• Karn Sinha

Our analysis of the new BYD blade battery

In this article, we will take a closer look at the BYD Blade Battery. We will examine their safety, cell and pack level energy densities, cost, the design choice of turning a prismatic cell into a structural member within the vehicle, and BYD’s claims of 1000km of range.

The best place to start with the BYD Blade Battery is safety because BYD’s claims of a non-flammable battery cell are what the Blade Battery is best known for. Let’s look at how BYD was able to achieve this.

Lithium iron phosphate (LFP) batteries are safer because the cathode crystal structure decomposes at high temperatures. Moreover, when it eventually does decompose, it releases less heat than Nickel based layered oxide cathodes like NCA and NMC.

The video above shows a prismatic NMC battery cell in a nail puncture test. It immediately blows out the burst diaphragm to release gases and then erupts into a white hot ball of fire.

Next is a prismatic LFP battery cell. The reaction here is lethargic in comparison to the NMC battery cell. It eventually blows out the burst diaphragm but doesn’t erupt into flames.

Finally, the Blade Cell.

As you can see nothing happens. How does the Blade Cell perform so well in this test? Even though it uses the same chemistry as the prismatic LFP battery cell?

According to BYD, this is due to two reasons: First, the Blade Cells are long and thin, which dissipates heat efficiently. When a battery cell short circuits, it begins a process called thermal runaway. Thermal runway is a feedback loop where heat leads to decomposition, which in turn leads to reactions that generate heat, which leads to more decomposition and heat.

The Blade Cell helps stop thermal runaway because the shape of the battery cell is like a heat sink. It has a large surface area in comparison to its volume. This is as opposed to prismatic batteries which have a low surface area in comparison to their volume. The boxy shape traps heat which exacerbates thermal runaway.

Second, BYD says that the electron circuit in the BYD battery is longer. To us, this explanation doesn’t make sense. When you run a nail through a battery cell it creates a short circuit between the anode and cathode, which is a tiny gap of about 14 microns, about the same diameter as a white blood cell. Rather than the electrons traveling a long circuit, through a wire, in a controlled way from the anode to the cathode, they travel in a shorter direct circuit in an uncontrolled way. This discharges the battery in a matter of seconds rather than a more typical 4-8 hours, releasing an enormous amount of energy, and therefore, heat. That is, the length of the short circuit from the anode to the cathode is the same no matter what the dimensions of the battery cell. However, we believe what BYD was attempting to communicate here is that, with a prismatic battery cell, the nail puncture goes through more layers, creating more short circuits. More short circuits release more energy more quickly and therefore overheat and explode.

The BYD Blade Cell is about 75% thinner than a prismatic cell, which means 75% fewer short circuits, resulting in a fraction of the heat generation. The reduced heat generation combined with a greater ability to dissipate that heat is what allows the Blade cell to run cooler during the nail puncture test compared to a typical prismatic LFP battery cell.

Could the blade form factor be applied to Nickel based chemistries to increase safety?

According to us, this is not possible. The Blade form factor works with LFP because LFP generates minimal heat during decomposition and resists generating that heat until it reaches far higher temperatures. High nickel chemistries store more energy in a smaller volume and generate quite a bit more heat at lower temperatures, so changing the form factor probably wouldn’t be enough to break the thermal runaway loop. Instead, for now, it appears the best way to improve the safety of Nickel based chemistries is to provide them with a path to vent pressure and flames safely.

Let’s move on to BYD’s energy density claims. There are two metrics for energy density. The first is gravimetric energy density, which is how much energy a battery can store for a given weight. The second is volumetric energy density, which is how much energy can be stored in a given volume. For lithium-ion batteries in automotive applications, using gravimetric energy density has tended to make sense because they’ve typically been nickel based lithium-ion batteries. Nickel based batteries tend to have good gravimetric energy density and great volumetric energy density. That is, volume wasn’t the primary limiting factor for nickel based battery pack size. LFP batteries are a slightly different beast than nickel based batteries.

Although gravimetric energy density matters for LFP battery cells, it’s not their primary handicap. The primary handicap of LFP battery cells is their low volumetric energy density. This is why LFP battery cells are usually produced in prismatic formats, which are cuboid. These cuboid shapes can fill the space in the bottom of a vehicle better than cylinders. LFP is compatible with these large prismatic cell formats because the chemistry is inherently safer than Nickel based cells.

Cell casings are typically made of steel, which offers fire protection due to their high melting point. LFP is safer, so less steel protection is required and larger cells can be used. High nickel chemistries are more volatile, and so the cells have to be smaller in comparison to LFP cells.

With all that in mind, let’s compare the energy density specs of BYD LFP Blade Cells and packs to CATL LFP battery cells and packs. There are reports that CATL is currently shipping batteries with a gravimetric energy density of 200 Wh/kg. I haven’t found confirmation of that, but it aligns with CATL’s development timeline. At 200 Wh/kg for gravimetric energy density, they were expecting to hit 450 Wh/L for volumetric energy density.

All the reports we've seen for BYD’s Blade cells indicate around 448 Wh/L and 165 Wh/kg. As far as we know there’s no publicly available data yet from a teardown. All the figures we are providing today were either from BYD, sources out of China, or government reports from China.

The volumetric energy density of Blade cells vs CATL’s cells is for all intents and purposes identical at 448 versus 450 Wh/L, while the gravimetric energy density of the Blade cells is much lower at 165 versus 200 Wh/kg. Since both these sets of numbers are for the same chemistry, why the difference? Time for some speculation. The BYD Blade Battery is a structural battery. The BYD blade cells replace the steel beams that are usually used in and around automotive battery packs. Typically, battery cell casings are around 125 microns thick, roughly 1/10 of a millimeter. We’d be surprised if that’s enough to provide the rigidity these cells will need as structural members in the battery pack and the vehicle, and so each cell might be using a slightly thicker steel casing to provide rigidity. Steel is dense, so even a slight increase in thickness to the battery cell casing would greatly reduce gravimetric energy density.

Let’s do a quick summary before moving on to pack level energy density. BYD’s claim of 448 Wh/L is industry-leading and appears to be on par with what CATL is rumored to be producing. On the gravimetric front, BYD Blade cells achieve 165 Wh/kg compared to 200 Wh/kg for CATL. My assumption is the lower energy density is due to thicker steel for the BYD cell casings. BYD can make up for that by removing steel elsewhere in the vehicle. More on that later in the video.

On to pack level energy density. BYD claims that the Blade Pack achieves a packing density of 62.4%. 62.4% times 448 Wh/l means a pack level energy density of 280 Wh/l. CATL is probably achieving a packing density of 43%, leading to a pack level energy density of 194 Wh/l. BYD’s claims would place their pack level volumetric energy density 44% higher than a CATL LFP pack.

Although we’re sure BYD’s made great leaps, we view a 44% improvement over CATL LFP as unlikely. If BYD’s numbers were correct, they would be achieving higher energy densities with LFP than Tesla is achieving in the Model 3 with Nickel based battery packs. The pack level energy density of the Nickel based Model 3 battery is 238 Wh/L versus the 280 Wh/L claimed by BYD for the Blade, or 18% better. Given LFP’s poor volumetric energy density and Nickel’s high volumetric energy density, we can’t see a pathway for LFP packs to outperform Nickel packs, even with a lot of engineering magic. Independent sources within the industry that have seen teardowns of the blade pack have indicated that Nickel based battery packs still have a 10% edge compared to BYD blade batteries. This would place the BYD Blade Battery at around d 214 Wh/L rather than 280 Wh/L. But, without any publicly available information to back that up, let’s look for other evidence to back up the reports that BYD is achieving 18% greater volumetric energy density with LFP than a battery pack with Nickel based battery cells.

BYD’s flagship EV, the BYD Han, is BYD’s longest range EV, at 605 kilometers of range. They appear to have vehicles with longer ranges, but from what we can tell, those vehicles are hybrids or concept vehicles. 605 kilometers is 375 miles of NEDC range, which equates to about 262 miles of EPA range, give or take about 30 miles. That 262 miles of range is provided by a 77 kWh battery back built on Blade technology. Again, that’s BYD’s longest range, flagship vehicle, in 2021. If you know of a vehicle we are missing here, let us know in the comments below.

77 kWhs is fairly low capacity in 2021 for a vehicle that’s the same length as the Tesla Model S. The new Model S has a battery pack that’s around 99 kWh and achieves an average range of about 381 miles across trims. It’s clear that BYD isn’t trouncing Tesla by 18%. It’s the opposite. The Model S has a range that’s 45% greater than the Han.

Let’s bring in BYD’s aspirations for the blade pack to try and level the field. BYD appears to be targeting 1000 km for their concept vehicle, the Ocean-X, which is a mid-sized sedan. When converted to an EPA test cycle and miles, 1000 km means about 434 miles of range. Is that possible?

Tesla fits an 82 kWh battery pack under the Model 3, which is also a mid-sized sedan. If BYD is claiming they can do 18% better with respect to the volumetric energy density of their pack, I estimate that to mean a 97 kWh battery pack in the same volume as Tesla’s 82 kWh pack.

This chart from Matty Mogul will help us put that in perspective.

The Model 3 delivers an energy efficiency of just over 4.5 miles of EPA range per kWh. Tesla has market-leading efficiency with their current generation of vehicles and pack architecture, which uses a Nickel chemistry. BYD is claiming that the Ocean-X would fall right in line with Tesla’s numbers with 4.5 miles of EPA range per kWh. This seems farfetched. Why? Their current generation flagship, the BYD Han, only gets about 3.5 miles of EPA range per kWh, which is 22% less than their projection for the Ocean-X.

We're not saying it’s impossible for BYD to hit 4.5 miles per kWh. In fact, I think BYD will eventually reach Tesla level efficiency with an LFP battery pack despite lower cell-level energy density. However, it will require not only the blade structural battery, but also power train, drag coefficient, and chemistry improvements. This could be possible before 2025, but it would be quite an achievement. 2025 or later seems more likely.

Time for another interim summary. At the pack level, BYD claims their blade battery can achieve a volumetric energy density of 44% higher than CATL LFP and 18% greater than the pack in a nickel based Model 3. This sounds too good to be true and it probably is.

From what we hear from those in the industry who’ve seen teardowns, the Model 3 with 2170 cells leads by 10% rather than trails by 18%. If we look to reality to confirm BYD’s energy density claims, things don’t add up. The BYD Han falls well short of 300 miles of EPA range rather than hitting the 434 miles that they’re touting with their marketing. In the meantime, the Tesla structural battery will be in vehicles soon, which should do for Nickel chemistries what the blade battery did for LFP chemistries. The structural battery will increase both the volumetric and gravimetric pack level energy density of nickel based batteries and I’m speculating that it will also be safer than other Nickel based battery packs.

As a final note on the blade battery, it looks to be 26% cheaper than competing LFP battery cells. CATL battery packs cost about $88/kWh, whereas the BYD blade battery pack costs about $65 per kilowatt-hour. This should make BYD’s vehicles quite profitable.

Interestingly, there are rumors that Tesla and BYD are courting each other. If they are, let’s hope those discussions are fruitful. Tesla needs all the battery cells they can get and the blade battery pack would be deadly in the hands of Tesla.

If the blade pack offered a 10% pack level energy density improvement and a 26% cost reduction over CATL LFP, Tesla could increase the range of their LFP standard range vehicles from 253 miles of range to around 278 miles of range while getting the battery pack about 18% cheaper. Additionally, $65/kWh is actually cheap enough for Tesla to make a $25,000 subcompact vehicle at roughly a 30% profit margin. While we’re on the topic of battery cost, it’s worth noting that we’re starting to see inflation in the battery industry. There are reports that BYD is increasing prices by 20%. But, even with a 20% increase, BYD’s prices at the pack level would still be just $78 kWh, $10 less than CATL. BYD certainly isn’t alone here.

Battery prices are expected to increase across the board in 2022.

For those who don’t have in-house cell production and a secure materials supply, prices will be relatively flat the next few years, possibly longer. How does BYD achieve such low manufacturing costs?

We're guessing it has a lot to do with the fact that the BYD blade battery pack is a structural battery pack. Its ladder frame structure will eliminate parts, accelerate production, and provide torsional rigidity to reduce weight and cost in other parts of the vehicle. Furthermore, blade battery cells are large. Large cells require less material and fewer manufacturing operations per kWh of cells produced, which should reduce costs at the cell level.

You may be wondering how the BYD structural pack compares to the Tesla structural pack. Our view is that each approach is optimized for the chemistry it’s been designed around. LFP for BYD and high nickel for Tesla. That’s not to say Tesla’s structural pack can’t be used for LFP, I think it will, it’s just optimized for high nickel. The risk of fire is minimal with the LFP based blade battery. Therefore, it may not need fire retardant material between the batteries. Instead, BYD can use cheap and abundant steel to reinforce their battery cells to create rigidity with a ladder frame design.

Tesla’s structural pack uses 4680 cylindrical battery cells because Nickel based battery cells are high energy and a smaller cell makes the energy easier to contain safely. However, even though the 4680 cell is smaller than blade cells, there’s still a risk of fire spreading between cells. So, Tesla needed to take a completely different approach. Their battery cells are bonded with fire retardant epoxy foam and two face sheets to form a contiguous honeycomb structure that takes advantage of the cylindrical shape of the 4680 to form a structurally unified slab that‘s lightweight, super strong and fire retardant.

In summary, the BYD Blade Battery and BYD Blade Battery Pack are both impressive engineering achievements that capitalize on the strengths of the battery chemistry they were designed around. LFP allows for large battery cells and BYD discovered that long and thin battery cells act as a heat sink to rapidly wick away heat, and appear to reduce heat generation by reducing the number of layers in the battery cells. These effects leverage the inherent safety of the chemistry to create what may be the safest vehicle battery pack that’s commercially available.

Furthermore, the form factor also enables a strong, lightweight ladder frame structure within the vehicle, that when combined with the safety of the blade, allows for a lower profile, lighter weight battery pack. Blade battery packs should allow for vehicles with more range than typical LFP battery packs through higher volumetric energy density. They also appear to be easier to manufacture, resulting in a pack level cost that’s 26% less than CATL LFP batteries. This makes the blade ideal for medium range, low-cost vehicles.

300 miles of range is still a stretch for LFP, despite BYD’s claims of 1000 km potential range with their battery and vehicle architecture. I’m sure they’ll get there, but I think it will take 3 or more years rather than being something we can expect in the next year or two.

Regardless, the blade is a step forward and we hope Tesla ends up working with BYD at some point. The brilliant engineering and low cost of the BYD Blade Battery pack coupled with Tesla’s superior hardware and software would add up to a $25,000 subcompact Tesla vehicle that ticks every box from cost to safety to range while potentially offering roughly 30% margins.

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