Force Discharge Testing Data Analysis,
Force Discharge,

▍Certification Overview

Standards and Certification Document

Test standard: GB31241-2014: Lithium ion cells and batteries used in portable electronic equipment―Safety requirements
Certification document: CQC11-464112-2015: Secondary Battery and Battery Pack Safety Certification Rules for Portable Electronic Devices

Background and Date of implementation

1. GB31241-2014 was published on December 5^th, 2014;

2. GB31241-2014 was mandatorily implemented on August 1^st, 2015. ;

3. On October 15th, 2015, Certification and Accreditation Administration issued a technical resolution on additional testing standard GB31241 for key component “battery” of the audio and video equipment, information technology equipment and telecom terminal equipment. The resolution stipulates that the lithium batteries used in the above products need to be randomly tested as per GB31241-2014, or obtain a separate certification. 

Note: GB 31241-2014 is a national compulsory standard. All the lithium battery products sold in China shall conform to GB31241 standard. This standard will be used in new sampling schemes for national, provincial and local random inspection.

▍Scope of Certification

GB31241-2014 Lithium ion cells and batteries used in portable electronic equipment―Safety requirements
Certification documents is mainly for mobile electronic products that are scheduled to be less than 18kg and can be often carried by users. The main examples are as follows. The portable electronic products listed below do not include all products, so products not listed are not necessarily outside the scope of this standard.

Wearable equipment: Lithium-ion batteries and battery packs used in equipment need to meet standard requirements.

▍Why MCM?

● Qualification recognition: MCM is a CQC accredited contract laboratory and a CESI accredited laboratory. The test report issued can be directly applied for CQC or CESI certificate;

● Technical support: MCM has ample GB31241 testing equipment and is equipped with more than 10 professional technicians to conduct in-depth research on testing technology, certification, factory audit and other processes, which can provide more accurate and customized GB 31241 certification services for global clients.

Force discharge testing is an item for testing safety of discharge. Normally the tested cell will be discharged in 1 ItA for 90 min. Figure 1 is a chart of force discharge testing from a kind of lithium-ion battery cell. Unlike normal ideal model (as shown in figure 2), the voltage and current fluctuate. We therefore try to analyze the principal behind the chart.
According to the trend of voltage, we can separate the discharge process into three stages. In the first stage, the voltage drops down from 3V to 0.65V. In the second stage, the voltage is around 0.65V to 0.5V. The voltage stops dropping, and there are fluctuations. In the third stage, voltage drops to 0V, and no fluctuation. Here the voltage refers to the potential difference between anode and cathode.
Voltage decreases with continuous discharge. This is because the potential of negative pole goes higher and that of positive pole gets lower, and li-ion runs out from negative pole to positive pole. As the test uses 1C current, the voltage drops down quickly. The SEI film may be broken down in this process, generating gas and heat.
Negative pole potential increases until an over potential that will causes copper foil dissolve. As there’s carbon coat, a higher over potential is needed to trigger copper foil dissolve, due to cu-ion transferring electric charge. In the negative pole, the copper foil is dissolved and oxidized to Cu+ and then Cu2+ and these cu-ion penetrate through separate film to positive pole, with Cu2+ redox to Cu+, and then redox to copper, depositing on the positive pole.