Lithium-ion battery charge and discharge safety and battery detection design

Lithium-ion battery charging safety of mobile phones is increasingly valued by consumers. Therefore, charger manufacturers must master the specifications and characteristics of lithium-ion batteries when designing products, and use charging chips with perfect battery detection and protection functions to reduce Danger due to conditions such as over current, over voltage or over temperature.

With the advancement of science and technology and the improvement of the quality of life, traces of electronic products are everywhere, and mobile phones are an indispensable necessity in human life. Whether it is the early Black King Kong phone or today's powerful smartphone, you need a power supply to operate.

There are two main types of batteries in early mobile phones, one is nickel-metal hydride and nickel-cadmium batteries, and the other is lithium-ion batteries. However, it is very rare to use nickel-hydrogen and nickel-cadmium batteries as power sources. Most of them are very rare. Lithium-ion batteries are used. In particular, consumers hope that mobile phones will have longer standby time and smaller size, so nickel-metal hydride and nickel-cadmium batteries have been slowly eliminated due to consumer expectations. Although nickel-metal hydride and nickel-cadmium batteries are superior to lithium-ion batteries in terms of price and replacement of batteries, nickel-hydrogen and nickel-cadmium batteries can still be seen in other electronic products; however, in terms of volume, weight and capacity, Nickel-metal hydride and nickel-cadmium batteries are not as good as lithium-ion batteries, so today's light and short electronic products are advertised, almost all using lithium-ion batteries.

Because of its powerful functions and high power consumption, the smart phone is a lithium-ion battery that requires large battery capacity and more durable power. When the battery of the mobile phone is low, the user usually charges the battery at any time with a charger or with a set of mobile power.

Volume/capacity combined with lithium-ion battery for electronic products

Rechargeable batteries can be divided into four categories according to their materials: lead-acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries and lithium-ion batteries.

Table 1 Comparison table of rechargeable batteries

From the advantages and disadvantages of Table 1, nickel-cadmium, nickel-hydrogen and lithium-ion batteries are more suitable for use in electronic products; lithium-ion batteries are superior to nickel-cadmium in volume, weight and capacity (time of use of electronic products). Nickel-metal hydride batteries also have no memory effect, so lithium-ion batteries seem to be much more convenient for use in electronic products.

Extend the service life of lithium-ion battery charging/discharging voltage

In general, lithium-ion batteries have a limited range of electrical safety. Due to the characteristics of the lithium-ion battery, when the battery voltage rises to the highest set voltage during charging, the charging should be stopped immediately to avoid the danger of the battery being damaged due to over-charging; when the battery is powered (discharged), the battery voltage is reduced. Discharge should be stopped below the minimum set voltage to avoid reducing the service life due to overdischarge.

In addition, in order to ensure the safety of battery use, lithium-ion batteries must also be equipped with short-circuit protection to avoid danger; even if most lithium-ion batteries have protective circuits installed, choose a good charger or mobile power supply. This is still an important consideration.

Lithium-ion battery charger is the first safety

A charger is a device that charges a battery to its rated voltage, and a lithium-ion battery charger must have the following features:

1) can provide a fixed current to the rechargeable battery

When the battery voltage reaches the maximum value and no longer rises, its charging current will start to drop. This will avoid overcharging the battery and causing battery damage. When the charging current drops to a certain level, the charger will stop charging.

2) Make sure the battery has a usable voltage

After the battery is fully charged, if it is left for a long time, it will naturally discharge. To avoid excessive battery self-discharge, the battery voltage will drop. When the battery voltage is lower than the set voltage, the charger will restart charging the battery. To ensure that the battery can maintain a certain voltage while in use.

Four-stage charging brief

The following is an introduction to Lithium-Ion battery charging using the charging integrated circuit (IC) of Pai Heng Semiconductor - AIC6511. Figure 1 shows the charging curve of the lithium-ion battery:

Figure 1 Lithium-ion battery charging curve

1) Trickle Charge or Pre-Charge

At this time, the lithium ion battery voltage is less than 3 volts (V). When the charger starts to charge the battery, the internal impedance of the lithium ion battery will be large due to the characteristics of the lithium ion battery, so the charger will first charge the battery with a small current. At this time, the battery voltage continues to rise.

2) Constant Current Charge (CC Charge)

When the battery charging voltage rises to about 3 volts, the charger changes the battery to a constant current charging with a maximum charging current, at which time the battery voltage continues to rise.

3) Constant Voltage Charge (CV Charge)

When the battery charging voltage rises to approximately 4.2 volts near the saturation point voltage of the lithium ion battery, the charger changes the battery in a constant voltage mode, at which time the charging current begins to drop.

4) Charge Full

When the charging current drops to a small current, the charger stops charging the battery.

After the battery is completed, if it is left for a long time, it will naturally discharge. If the battery voltage drops due to excessive battery discharge, the power IC will restart the battery when the lithium-ion battery voltage drops to 4 volts. The Charge mode is charged to ensure that the battery maintains a certain voltage while in use.

Lithium-ion battery charging cycle

Due to the characteristics of the lithium-ion battery, if the lithium-ion battery is deeply discharged before charging, the charger will first perform Pre-Charge charging with the weak current (the setting values of different manufacturers are different. The set value is about 10% of the maximum charging current), and the battery voltage rises during charging.

When the battery voltage rises to about 3 volts, the charger changes the battery to CC Charge with the maximum charging current, and the battery voltage continues to rise.

When the battery charging voltage rises to approximately 4.2 volts near the saturation point voltage of the lithium ion battery, the charger changes the battery with CV Charge, and the charging current begins to decrease. When the charging current drops to approximately equal to the Pre-Charge current, The charger stops charging the battery, that is, it completes charging.

Whether charging the battery with a universal serial bus (USB) or AC power converter (Adapter) input power, when the battery starts charging, if the charging time exceeds its set time, the charger still operates in Pre-Charge mode without entering In the CC Charge mode, or still operating in the CC/CV Charge mode without entering the charging completion state, the charging timer protection function of the IC stops the charger from charging the battery.

Charge timing protection ensures battery safety

Figure 2 is a schematic diagram of the pin of the example charging IC. The charging timing protection time is set by the capacitance CTMR of the external TMR pin (Pin 15) of the IC. 

If the battery is in the charging state, the charging time has exceeded the charging time protection time set by the user, but the charger has not yet deviated from the current charging state or ended charging, then the charging and timing protection function of the IC will start immediately. Force the charger to stop charging the battery (Figure 3). At this time, the STAT1 (Pin 12) level is High and the LED1 indicator is off (Figure 4). If the TMR (Pin 15) pin is connected to GND (Pin) 6) The pin position can be used to release the charge timer protection function.

Figure 3 Whether the charger correctly detects the battery charging situation is critical to the safety of use.

When the input power is reset and the EN signal is triggered, the charge timing protection time can be released and re-timed.

Charging indication status

In Figure 4, STAT1 (Pin 12) and STAT2 (Pin 13) are internally two Open-Drain N-type metal-oxide-semiconductor (NMOS) switches, which must be connected to VREF33 pin (Pin 7) or other Pull-Up The bias voltage supply of the resistor is connected, and its operation is shown in Table 2.

Figure 4 AIC6511 typical application circuit

Table 2 Charging indication status table

Input power detection prevents battery leakage current from flowing back

The battery can be charged by two different input power sources, AC Adapter or USB. If the AC Adapter and USB power supply are connected at the same time, the IC internal switch will preferentially select the AC Adapter terminal as the input power of the charger; however, this should be avoided.

In Figure 4, the Adapter power supply for the general socket is input at the VIN pin (Pin 2). In the ACIN charging mode, the battery can be charged with a charging current of up to 2 amps (A). The maximum charging current is set by the RS1 resistor.

The USBIN pin (Pin 5) is for USB power input. When USBIN charging mode is selected, its input limit current is set by RILIM resistor, setting 500 mA (mA) for USB 2.0, 900 mA for USB 3.0.

When using USBIN mode, the CC Charge current will vary with different input voltages and battery voltages. By detecting the current flowing through the RS1 resistor during CC Charge, the fixed input limit current IUSB_LIM is adjusted. During charging, if AC AC Adapter is used And the USB power supply is removed, the IC internal switch will be turned off and the anti-backflow protection function is activated to prevent the battery leakage current from being reversely poured back to the input power terminal.