Basics about cell phone batteries

Lithium batteries are widely used in portable consumer electronic devices. Most cellphone batteries are lithium batteries, which have a high energy density, small memory effect and only a slow loss of charge when not in use. Lithium ions move from the negative electrode to the positive electrode during discharge and back when charging.

Generally speaking, Li-on batteries consist of a protective circuit board, battery cell, Al-plastic film wrapping outside and other films inside.

To make sure our batteries are safe before the sale, there are many steps that need to be tested for a good quality battery, such as a visual inspection, functionality test, and reliability test.

1.     Visual Inspection:

aCheck whether the battery is swollen or not.

b)Check whether the battery is worn or has scratches on it and if the logo has disappeared or not.

c)Check if the connectors have signs of corrosion and if there is evidence of effusion.

d)Check if the contacts on the connector and board are shining and no residues are present. Meanwhile, check if the contacts are soldered properly on the connector or on the protective circuit board.

2.     Functionality test:

a)Check if the battery‘s internal resistance is under 150mΩ

b)Check if the battery voltage is above 3.8V

c)Check if battery overcurrent is consistently between 2A and 6A for 2G and 3G phones, but for 4G phones, the battery overcurrent will surpass 5.5A.

d)Check battery capacity: after charging the battery with a constant current and constant voltage, then allow it to rest for 10 minutes and discharge the battery with a constant current and constant voltage discharge until the protective circuit cuts it off. Usually, the discharge time should be longer than 5 hours under 0.2C current or it should be longer than 51 minutes under 1C.

*C means Capacity, if a battery has 3000mhA capacity, then the current 0.2C should be 6A. i.e., the discharge time should be longer than 5 hours with 6A current.

3.     Reliability test:

a)Overcharge protection test: after being fully charged a power source is used with a 7.4V and 2C output current to charge the battery for 8 hours consistently. The protective circuit should cut off the battery, and the battery shouldn’t deform or catch on fire, otherwise, the battery will be denied.

b)Short-Circuit protection test: after a full charge, a 1.5mm wire, its resistance is under 50mΩ, is used to connect the positive electrode and negative electrode for an hour. If the battery doesn’t get damaged or burst and still functions well, then it passes.

c)Over-discharge protection test: when a battery is discharged over a long period of time its internal voltage should stay under 2.75V to protect the battery from damage. If the voltage surpasses 2.75V the protective circuit should cut power to protect the battery.

d)High temperature and low-temperature test: A fully-charged battery is heated at high temperatures around 70℃ for 24 hours, then checked to see if the battery will be ok after 2 hours at room temperature. The battery is also tested to see if it will be ok after sitting at -40℃ temperatures for 24 hours, and then checked again after sitting for 2 hours at room temperature.