At this point we’ve talked about the hazard of lithium-ion batteries in addition to a few possible solutions, so let’s analyze a specific situation and see what conclusions we can draw from it. A few facts and assumptions about NYC:
- NYC Population is 8.5 million people.
- Assume 2 handheld electronic devices (cell phones, tablets or laptops) per person, for a total of 17 million devices.
- There are 60,000 e-bike delivery people in NYC.
- There are 25,000 rental e-bikes in NYC.
- It’s unknown how many personal e-bikes are in NYC, but we’ll estimate 115,000.
- In total, we’re assuming 200,000 e-bikes in NYC for all three categories. (The total in the US is only a few million, so this passes the sniff test.)
Hazard Calculations
Let’s evaluate the hazards associated with each, assuming the probability of a cell catching fire is the same for both an e-bike battery and a cellphone battery. This will simplify the math.
Cell Phone Hazard
About 1 in 5 million cell phones will spontaneously catch fire each year, resulting in 3 fires in NYC per year. For comparison, Canada, which has 38 million citizens, experienced about 37 fires per year for five years. Assuming the additional fires come from abuse, there would be about 8-10 cell phone fires per year in NYC. Since cell phone fires are relatively small and unlikely to ignite the surrounding materials, the hazard is minor.
Cellphone hazard: N * F * I * D = 10 fires x 10 Wh = 100 Wh
E-bike Hazard
There are 200,000 e-bikes in NYC, with each e-bike battery pack containing about 50 cells. This means that there are about 10 million e-bike battery cells in NYC. If 1 in 5 million cells catch fire, then there would be…well, two fires every year. But last year alone there were 216 e-bike fires, meaning they are occurring 100 times more frequently than they are for cell phones. And since we know the number of fires, we can calculate the hazard: N * F is 216, and I * D (which is just the size of the battery) is about 500 Wh.
E-bike hazard: N * F * I * D = 216 fires * 500 Wh = 100,000 Wh
Thus, 200,000 e-bikes are creating 1000 times as much hazard as about 17 million electronic devices. So, you might be wondering “is 1000 the right number?”. While it’s hard to know for sure, a hazard number that is 1,000 times worse for e-bikes compared to cellphones would match the media attention.
The bottom line is that e-bike fires are occurring 100 times more frequently than cell phone fires. What gives?
Why Are E-bikes So Much Worse?
It’s hard to give an exact answer without examining the situation more closely. For now, the pundits are finger-pointing at cheap Chinese battery cells, but nobody is offering any real analysis or evidence. Could the Chinese cells catch fire 100 times more often? I don’t think so—because they would be catching fire in their factories and in many other devices as well.
Let’s take a closer look through the lens of our structure for battery safety and see what the culprits could be.
Protection
Each battery cell is encased in steel, so there is adequate protection for the cell. The battery itself is also in a case, but with some abuse and damage, there could be the ingress of water, dirt, conductive particles and other debris that could cause short circuits inside the pack.
Mitigation
The cases I’ve seen will not contain the fire for long, allowing the fire to ignite the surrounding materials quickly. Flat zero for mitigation with lots of room for improvement.
Detection
Are there temperature, voltage and other sensors on the batteries? We’ll have to dig in to see. If not, then a zero here as well.
Reduction
It’s likely that the cells have no special fire protection and are standard 18650s. And although the packs are often made with plastic parts that will contribute to the fire, this would be minor compared to the batteries themselves.
Perfection
This could be lower for cheaply built cells, but the cells from Samsung, LG, Sanyo and others are all top notch, making up a significant portion of the e-bike battery market.
Control
It’s likely that there are no special features built into these cells to help control the flow of energy. There is significant opportunity for improvement here.
The “Good” and the “Bad”
The bad news is that the fires are happening and they are big enough to cause a city-wide uproar in NYC.
The good news is that the technology exists to make safe packs, and just needs to be applied broadly. The opportunities include using better casing materials, thermal protection, sensors, and safer cells.
The Silver Lining
Though the current situation is bleak, the industry can better itself. To help, Soteria Battery Innovation Group will sponsor a project to understand the current state-of-the-art and work with the industry to develop best practices guidelines to build safe e-bike batteries. The project will involve disassembling batteries, testing cells, profiling e-bike users, measuring the abuse e-bike batteries receive and giving the industry opportunities to speak into the best practices. We will outline it here in our next newsletter. If you are interested to participate, stay tuned, message me or leave a comment below.
0 comments
