The side voltage of the battery specifically refers to the voltage of the aluminum layer between the cathode tab and the aluminum laminated film of the polymer battery.

The side voltage of the polymer lithium battery refers to:

1.The voltage of the aluminum layer between the cathode tab and the aluminum laminated film; 2. The voltage of the aluminum layer between the anode tab and the aluminum laminated film.
In theory, the aluminum layer between the cathode tab and the aluminum laminated film is insulated, which means that their voltage should be 0.
In fact, during the processing of the aluminum laminated film, the inner PP layer may be locally damaged, resulting in local conduction (including electronic channels and ionic channels) between them, forming a micro-battery and thus a potential difference (voltage).
The side voltage standards vary among manufacturers, but most of the industry sets it below 1.0V. The standard of voltage is based on the dissolution potential of aluminum-lithium alloy

Side voltage testing:

Side voltage testing is primarily used to inspect the sealing effect of lithium battery packaging films and detect short circuits between the tab and the aluminum laminated film of the packaging film. Short circuits can cause corrosion of the aluminum laminated film, electrolyte leakage, gas swelling, low voltage, and a series of other issues, posing safety hazards.

The side voltage of lithium polymer batteries specifically refers to the voltage across the aluminum layer between the positive tab and the aluminum-laminated film of a polymer lithium battery. In theory, the aluminum layer between the positive terminal and the aluminum-laminated film should be insulated, meaning that their voltage should be zero. However, during the processing of the aluminum-laminated film, the inner PP layer can suffer from localized damage, resulting in partial conduction (including both electronic and ionic channels) between them. This creates a micro-battery, leading to a potential difference (voltage). The side voltage standards vary across manufacturers, but the industry generally sets it below 1.0V. The basis for this voltage standard is derived from the dissolution potential of the aluminum-lithium alloy.

The potential difference between the positive tab and the aluminum-laminated shell is used to check if there are electronic channels between the negative tab and the aluminum-laminated film. If there are electronic channels between the negative tab and the aluminum-laminated film, and the inner PP layer of the aluminum-laminated film is damaged, corrosion may occur.

One of the reasons for gas swelling: packaging corrosion.

Gas swelling can be quite troublesome. Without effective detection methods, it's difficult to control defective products within the company and prevent them from reaching customers. The issue may manifest months later as gas swelling, leading to returns.

In such cases, the aluminum-laminated film of the battery cell has already been corroded. Moisture enters the battery through the corroded aluminum-laminated film and reacts with the electrolyte, causing gas swelling.

Why is it necessary to control side voltage?

If the inner PP layer of the aluminum-laminated film is damaged, corrosion and damage can occur.

Corrosion requires two conditions: 1) an electronic path, where the negative electrode and the aluminum layer of the aluminum-laminated film form an electronic path; 2) an ionic path, where the aluminum layer of the aluminum-laminated film and the electrolyte form an ionic path. Without either of these conditions, corrosion cannot occur.

When both conditions are met, lithium ions react with the aluminum layer of the aluminum-laminated film, forming a lithium-aluminum alloy. This lithium-aluminum alloy is a powdery substance that can puncture the aluminum-laminated film, resulting in the black spots often seen inside the film. These black spots become more prominent over time and with increasing charging and discharging cycles.

How to select?

Currently, the detected methods include:

1. Testing the resistance between the aluminum-laminated film and the negative tab. A resistance greater than 5M ohms is considered relatively safe. Some companies set lower standards, depending on the final ppm tolerance for defective products. You can calculate some data and define your own standards as well. This resistance test is primarily used to screen out electronic paths.
2. Testing the current between the positive electrode and the aluminum film, which can be done using a direct current source. Generally, if the current is greater than 0.001mA, it is considered a defective product and needs to be selected out.
3. Testing the voltage between the positive electrode and the aluminum film. Generally, if the voltage is greater than 1V, it is considered a defective product.

You can choose to combine methods 1 and 2, or 1 and 3 for testing.

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