This article presents the different categories of batteries and the safety issues associated with each one.
What are the main types of batteries?
There are two main types of batteries, primary and secondary batteries. The latter are rechargeable, i.e., have a multi-charged discharge life cycle, allowing the user to recharge and reuse the battery. On the other hand, the primary batteries have a one-time discharge life cycle, and the user can use the battery once before its disposal. Safety is an essential requirement for both primary and secondary batteries.
Primary batteries: Chemistry Designs & Safety Concerns.
As mentioned above, primary batteries aren’t rechargeable. This battery type can have many different forms, among which are the following:
- Button cells
- AAA, AA, B, C and D forms
- 9-V batteries.
Their chemistry design often consists of either zinc-carbon, alkaline, lithium or lithium-ion disulfide.
The oldest batteries still in production are zinc-carbon batteries. They have 1.5-V chemistry and are less costly than alkaline or lithium-ion disulfide batteries. However, their capacity is less than that of the others, and they don’t last as long. The zinc-carbon batteries may also leak corrosive electrolytes when fully discharged.
The alkaline batteries have a 1.5V output voltage and a very low internal discharge rate. Users can store them for extended periods of time. These batteries may also leak corrosive electrolytes.
Lithium-ion disulfide batteries usually have an output voltage of between 3V and 4V. They can power energy-intensive electrical products, such as digital cameras. However, there are some safety concerns when it comes to these batteries. They’re heat-sensitive and may fail exothermically when heated by an external source or rapidly discharged. Manufacturers should equip such batteries with a current-limiting device to ensure user safety and prevent excessive discharge current.
From a safety perspective, primary batteries are very hazardous. Some of the electrical product hazards and risks we can mention are as follows:
- their cells can overheat and become a burn risk for the user,
- they may fail, causing the cell to disassemble rapidly,
- their electrolytes may be corrosive and may hurt the user’s skin and eyes when in contact,
- they may vent hot gases.
Additionally, button cells are associated with the risk of being swallowed by children due to their size and shape.
Secondary Batteries: Chemistry Design & Safety Issues
Secondary batteries are rechargeable and often used in mobile products. Their chemistry design may include lead-acid, nickel-cadmium, nickel-metal hydride or lithium-ion.
Today, the predominant rechargeable battery type, especially in mobile devices, is lithium-ion. Lithium-ion batteries consist of a set of electrodes housed in a casing. They can have various dimensions, and their shape is cylindrical or prismatic. Lithium-ion batteries have a lightweight, high-energy density of the cells and are increasingly low cost.
A typical battery system includes four elements:
- a power adapter (e.g. USB or 120-V/230V AC) which conditions the power that is routed to the product charger circuit to charge the battery,
- a charger circuit that ensures that the battery is not overcharged,
- a secondary set of current, voltage and temperature controls, which act as an independent safety circuit that is called upon to operate only when the control circuit fails,
- a battery that provides the power needed to power the electrical product.
All four system elements are subject to compliance testing and safety certification at both subsystem and system levels.
From a safety perspective, rechargeable batteries, especially lithium-ion cells, may go into thermal runaway. The severity of the thermal event depends on the cell state of charge and may lead to, as follows:
- venting of potentially combustible gasses
- rapid disassembly
- ignition and flaming.
There is a higher risk for flaming combustion in multicell batteries due to the presence of flammable electrolytes or sustained exothermic chemical reactions.
Generally, failure of secondary batteries may be because of a poor cell design, a cell manufacturing defect, battery management design defect or failure, electrical abuse, thermal abuse, mechanical abuse, or end-of-life.
Battery Safety Design
A battery system (in particular, a lithium-ion battery system) should have a design that ensures the cells in the battery comply with all legislative battery requirements. The system should be tested against all relevant battery safety standards to evaluate the product’s level of regulatory product compliance and safety. Besides conducting compliance testing, manufacturers should also have a quality control system to avoid any manufacturing defects in the cell that can make the entire battery system unsafe.
An effective way to manage product quality and design safety is using a risk management tool such as SFMEA. This tool can provide an overview of all identified risks in the battery and its components and facilitate minimizing product safety risks.