foreign [Music] systems insights video series I'm Eric stoffel president of stoffel systems today's topic is disconnect switches as used in a Lithium-Ion battery pack so every battery pack consists of a group of cells as described in a previous video battery management system or BMS also described in a previous video and a disconnect switch which we'll dive into in today's video so what is the purpose fundamentally for a disconnect switch remember the primary purpose of a BMS or battery management system is to ensure safe and reliable operation of a Lithium-Ion battery pack at the end of the day how does it do that well the BMS is taking in lots of information including the voltage of the cells the temperature of the cells the amount of current flowing in or out of the pack and a variety of other information that's useful for it to determine if the battery pack is operating in a safe envelope or safe manner and if the BMS decides or determines that the battery pack is starting to exceed that safe envelope so for example say the cells are getting too hot or being overcharged a very dangerous condition it needs a means of stopping that from occurring or getting worse and the way it does that is it controls a disconnect switch to prevent further current flow into the battery pack and then the BMS will communicate to the overall application System Controller that something's wrong perhaps it'll say no more current flow I'm overheated or it'll indicate via some sort of light or something like that but fundamentally the BMS needs the ability to interrupt current flow to prevent the pack from getting into a worse condition so let's discuss what the two primary types of Disconnect switches that are first is electromechanical and this could be a relay or another term is a contactor so what this is diagrammatically a contactor has two high current contacts and then an electrically isolated coil which then can determine whether or not this switch is closed or open closed would be for current flowing and open would be to interrupt all the current flow and so the BMS controls it via these aux or this um coil drive line here the other means of a disconnect switch or other method of achieving a disconnect switch is what's called solid state and as its name suggests this means that there's no moving parts and it's done completely in semiconductors or silicon so generally speaking this is done with n mosfets which are transistors that have low resistance and there are some important differences between how these systems operate so let me draw this out in terms of the mosfet approach so the solid state switch would typically have two Source coupled mosfets configured in this manner with a Gate Drive that can determine whether or not current is allowed to flow or if it is blocked in both directions so these are the two fundamental disconnect methods that a battery system can use so going back to the original diagram I drew this battery pack has only a top side disconnect switch but in many applications particularly high voltage applications such an electric such as an electric vehicle it's actually important to have both top and bottom side disconnect switch capability and this is important because it allows the BMS to have some redundancy in disconnecting itself in case there's a failure in one of the switches and it also creates an additional factor of safety for any sort of shock hazard there is another architecture Which is less common today but this would be having a low side switch only and this typically would be done with a solid state switch because it's easier to drive the Gate of these n mosfets on the low side of the battery however one challenge with this is if you have a BMS that is communicating with an external system it needs to be referenced to some sort of ground and if the disconnect switch is on the bottom leg or on the negative leg of the battery then you cannot use this negative leg as the reference point for the interface so you might need an isolated communication interface or some sort of other way of communicating information from the BMS the application System Controller so for most purposes now the recommendation is to try to avoid this approach because it has that issue I just described instead what you can do is you can take a solid state switch put it on the salt the positive side of the battery and then just have a gate driver that's either isolated or has the capability to do a high common mode voltage so you can drive it on the positive side just like as is shown in this diagram so let's discuss some of the benefits and the pros and cons essentially of each of these approaches so starting with electromechanical what are some of the characteristics that are beneficial of that one of them is that you have very high current carrying capacity so for example you could easily have a contactor that can handle 500 amps 600 amps even a thousand amps whereas doing that in the solid state would be very challenging it also allows you to have electrical isolation because remember over in this diagram I mentioned that the coil Drive is electrically isolated from the actual contacts so for example in a high voltage electric vehicle battery such as this diagram here the BMS can be referenced to the low voltage system ground the 12 volt system and drive these contactor coils without having to have a separate electrical isolation circuit and that's actually very helpful [Applause] okay so then where are the benefits or why would you use a solid state switch instead of an electric mechanical switch well in this case a solid state switch is generally for slow currents in particular it's smaller and light weight because depending on the size of the battery if you have say 10 amps flowing in most use cases doing that with two fets is much simpler much smaller much lighter and lower cost than doing it with a contactor or relay the other benefit of an electric of a solid state switch instead of an electromechanical switch is you get a rapid response time what do I mean by this well because it's solid state you can open this connection in less than sometimes five microseconds or even less than a microsecond depending on how you design the gate current or the Gate Drive whereas with electromechanical relays the operating time is generally in the 50 to 200 millisecond range and that can mean that there's differences in the types of faults you can protect against for example this allows you to have what we call an e-fuse type functionality so for example if the BMS has a current sensor that has bandwidth high enough to detect a rapid increase in the current you can quickly have the BMS execute an opening of this switch to prevent that current flow sometimes so quickly before it even passive fuse can blow in the system and that's really strong benefit one of the other things to think about is the form factor is much more flexible with a solid-state switch you can put it on the BMS board itself or you can make it flat on a separate board somewhere else whereas the electromechanical relay is typically a larger cylinder that needs to be physically placed and mounted within the battery pack however one thing to consider with the another thing to consider with the solid state switch versus the electromechanical is the electromechanical actually allows higher voltage so for example solid-state switches are appropriate for 24 volt systems perhaps 48 volt battery pack systems maybe even up to 96 volt systems but certainly are very uncommon or very challenging to do in a 400 volt system or an 800 volt system that is much more appropriate for a contactor because it is able to have lower contact resistance at these higher blocking voltages so there are a number of factors to consider when looking at the different disconnect circuits and disconnect switches that you could include in your battery pack design one final comment I would like to make is regarding a manual service disconnect which is somewhat related but not identical to the concept of what I'm mentioning here so a manual service disconnect or MSD is typically placed just drawing out the overall architecture of the battery pack for example for an electric vehicle manual service disconnect is placed typically in between two halves of the battery so that it can be disconnected during assembly or service of the battery pack that does two things it allows the maximum voltage present in the battery pack to be half because it allows you to disconnect this electrically from this module as well as it allows prevents a circuit from being completed where if someone's working on a system up here they don't know they don't no longer have to worry about current flow because the MSD is disconnected now mind you the MSD is typically not controlled by the PMS it is controlled as a separate unit that's physically removed from the battery prior to service or assembly and it's the last connection that's made before the battery pack is complete so in summary these are the different types of Disconnect switches to consider when making a Lithium-Ion battery pack thanks for watching see you next time [Music]