In-depth analysis of T BOX system solution: charging and discharging management

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In-depth analysis of T BOX system solution: charging and discharging management [Copy link]

 

Section 2 Charge and Discharge Management

Under normal circumstances, VBAT will charge the backup battery while supplying power to the load; when encountering an emergency (such as a car crash), VBAT cannot supply power normally, and the backup battery will be used to supply power to the load. As shown in the blue shaded box in the figure below, the charge and discharge management of the backup battery is mainly divided into three parts: battery pack, charger, and pre-boost.

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Figure-1

1. Battery Pack

Commonly used batteries include Ni-MH, LiFePO4 and Li-Lon. Their characteristics are listed in the following table.

Type	Ni-MH	LiFePO4	Li-Lon
Advantages	Reliable and durable; Excessive charging will not produce high temperature; Low cost;	left]High rated current; Long life; High thermal stability; Safe;	High energy density; High voltage (3.6V); Long life;
Disadvantages	Fast self-discharge, resulting in frequent charging; Low voltage (1.2V), resulting in large size;	Low voltage (3.2V); High self-discharge, leading to balance problems and aging;	Fragile, transportation requires protection circuit; Peak current needs to be limited when charging; Temperature needs to be monitored; Expensive;

Table-1

Ni-MH and Li-ion are commonly used in the automotive field. Ni-MH is relatively safe and reliable, and will not generate high temperatures even if overcharged. Li-ion can achieve higher energy density in a smaller volume and at a lower price. The world's leading automakers have already applied lithium-ion batteries to E-Call.

2. Charger

1) Solution for lithium batteries

The charging process of lithium batteries can be divided into three steps: when the voltage is less than 3V, pre-charge is performed first with a current of 0.1C; when the voltage rises to 3V to 4.2V, constant current charging is adopted with a current of 0.2-1.0C; when the voltage is greater than 4.2V, constant voltage power supply is adopted. At this time, the current decreases with the increase of voltage until the battery is fully charged. TI's main device is the BQ24081-Q1, which has the following advantages:

• Highly integrated linear charging device, integrating power FET and current sensor, can provide high-precision current and voltage regulation and read charging status;

• Charging current can be adjusted by setting external resistors;

• Battery capacity, cycle period and safety can be maximized;

• Very suitable for low voltage difference charging applications;

• Sleep mode can be provided to reduce power consumption;

• Small package, 3mm × 3mm VSON10.

2) Solutions for NiMH Batteries

The charging process of NiMH batteries can also be divided into three steps: first, fast charging (Fast Charging) to restore most of the capacity; then, optional top-off charging (Optional Top-off Charge) is used to restore all the capacity to complete the charging; finally, continuous pulse trickle charge (Pulse-Trickle Charge) is provided to maintain the charging, thereby compensating for the self-discharge of the battery. The end point control during the charging process is very important, and the charging can be terminated by detecting the voltage drop (-), detecting the voltage peak, and detecting the maximum temperature. TI has a charging management solution specifically for NiMH batteries, such as BQ2002. However, this method is more expensive.

When the cost is limited, LDO can also be used to directly charge NiMH batteries. Because LDO has a maximum output current limit, this will not cause the current to exceed the standard. TI's main device is TPS79801-Q1, which has the following advantages:

• Wide input voltage range: 3V to 50V;

• No input protection diode is required;

• Output current reaches 50mA;

• left]Low voltage drop, typical value is 300mV.

3) Solution for lithium iron phosphate battery

Lithium iron phosphate battery is a lithium-ion battery that uses lithium iron phosphate (LiFePO4) material as the positive electrode of the battery. It is a new member of the lithium-ion battery family. Because its performance is particularly suitable for power applications, it is also called lithium iron phosphate power battery.

Its charging algorithm is different from that of Li-Lon battery: first, the battery is quickly charged to the overcharge voltage, and then slowly drops to a lower floating charge voltage threshold. The charger integrates sensors to achieve higher accuracy in the current and voltage regulation loop. The internal control loop also monitors the chip junction temperature through the charging cycle. If the internal temperature threshold is exceeded, the charging current will be reduced. The recommended chip is BQ25071-Q1. 3. Boost circuit Since the voltage output from the battery pack is low (such as 4.2V for Li-ion), a pre-boost chip is usually required to boost the voltage to 5V for the subsequent secondary power supply. The TPS61085-Q1 recommended by TI is an automotive 18.5V, 2A, 650kHz/1.2MHz boost converter with forced PWM mode.