EV Fundamentals: Causes of Battery Fires
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Causes of Battery Fires
07.01
| 00:00 | Over the years, a lot has been made of EV batteries and their risk of spectacular failure, usually resulting in an intensely hot fire that's extremely difficult to extinguish. |
| 00:09 | The reality is that any energy storage device carries with it some level of risk, whether that be a battery, a fuel tank, or a steam engine boiler. |
| 00:16 | And just like how gas tanks and steam boilers became far safer as time went on, battery safety technology has also come a long way in a really short period of time, thanks in part to stringent quality control standards imposed on all manufacturers. |
| 00:28 | These standards have had a big effect on the safety of electric vehicles, to a point now where a runaway thermal event is extremely rare. |
| 00:35 | And with that said, rare doesn't mean impossible. |
| 00:38 | So, in this module, we're going to explore the causes of lithium-ion battery fires and how they're best avoided. |
| 00:44 | Before jumping in, it is important to note that while it's true that EV fires are much harder to put out than gasoline fires, they occur far less frequently, and they typically build up and expand much more slowly as the fire propagates from the initial culprit cell outwards, giving you much more time to do something about it. |
| 01:01 | There are three main culprits when it comes to lithium-ion battery fires. |
| 01:05 | Too much heat, puncture damage, and not enough heat when charging. |
| 01:08 | Let's look at excessive heat first. |
| 01:11 | The reality is that if any battery cell reaches a certain temperature, it'll ignite just like nearly any other energy source. |
| 01:17 | In the case of lithium-ion, this happens because as the heat rises, the electrolyte solution inside the cell that promotes the efficient transfer of electrons between the positive cathode and negative anode expands. |
| 01:28 | Given enough pressure, the cell will rupture, and if it's hot enough, around 420 degrees Celsius, all the solution needs to ignite is oxygen. |
| 01:37 | This, as you can imagine, starts a chain reaction as the surrounding cells follow the same fate as the fire heats them. |
| 01:43 | Again, the various fail-safes found in electric vehicles make this extremely unlikely to happen, but in some really rare cases, say uncontrolled currents from a charger pushing the voltage up to critical levels and a BMS somehow not detecting them, this can happen. |
| 02:00 | So, at that point, a cell can start to experience thermal runaway, where it continues to heat up even when it is no longer actively charging or discharging. |
| 02:08 | If it gets hot enough to ignite the electrolyte solution, the fire will likely grow and engulf the entire battery. |
| 02:14 | It's worth noting that many cells don't experience thermal runaway no matter what voltage they're charged to. |
| 02:19 | Every battery cell is different, and that's why the data sheets explaining the test results of each cell type are so important to know and understand. |
| 02:26 | The next cause of battery fires is puncture damage. |
| 02:29 | Like we just touched on, for all the pros of lithium-ion technology, one of the big cons is just how flammable that electrolyte solution inside each cell is. |
| 02:38 | In the event that a cell is ruptured, the cathode and anode will short-circuit, which leads to tremendous heat generation as a cell nearly instantly discharges its stored energy. |
| 02:47 | If the heat is allowed to build up to a certain level, the electrolyte may ignite. |
| 02:52 | Combine that with the solution acting as a fuel source, mixing the sudden rush of oxygen coming in from the damaged area, and you can see there's an opportunity for fire. |
| 02:59 | Again, the problem of course is that if one cell in a battery pack ignites, it's very tightly packed in with thousands of other cells, in the case of a pack with cylindrical cells at least. |
| 03:08 | They'll all react the same way if they get hot enough. |
| 03:11 | Fire of course results in more heat and more fire, and that's just slowly going to spread out to more and more cells, usually affecting an entire battery pack. But individual cell data varies. |
| 03:21 | Many cells can be punctured without any kind of thermal event occurring, and some are much more sensitive to this type of event. |
| 03:27 | Our third cause of fire on the list, and admittedly the least likely, is charging at too low of a temperature, particularly DC fast charging. |
| 03:35 | Now, this used to be a bigger problem than it currently is, thanks once again to modern safeguards that will a. preheat the battery before it receives any charge in cold environments, and b. BMS that won't allow a battery to be charged in the first place if it's not warm enough. |
| 03:50 | So, what's the problem with charging in cold temperatures? Well, a lithium ion battery can discharge at a wide range of temperatures, but charging is more delicate and requires a narrower temperature range, usually between 0 and 45 degrees celsius. |
| 04:03 | Anything below freezing, and the lithium ions will plate the cell's anode with metallic lithium rather than charging the cell. This is extremely bad for the battery and damages it permanently. |
| 04:13 | At best, we're going to see a serious reduction in the charge capacity, but at worst that metallic plating puts pressure on the cell's separating membrane, eventually causing it to fail, , which then in turn creates a short circuit that can lead to a fire. |
| 04:27 | The scary part in this scenario is that there's a good chance the short circuit won't actually happen when charging, but instead kick off a chain reaction at some point later down the road when the vehicle sees heavy vibrations or sudden shock that could come from something as simple as attacking a speed bump too fast. |
| 04:43 | So, what can we do to avoid any of these scenarios? Well, thankfully, these days manufacturers, and by that I mean both OEM and aftermarket, have these potential problems well and truly covered and plan for protections through both the battery management system and the VCU. |
| 04:58 | If we're working on a conversion project, however, there are a few things to consider. |
| 05:02 | Firstly, if you're using aftermarket cells, test data is usually available, which indicates how sensitive the cell is to being overcharged, heated, or punctured. |
| 05:11 | This is a really useful way to understand the risks involved with the specific battery cells you're working with. |
| 05:16 | Secondly, we should make sure that these safety protocols are set up and working correctly, and thirdly, it's very important that our battery pack is protected as well as possible. |
| 05:25 | Most OEMs employ fairly serious underbody protection due to their batteries usually being fitted under the floor of the vehicle, and while that's probably not where we'll be mounting our batteries in an EV conversion, the same idea still applies. |
| 05:38 | When planning the battery pack mounting location, it's important to consider where to mount your battery packs so that the risk of damage is minimized in the event of an accident, and so no matter where they're mounted, we need to ensure that they're adequately protected. |
| 05:51 | Having some level of containment is also important so that in the event of a thermal runaway, the occupants are isolated from the battery modules. |
| 05:57 | This is basically just the same thing as saying you want to have a firewall between the driver and the fuel tank of a gasoline-powered vehicle. |
| 06:04 | Before finishing up with this module, let's just quickly cover off the main points we've discussed. |
| 06:08 | Much has been made in the mainstream media over EV fires, but the reality is that they're far less likely to occur than ICE vehicle fires. |
| 06:14 | The quality control methods used when manufacturing modern EV battery systems, plus various fail-safes employed by the BMS and VCU, makes these extremely unlikely to happen. |
| 06:24 | There are three main culprits. High temperature, which causes the electrolyte solution inside the cell to expand, eventually rupturing the casing and self-igniting. |
| 06:33 | Puncture damage, which causes a short and introduces oxygen to the very flammable electrolyte solution. |
| 06:38 | And lastly, charging at sub-zero temperatures, which plates the cell's anode in lithium that has the potential to rupture the membrane separating the positive and negative causing a short. |
| 06:47 | The problem with any one of these situations is that once one cell goes, there's a chance that the heat from that fire will cause other cells to react in the same way, creating what's known as a thermal runaway event. |
