In portable consumer devices, lithium-ion battery technology has won the day. Every manufacturer of mobile phones, tablets and notebook computers looks to lithium-ion technology to provide a lightweight, long-lasting and reliable energy storage capability that can fit easily inside a small enclosure or housing. Crucially, after billions of hours of use worldwide, lithium- ion batteries in these applications have proved to be safe.

In larger devices, however, the adoption of lithium-ion batteries has been slower. Motor-driven devices such as forklifts, AGV’s or other small or medium sized vehicles commonly use sealed lead-acid (SLA) batteries, which are highly tolerant of abuse conditions of charging and temperature. SLA batteries have an extremely good safety record stretching over decades of use.

Given the advantages of lithium ion batteries and the dramatic cost fall of these battery packs, this will change.

VARTA has recently launched a range of standard lithium-ion batteries which are especially made for demanding environments like logistics, agriculture and others. The application specific batteries (ASB) will be available from 580 Wh to 2000 Wh and fulfill all requirements for these sectors, including a design with high safety.

The new VARTA standard batteries: Easy Block and Easy Blade

Treated with the correct care in design and construction, large lithium batteries can be safely managed just like their smaller counterparts. But the requirement for the OEM designer is to show a verifiable process flow which guarantees that the risk of danger or harm arising from battery malfunction is eliminated as much as possible.

This article describes the elements of a safety design that has demonstrated that it delivers safe lithium batteries with very large capacities, including packs offering more than 500Wh capacity. This safety design can be implemented into any large lithium battery pack.

Multiple levels of safety protection at system level

The architecture of a large lithium-ion battery offers the system designer three levels at which safety protections can be implemented: the cells, the protection circuit, and the complete battery pack. Good design practice calls for protection to be built in at all three levels.

Safe design should start with the cell. A large battery pack normally consists of multiple cylindrical cells packed together in such a way as to meet the mechanical and thermal requirements of the end product.

VARTA Application Specific Batteries deliver industrial-standard batteries for a range of OEM customers with demanding power and intelligence requirements. The Easy Blade is a CAN-Bus enabled modular battery with >1600Wh per battery module, capable of being used alone or combined together in parallel connection for up to 25 modules in total.
 

Different cells display different characteristics: the best cells from reputable manufacturers can support more charging cycles, have higher capacity, and offer greater tolerance of high-temperature operation than lower- quality cells.

Even within the product ranges of these market leaders, some lithium cell types are better suited to some applications than to others. The battery system design process should start, therefore, by specifying the use conditions of the battery in the end application. This specification should include:

• The typical and maximum number of charging cycles
• The typical load profile, including peak power requirements
• Charging conditions: how quickly must the battery be charged?
• Typical, minimum and maximum ambient temperatures

The cell manufacturer will then be able to work with the battery assembler to identify which cell offers the characteristics best suited to the needs of the application.

This is an important step which should be included in all design-for-safety processes. In addition, all cells should carry appropriate certifications, such as a UL1642 listing or IEC 62133 certification.

Importantly, VARTA Storage additionally carries out sample testing of the cells to be used under normal and abuse conditions to verify the performance as rated by the manufacturer. Crucially, this additional testing measures the behaviour of the cell in a forced failure. This includes testing for the ability to safely withstand:

• long charging periods at multiples of the maximum rated charge current
• a short-circuit at low resistance
• oven-testing at a temperature above the maximum prescribed in safety regulations

For large battery packs, specific safety tests are often devised to show the battery’s behaviour in real-world failure conditions relevant to the application, and to prove the safety performance of the final design. Therefore the first step when designing for safe operation is to ensure that the cells will perform predictably in all foreseeable use and abuse conditions, independent of any protection or control systems that supervise them.

In many batteries, each cell will also contain its own independent safety cut-off: the Current Interruption Device (CID)which is a fuse that automatically trips in extreme temperature conditions, rendering the lithium cell electrically inert before it reaches the point of combustion.

Four levels of safety in the BMS

Each cell, then, is tested for catastrophic failure, to ensure that, even in the worst possible case, the cells will fail safely. More sophisticated electronic controls, on the other hand, can monitor operating conditions and temporarily shut down the complete battery pack if necessary in such a way that it remains available for use when safe.

The best systems provide four levels of safety protection that operate across the entire battery pack: each of these protection mechanisms should take effect before there is any possibility that cell safety could be compromised.

These may be software or hardware mechanisms. In battery packs manufactured by VARTA Storage, the lower two levels are implemented in software, and the higher two levels in hardware.

The first level of protection calls on circuitry monitoring the battery’s voltage and state of charge (SOC) (see Figure 2). The system designer will set thresholds for over-discharging, over-charging and over-current (short circuit) and over-temperature conditions: if any of these thresholds is exceeded, the controller disables the power on the battery output. The controller continues to monitor operating conditions, and when the over-discharge, over-charge or short-circuit condition no longer applies, it will re-set the battery for use.

The second level of protection is triggered if the fault conditions recur: at a programmable threshold, the controller ceases to re-set itself. The battery is thus disabled, and can only be re-set if approved by VARTA following a review and safety check.

The first two levels of safety protection are extremely robust: they draw on voltage, current and temperature measurements that may be made accurately by familiar electronic components with a long record of reliable operation.

The failure of any one of the sensing components or of the microcontroller itself is extremely unlikely.

Nevertheless, the risk of such a failure needs to be taken into account. This is why VARTA Storage also implements two higher levels of safety protection that do not rely on any other component or on the execution of software algorithms. First, an active fuse monitors the battery voltage, and trips when a pre-set over-voltage threshold is crossed. This permanently disables the battery.

Last, a passive fuse or thermo-fuse is tripped either by over-current or over-temperature conditions. Again, the battery is permanently disabled.

System-level safety assurance

As the above has shown, safety mechanisms operate inside each cell, and in the battery pack as a whole.

The final level of safety assurance applies to the assembled battery pack. First, the mechanical design is optimized for thermal performance, and may be checked and tested for the presence of hotspots with a thermal imaging camera. In addition, the housing and all internal mountings and fixtures are specified to withstand the rated operating conditions of the end product. All VARTA Storage battery packs are also subjected to a series of 1m drop tests as standard, although more severe tests may apply to a specific application.

Finally, the battery pack must gain all relevant industry standard certifications when tested independently. This might include for instance a requirement for UL2054 and IEC 62133 certifications. Large battery packs must also be certified as permitted aircraft cargo where appropriate.

A fully equipped battery testing laboratory will provide for a range of additional abuse tests to be performed if required. This enables an OEM to simulate the conditions that its end product might encounter in the field, and to build a predictive model of the behaviour of their battery in these conditions.

Reaping the rewards of using advanced lithium technology

Any large energy concentration, such as a lead-acid or lithium battery pack, has the potential to cause harm if the energy is released in a sudden and uncontrolled manner. This, however, does not mean that the pack poses a risk to safety. The risk depends on the strength of the safety mechanisms that prevent an uncontrolled release of energy.

In a lithium battery pack that has safety designed in from the outset, this risk is made negligible through the implementation of four levels of protection of the battery pack itself: if any one level should fail, any of the others can independently shut off the battery to prevent any risk of harm or damage. The battery pack’s safety mechanism is reinforced by each individual cell’s over- current fuse.

With safe operation assured, lithium battery packs can compete on equal terms with lead-acid batteries. And since lithium batteries are lighter, smaller, offer longer cycle life and impose less onerous disposal arrangements, they look set to become increasingly popular in automotive, industrial, medical and transportation applications.

For more information about the ASB range, go to the ASB product overview or write an email to power@varta-storage.com.