Expert knowledge of battery management systems

A PTC thermistor is a heat-dependent resistor that trips when the current is too high and reversibly minimizes the current flow.

A CID causes a current-carrying bridge to deform irreversibly in the event of a misuse, so that the current flow is interrupted.

A rather new method is a fusing separator. The separator consists of two different plastics, both of which are porous. One of the two plastics melts in the event of an overtemperature and thus fills the gaps in the other plastic, which in turn leads to the current flow being minimized or coming to a standstill (shut down).

A safety valve is a predetermined breaking point which breaks open in the event of a pressure increase caused by temperature development and allows controlled blow-off of the electrolyte gas. This prevents a violent bursting.

Cell Balance is used for battery packs with more than one serially connected cell in order to keep the cells or the charge states in a balanced state. In particular, Cell Balance is used to compensate for charging states caused by different temperatures and internal resistances. It prevents the battery pack from shutting down prematurely due to under- or overvoltage of a cell, whereby the weakest cell determines the performance of the overall system.

To determine the capacity of the battery pack, a BMS has a gas gauge system. The gas gauge determines the total capacity and the remaining capacity available. It is probably the most complex of the available functions. To achieve the most accurate results, it must be developed to fit the application. In particular, the current profile of the end application and the user behavior are relevant.

A BUS system is available to the battery pack for reading out the data. The most common BUS systems are I2C, SMBus, CAN, LIN or RS232, but all other buses are also possible.

For low-power systems, it may make sense to integrate the charging part for the battery pack on the BMS. This has the advantage that the charger can be adapted in the event of a cell change, thus always ensuring backward compatibility with older systems.

In systems where the battery is the central element, system functions can be relocated to the battery. This can save a second intelligence (microcontroller) in the system. For example, motor control and battery protection circuitry are combined in the battery. This saves the control microcontroller (μC) and the double design of the power FETs (FET = field effect transistors).

All lithium-based battery packs must be qualified in the UN transport test to obtain approval for transport by road, rail, inland/ocean shipping and air. Battery packs with capacities no greater than 100 Wh can be transported more easily after passing the UN test. Battery packs above 100 Wh are always shipped as class 9 dangerous goods. The UN test includes vibration, shock, temperature change, altitude simulation, short circuit and overcharge. Without passing the UN test, transport is not allowed. The only exception: During the evaluation, up to 100 battery packs can be transported as dangerous goods in a wooden crate for test purposes. A UN approval must be available if more than 100 battery packs are shipped or battery packs are shipped to the end customer. The approval is issued to the product and the production facility. Eight samples are required for this test.

Approval IEC 62133 is an international standard for portable rechargeable batteries. Approval with CB Report is not mandatory unless explicitly required by a device standard. Many other standards (e.g. 60601 Medical) require IEC 62133 approval for battery packs. 13 samples are required for this test.

UL62133 is the North American harmonization of IEC 62133 and is of interest to anyone planning a market launch in North America. With the approval according to UL 62133, the CB Report for IEC 62133 can also be obtained. If approved by Underwriters Laboratories (UL), a quarterly manufacturing inspection is required to maintain the certificate. In addition, a sample must be sent to UL annually for requalification. 25 samples are required for this test.

UL 2054 was the only option for UL approval of batteries before the harmonization of IEC 62133. It differs primarily in that tests are also carried out on battery packs with a built-in first fault, which significantly increases the time and financial outlay. A first fault is, for example, a bridged protective FET. Approximately 55 samples are required for this test.

We implement Functional Safety in accordance with IEC 61508 and its application-specific daughter standards (including ISO 13849 for machinery / equipment, ISO26262 for automobiles and motorcycles). The implementation of functional safety serves as proof of development in accordance with the current state of the art in product liability cases pursuant to the German Product Liability Act (ProdHaftG). Implementation begins with the identification and quantification of potential risks due to malfunctions of the system under consideration. Based on the potential risk, technical and methodical measures are implemented in the development to provide proof of functional safety.