At present, most of the safety accidents of lithium-ion batteries occur due to the failure of the protection circuit, which causes the battery thermal runaway and results in fire and explosion. Therefore, in order to realize the safe usage of lithium battery, the design of protection circuit is particularly important, and all kinds of factors causing the failure of lithium battery should be taken into account. In addition to production process, failures are basically caused by changes in the external extreme conditions, such as over-charge, over-discharge and high temperature. If these parameters are monitored in real time and corresponding protective measures will be taken when they change, the occurrence of thermal runaway can be avoided. The safety design of lithium battery includes several aspects: cell selection, structural design and functional safety design of BMS.

Cell selection

There are many factors affecting cell safety in which the choice of cell material is the foundation. Due to different chemical properties, the safety varies in different cathode materials of lithium battery. For example, lithium iron phosphate is olivine-shaped, which is relatively stable and not easy to collapse. Lithium cobaltate and lithium ternary, however, are layered structure that is easy to collapse. Separator selection is also very important, as its performance is directly related to the safety of the cell. Therefore in the selection of cell, not only detection reports but also manufacturer’s production process, materials and their parameters shall be considered.

Structure design

The structure design of the battery mainly considers the requirements of insulation and heat dissipation.

• Insulation requirements generally involve the following aspects: Insulation between positive and negative electrode; Insulation between cell and enclosure; Insulation between the pole tabs and enclosure; PCB electrical spacing and creepage distance, internal wiring design, grounding design, etc.
• Heat dissipation is mainly for some large energy storage or traction batteries. Due to the high energy of these batteries, the heat generated when charging and discharging is huge. If the heat cannot be dissipated in time, the heat will accumulate and results in accidents. Therefore, the selection and design of enclosure materials (It should have certain mechanical strength and dustproof and waterproof requirements), the selection of cooling system and other internal thermal insulation, heat dissipation and fire extinguishing system should all be taken into account.

For the selection and application of the battery cooling system, please refer to the previous issuance.

Functional safety design

The physical and chemical properties determine that the material cannot limit the charging and discharging voltage. Once the charging and discharging voltage exceeds the rated range, it will cause irreversible damage to the lithium battery. Therefore, it is necessary to add the protection circuit to maintain the voltage and current of the internal cell in a normal state when the lithium battery is working. For BMS of batteries, the following functions are required:

• Charging over voltage protection: overcharge is one of the main reasons for thermal runaway. After overcharge, the cathode material will collapse due to excessive lithium ion release, and the negative electrode will also have lithium precipitation occur, which leads to the decrease of thermal stability and the increase of side reactions, which have potential risk of thermal runaway. Therefore, it is particularly important to cut off the current in time after the charging reaches the upper limit voltage of the cell. This requires the BMS to have the function of charging over voltage protection, so that the voltage of the cell is always kept within working limit. It would be better that the protection voltage is not a range value and varies widely, as it can cause the battery fail to cut off the current in time when it is fully charged, resulting in an overcharge. The protection voltage of the BMS is usually designed to be the same or slightly lower than the upper voltage of the cell.
• Charging over current protection: Charging a battery with current more than the charge or discharge limit can cause heat accumulation. When heat accumulates enough to melt the diaphragm, it can cause an internal short circuit. Therefore timely charging over current protection is also essential. We should pay attention that over current protection cannot be higher than the cell current tolerance in the design.
• Discharge under voltage protection: Too large or too small voltage will damage battery performance. Continuous discharge under voltage will cause the copper to precipitate and the negative electrode to collapse, so generally the battery will have discharge under voltage protection function.
• Discharge over current protection: Most of the PCB charge and discharge through the same interface, in this case the charge and discharge protection current is consistent. But some batteries, especially batteries for electric tools, fast charging and other types of batteries need to use large current discharge or charging, the current is inconsistent at this time, so it is best to charge and discharge in two loop control.
• Short circuit protection: Battery short circuit is also one of the most common faults. Some collision, misuse, squeeze, needling, water ingress, etc., are easy to induce short circuit. A short circuit will immediately generate a large discharge current , resulting in a sharp rise in battery temperature. At the same time, a series of electrochemical reactions usually take place in the cell after external short circuit, which leads to a series of exothermic reactions. Short circuit protection is also a kind of over current protection. But the short circuit current will be infinite, and the heat and harm is also infinite, so the protection must be very sensitive and can be automatically triggered. Common short circuit protection measures include contactors, fuse, mos, etc.
• Over temperature protection: The battery is sensitive to the ambient temperature. Too high or too low temperature will affect its performance. Therefore, it is important to keep the battery operating within the limit temperature. The BMS should have a temperature protection function to stop the battery when the temperature is too high or too low. It can even be subdivided into charge temperature protection and discharge temperature protection, etc.
• Balancing function: For notebook and other multi-series batteries, there is inconsistency among cells due to the differences in the production process. For example, some cells internal resistance is larger than others. This inconsistency will gradually get worsen under the influence of external environment. Therefore, it is necessary to have a balance management function to implement the balance of the cell. There are generally two kinds of equilibrium:

1.Passive balancing: Use hardware, such as voltage comparator, and then use resistance heat dissipation to release the excess power of high-capacity battery. But the energy consumption is large, the equalization speed is slow, and the efficiency is low.

2.Active balancing: use capacitors to store power of the cells with higher voltage and releases it to the cell with a lower voltage. However, when the pressure difference between adjacent cells is small, the equalization time is long, and the equalization voltage threshold can be set more flexibly.

Standard validation

At last, if you want your batteries successfully enter international or domestic market, they also need to meet related standards to ensure the safety of lithium-ion battery. From cells to batteries and host products should meet corresponding test standards. This article will focus on the domestic battery protection requirements for electronic IT products.

GB 31241-2022

This standard is for batteries of portable electronic devices. It mainly considers term 5.2 safe working parameters, 10.1 to 10.5 safety requirements for PCM, 11.1 to 11.5 safety requirements on system protection circuit (when the battery itself is without protection), 12.1 and 12.2 requirements for consistency, and Appendix A (for documents).

u Term 5.2 requires of cell and battery parameters should be matched, which can be understood as the working parameters of battery should not exceed the range of cells. However, do battery protection parameters need to be ensured that battery working parameters do not exceed the the range of cells? There are different understandings, but from the perspective of battery design safety, the answer is yes. For example, the maximum charging current of a cell (or cell block) is 3000mA, the maximum working current of the battery should not exceed 3000mA, and the protection current of the battery should also ensure that the current in the charging process should not exceed 3000mA. Only in this way can we effectively protect and avoid hazards. For the design of protection parameters, please refer to Appendix A. It considers the parameter design of cell – battery – host in use, which is relatively comprehensive.

u For batteries with a protection circuit, a 10.1~10.5 battery protection circuit safety test is required. This chapter mainly investigates charging over voltage protection, charging over current protection, discharging under voltage protection, discharging over current protection and short circuit protection. These are mentioned in the above Functional Safety Design and the basic requirements. GB 31241 requires checking for 500 times.

u If the battery without protection circuit is protected by its charger or end device, the safety test of 11.1~11.5 system protection circuit shall be conducted with the external protection device. Voltage, current and temperature control of charge and discharge are mainly investigated. It is worth noting that, compared with batteries with protection circuits, batteries without protection circuits can only rely on the protection of equipment in actual use. The risk is higher, so the normal operation and single fault conditions will be tested separately. This forces the end device to have dual protection; otherwise it cannot pass the test in Chapter 11.

u Finally, if there are multiple series cells in a battery, you need to consider the phenomenon of unbalanced charging. A conformance test of chapter 12 is required. The balance and differential pressure protection functions of PCB are mainly investigated here. This function is not required for single-cell batteries.

GB 4943.1-2022

This standard is for AV products. With the increasing use of battery-powered electronic products, the new version of GB 4943.1-2022 gives specific requirements for batteries in Appendix M, evaluating equipment with batteries and their protection circuits. Based on the evaluation of the battery protection circuit, additional safety requirements for equipment containing secondary lithium batteries have also been added.

u The secondary lithium battery protection circuit mainly investigates over-charge, over-discharge, reverse charging, charging safety protection (temperature), short circuit protection, etc. It should be noted that these tests all require a single fault in the protection circuit. This requirement is not mentioned in the battery standard GB 31241. So in the design of battery protection function, we need to combine the standard requirements of battery and host. If the battery has only one protection and no redundant components, or the battery has no protection circuit and the protection circuit is provided only by the host, the host should be included for this part of the test.

Conclusion

In conclusion, to design a safe battery, in addition to the choice of the material itself, the subsequent structural design and functional safety design are equally important. Although different standards have different requirements for products, if the safety of battery design can be fully considered to meet the requirements of different markets, the lead time can be greatly reduced and the product can be accelerated to market. In addition to combining the laws, regulations and standards of different countries and regions, it is also necessary to design products based on the actual use of batteries in terminal products.