Contactor selection is an iterative process that includes consideration of system specifications as well as failure scenarios. Most of the contactor manufacturers share necessary information in datasheets. Thus, a wise approach would be making a comparison table to cover basic specifications like thermal and mechanical initially.
System Specifications
After pre-selection based on datasheet values, we should consider system specifications. Let’s assume that we have 430V maximum system voltage (100 NMC cells in series), and the moving average current profile for the battery as:
- 500A for 10s,
- 350 A for 100s,
- 200 A for continuous operation.
We can compare the load profile with the carry current profiles on the datasheet [1].
Time-Current graph shows the current profile of the system (orange) with contactor carry current capability. So, the comparison shows that the contactor can carry our systems’ normal currents.
We could also collate different current-time characteristics for comparing different contactors. This could give the opportunity to downsize and to find smallest contactor.
Failure Currents
What about failure currents? We should also consider several failure scenarios at this stage. To find out if this contactor is also suitable for failure current handling, we can start with the worst-case failure current.
The worst-case failure occurs when the short circuit is at the closest point to the battery HV connector terminals. If we consider electrical system resistances including cell internal resistances, the short circuit resistance is approximately 0.15 Ohm [2]. Therefore, the short circuit current will be 2867 A. However, This is quite a rough approach. Battery design engineers should analyze the failure current map considering:
- Cell OCV based on temperature, SOC and C-rate
- Cell DCIR based on temperature, SOC and C-rate
- System inductance
A similar calculation can be done for charge current. Short circuit events can happen also during regenerative braking and charging. However, in most cases, the worst failure event is during discharge. This is because there are intermediate elements like inverters, DC-DC converters, or external chargers with their protection circuits during charging.
If we look at the datasheet, we can see that the contactor can handle short circuit currents up to 4000A for 20ms. Therefore, this contactor option is suitable for our application.
Contactor-Fuse Relation
As mentioned in contactor basics, the minimum breaking current of the fuse is an important parameter for contactor selection. The maximum breaking current of the contactor must be greater than the minimum breaking current of the fuse. For instance, if there is an incomplete short circuit as 2000 A, the contactor can switch-off the circuit. Because the maximum breaking current is 3000 A according to the above table, the incomplete short circuit can be safely handled by contactor. Therefore, giving a 15% safety margin, the minimum breaking current of the fuse must be 2550A at maximum.
Temperature Derating
Temperature affects contactor selection significantly. Contactor manufacturers make tests to specify how the temperature affects contactor performance. The current carry performance reduces at higher temperatures and with a lower cross-section of busbars. Therefore, battery designers should design busbar or cable sizes according to contactor capability.
5 Steps for Contactor Selection
- Check if the contactor is suitable for thermal and mechanical loads
- Identify the battery system absolute maximum voltage and moving average currents so that the battery system current carry-time curve is below the contactor curve
- Analyse the maximum short circuit current if the contactor can withstand
- Check the fuse selection so that the contactor can break incomplete short circuits if the fuse cannot
- Check busbars and neighbor components to identify if temperature derating can change the selection
References
- “GV240 Series“, Gigavac LLC, 2018, accessed 07 Jan. 2022.
- Ozguc, Murat Kubilay, Kadir Aras. “Selective Shut-off Strategy in Distributed Battery System“. International Journal of Automotive Science and Technology 5.1 (2021): 27-33.
- Kroeker, Matthias, Hans-Joachim Faul, Roman Dietrich. “EVC 250 Main Contactor“, TE Connectivity, 2014. 04 Jan. 2022.
Hi Eymen,
This page was super useful – thanks a lot for putting it together! Contactor selection looks trickier with 800V systems because the max break current in your example would be only 900A. Combined with the higher system voltage, the short circuit current is likely to be multiple times greater than the maximum break current! In this instance, would it suffice to size the contactor such that the short circuit current (20ms) was greater than the actual short circuit current, and such that the fuse clears faster than 20ms (e.g. 10ms)?
Cheers,
Will
Totally agree with you.
I would say your customer’s pockets will always prefer to service a fuse than a contactor, so you really want to blow the fuse first to protect your circuit rather than open up contactors under these loads even if they can (remember that at those sort of currents the contactor can open under load but only once or twice best case without damaging the gold or silver plating in the contacts).
Most discharge short-circuit currents at 800V will be well above 4 kA, so you’ll find good range of fuses blowing within a few ms. Then there is the case in which you want to protect your wiring harness under “low current overloads” e.g. 2 kA or lower. In this case you might struggle to find fuses that can open circuit fast enough, so opening with contactors is a good shot.
Some manufacturers for battery junction box might actually have two contactors in parallel in their +ve and -ve lines because the current rating needed is lower and they can buy two or three for the price of a high current rating contactor. This will increase packaging space and weight but gives you safety margin if you want to open up contactors under load as the current splits.