The Basics
Fuses are protection devices that protect electrical circuits against undesired high currents. We can use passive fuses and pyro fuses in battery design.
Selecting a Fuse
- Select a fuse rated double as continuous current (e.g. initially take 400A fuse for 200A continuous current) and draw the load profile next to 50% of the fuse breaking current-time chart to check if pulse currents can be carried by the fuse without aging.
- Calculate the components and the fuse breaking energy (I2Rt) to ensure that the weakest component is the fuse.
- Identify the minimum breaking current for the failure cases when the contactor cannot switch-off.
- Analyze the short circuit clearing time and check if the contactor can withstand until the fuse breaks the circuit.
- Check the contactor-fuse coordination for normal operation, overloads and failure currents.
Manufacturers
The fuses in a battery pack protect the battery and the other electrical components against high currents. There are special off-the-shelf components similar to 12V starter battery fuses. However, EV fuses are rated for high voltage and traction currents. The page has a list of EV fuse manufacturers.
Cell Level Fusing
When we connect cells in parallel to increase the capacity we might also want cell level fusing. This fusing being by definition designed to disconnect a cell that for some reason is sinking or delivering high currents.
The fusing can be inside the cell and sealed or external to the cell, sometimes both internal and external fuses are used.
Fuse Coordination (Selective / Layered Fusing)
Cell, module and pack fuses only do their job if they open in the right order. The aim, usually called selectivity or discrimination, is that a fault confined to one cell trips only that cell’s fuse and leaves the module and pack fuses untouched, so the rest of the pack keeps running.
Getting the order right is a question of energy, not just current rating. Each fuse has a pre-arcing I²t – the thermal energy it absorbs before its element melts and it starts to clear. For a cell-level fuse to reliably clear before a module or pack-level fuse even begins to melt, its total clearing I²t has to sit below the minimum pre-arcing I²t of the fuse tier above it, and that comparison needs to hold across the fault current range the pack can actually see, not just at one test point. As a rule of thumb, published selectivity guidance looks for an I²t ratio of at least 1.5:1 between a downstream fuse and the one above it before treating the pair as reliably selective [1].
In practice, layered fusing on a pack needs:
- A cell fuse whose melting curve sits comfortably inside the pre-arcing curve of the module fuse above it, across the expected fault current range
- A module fuse whose clearing energy at the pack’s maximum fault current stays below the pack fuse’s pre-arcing threshold
- Margin for manufacturing tolerance between fuse batches and for the way ambient and local heating shift each tier’s time-current curve in service
Where a contactor also has to clear above some fault current, the same discrimination logic applies between it and the pack fuse – see the contactor-fuse coordination point above.
References
- Fuse Selective Coordination, Eaton / Cooper Bussmann technical library
- IEC 60269 (Low-voltage fuses) and IEC 60364-5-53 (Selection and erection of electrical equipment – protective devices)
Pyro Fuse
Pyrotechnic or Pyro Fuses can actively interrupt circuits. Just like airbag systems, pyro fuses may be a part of HV systems. Electrical designers use pyro fuses in case the electrical circuit needs to be broken via an external signal. However, pyro fuses can also interrupt the circuit during events where current flow is higher than the current limit.
See also: Contactors, Contactors: The Basics