin. The chip automatically selects an external power source (primary adapter or USB). If both power supplies are present, it will select the primary adapter to charge. Any power supply must be able to provide a minimum voltage of 4.5V. The DS2711/DS2712 and MAX8601 are stand-alone chargers with multiple monitoring functions (voltage, current, temperature, timing, etc.) that do not require microcontroller monitoring or power surge protection, and provide clear, simple External switching.