In contrast to mobile applications, stationary energy storages based on lithium-ion batteries which are often connected to PV-installations commonly seen in residential buildings, barely have customized safety-relevant norms and guidelines. Technical errors which cannot be avoided to 100% in these systems can lead to fires. The related loss of trust on side of the consumers, as well as the non-insurability of these home storages due to the high risk of fire could lead to a collapse of the young market.
For this reason, it is essential to develop and norm novel and adequate safety concepts for these technologies. Our concept sees to a diverse-redundant safety on three levels for all battery systems:
Level 1 – Sensor Technology
The battery management system (BMS) serves the permanent and redundant supervision of the battery’s critical values. Critical operating states, such as overload, deep discharge, excess current, short circuit or temperature, are permanently monitored and are avoided with a corresponding charge control on cell level. When the voltage metering malfunctions, the whole system is shut down. Thus, a diverse-redundancy of the system is necessary and therefore also a supervision of the management system on level 2.
The use of glass fiber senor technology developed by Fraunhofer HHI for the recording of current, temperature and strain enables a monitoring which surpasses conventional metering technologies and can be integrated into common BMS.
Level 2 – Electrics
In case of an error, especially in the case of a short circuit, the battery system is shut down quickly and securely. Lithium-ion batteries in particular have a low internal resistance which, in case of a short circuit, leads to high short circuit currents in very little time, accompanied by a high hazardous potential. A fast shutdown of high DC-charging currents is therefore essential. This poses high technical demands. The integrated switches have to be able to shutdown high voltages and currents quickly. Additionally, an all-pole disconnection of the battery is intended in case of a short circuit. If the switch should fail, safety fuses have been integrated on this level as a safety reserve. In case of an incorrect charging of the system, the close meshed glass fiber metering system is equipped with space and time profiles of temperature and pressure measurement values on cell level, in order to detect errors at an early stage and to initiate a shutdown of the relevant parameters on time.
Level 3 – Protective Casing
Lastly, the batteries have to be integrated into flame-retardant casings, their main purpose being the protection of the inboard lithium cells from thermal runaway and mechanical influences. Elements on this safety level provide among other things the protection of cells and modules from contact / electric shocks and temperature conditioning / thermal insulation. They also constitute the last safety layer to delay/contain combustion. A coating of components of the casing with flame-retardant materials, as well as a customized active fire-extinguishing device, complete the safety measures implemented on the third level.
In summary, for the safe installation and secure operation of a battery system, extensive measures have to be taken.
Aside from the mere supervision and monitoring of the battery parameters, a safe operation requires the examination of further influencing factors – under certain circumstances also external factors – which are necessary for the containment of hazardous situations. This can vary with, for instance, the location of implementation, the application and the technical framework conditions. Therefore, for the secure operation of a battery system, a corresponding battery-risk-management has to be provided. If you have any questions on this regard please feel free to contact us.