How does overcharge relate to thermal runaway?

Damage is not the only cause of thermal runaway. Another potential cause is overcharge.

Safety note

Safety scientists test batteries and other products in order to understand what happens when a product fails. These tests are always performed in controlled laboratory settings. This is important in order to be certain that the test results are valid and the scientists are safe as they perform the tests.

Do not attempt to recreate these tests yourself: instead, this pathway will take you to the lab virtually. If you are interested in performing similar tests yourself, consider pursuing a career as a safety scientist!

Extreme Test

Abusive Overcharge

Safety scientists use a test known as the abusive overcharge test. This test determines whether protective components will prevent a battery from overcharging. It also determines whether a charger recognizes a battery’s chemistry so that it can charge the battery correctly.

Overcharge is when a cell or battery continues to receive current even after the cell has been fully charged. Most commonly this is due to a lack of communication between the cell and an incompatible charger.

Asset: The Overcharge Test

Well-made batteries and chargers are built with safety features to prevent overcharge. One of those features is a battery management system (BMS).

Battery Management System

Lithium-ion batteries, whether they are single cell or multi-cell, contain an electrical component called a battery management system (BMS)

The BMS is built to constantly monitor and control what voltage and current the battery receives from a charger. Some BMSs have thermal (temperature) control as well. The sophistication and capabilities of a BMS vary from battery to battery. If the BMS in a battery is not built correctly, then the lithium-ion cell can receive an inappropriate amount of current or voltage, triggering thermal runaway.

If the BMS in a battery is not built correctly, then the lithium-ion cell can receive an inappropriate amount of current or voltage, triggering thermal runaway. 

Overcharging a hoverboard

With an abundance of portable electrical devices comes an abundance of chargers. You may have noticed that many chargers look the same and even fit multiple devices. However, using the wrong charger for a device could lead to catastrophic results.

In this test, you’ll examine the impact of two different chargers on the battery system of a hoverboard. In the test below, the hoverboard battery has a BMS but no overcharge protection. 

Asset: Overcharge

When you selected the charger, you worked to match the correct voltage (volts) and current (amperes) between the charger and the device.

  • Voltage: Voltage is the amount of electrical force measured in a system. It is measured in volts (V).
  • Current: Current is the flow of electricity. It is measured in amperes (A), which is calculated as volts per ohm.
  • Internal Resistance: Internal resistance is a force in opposition to the electric current. Internal resistance reduces the rate of the current. It is measured in ohms (Ω). Various factors contribute to internal resistance.

Overcharge can also lead to thermal runaway over time. When a lithium-ion cell is charged past its capacity, the area at the anode that receives lithium ions ends up with too much lithium stored there. The extra lithium begins to form sharp, solid metal formations known as dendrites. These dendrites can eventually puncture the battery’s separator and create an internal short circuit that can lead to thermal runaway.


How do engineers design batteries and chargers to protect against thermal runaway?


Building a problem definition for thermal runaway

You may have noticed that many devices seem to have the same charging port but come with special chargers. Some consumers don’t notice that different chargers have specific uses.

If you were designing a new rechargeable speaker, what risks of using the wrong charger should you be aware of while you design the speaker’s charging port? 

Do you have a design idea for mitigating that risk? Continue through the pathway to learn more about the problem of thermal runaway so you can solve this engineering design challenge.

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