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

The telematics control unit (TCU or T-BOX) is an embedded vehicle system that can be used in areas such as wireless tracking and communication of vehicles.

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.

Figure-1

1. Battery Pack

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

Table-1

In the automotive field, Ni-MH and Li-ion are commonly used. 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 is cheaper. The world's leading automobile manufacturers have already applied lithium-ion batteries to E-Call.

2. Charger

1) Solutions for lithium batteries

The process of charging a lithium battery 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 used, with a current of 0.2-1.0C; when the voltage is greater than 4.2V, constant voltage power supply is used, at which time the current decreases as the voltage increases until the battery is fully charged. TI's main device is BQ24081-Q1, with the following advantages:

Highly integrated linear charging device with integrated power FET and current sensor to provide high-precision current and voltage regulation and read charging status;

The charging current can be adjusted by setting an external resistor;

It can maximize battery capacity, cycle life and safety;

Very suitable for low voltage drop charging applications;

Can provide sleep mode 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, timed charging (Optional Top-off charge) to restore all the capacity to complete the charging; finally, by providing continuous pulse trickle (Pulse-Trickle Charge) to maintain 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, detecting the maximum temperature, etc. TI has a charging management solution specifically for NiMH batteries, such as BQ2002. However, this method is more expensive.

When cost is limited, LDO can also be used to directly charge NiMH batteries. Because LDO has a maximum output current limit, it will not cause the current to exceed the limit. 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 up to 50mA;

Low dropout voltage, 300mV typical.

3) Solutions for lithium iron phosphate batteries

Lithium iron phosphate battery is a lithium-ion battery that uses lithium iron phosphate (LiFePO4) 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 batteries: the battery is first quickly charged to the overcharge voltage, and then slowly dropped to a lower floating charge voltage threshold. The charger integrates sensors to achieve high 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 is 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.