· By Austin Duggan
Thermal potting for l-ion battery fire safety
Lithium ion battery packs are dangerous. A lithium ion cell will erupt into flame if it's overcharged, over-discharged, overheated, punctured, or penetrated with liquid. And if one cell ignites, that eruption is likely to ignite the neighboring cells, and those cells erupting their neighbors. This is called "thermal runaway". It's what causes the sorts of fires that have tragically burned down NYC apartments. The fires spread incredibly quickly and are extremely difficult to put out. Here's a video of some examples.
The following five practices are (or at least should be) standard for responsible battery manufacturers to decrease the chances of runaway battery fires.
Reputable cells: A battery pack can only be as safe as the cells it's comprised of. Off-brand cell manufacturers cut corners on quality control and, as a result, make a product that is unsafe. Responsible battery manufacturers exclusively use cells from reputable brands that have been thoroughly tested (e.g. Samsung, Sony, Molicel, etc). We use the Molicel P42a in our "Competitor" battery, and the Samsung 50S in our new 36V "Commuter" battery.
Fuse: If the wires coming from the negative and positive terminals of a battery pack come into contact, they will emit extremely high amounts of current all at once. It's enough to cause a fire. So responsible battery manufactures make sure that there is a fuse between one of these terminals and any wiring connected to it. We use an input and output fuse.
Cell-spacing: Thermal runaway is more likely when the cells are touching each other. So battery manufacturers will space the cells apart, often using fire resistant cell-holders. We space our cells 2.2mm apart using a thermal epoxy that has proven incredibly resilient against thermal runaway (more on this below).
Encasing: Battery manufacturers encase their packs in a way that provides some protection against puncture and liquids. For electric bicycles, the most common material is plastic. Because lithium ion cells emit gas when they erupt, these cases must allow "venting" so as not to build up enough pressure during battery eruption to cause explosions. We use a 6061-T6 aluminum case with 7075 aluminum end caps, which offer far better fire and puncture resistance than plastic. It has sealed seems throughout that can expand in case of extreme venting.
BMS: Every battery pack has a minimum and maximum voltage, as well as a maximum charge and discharge current. Responsible battery manufacturers use a battery management system (the acronym is "BMS") in each of their battery packs. At a minimum, the BMS is designed to shut the output of the pack off when the battery reaches its minimum voltage or maximum output current, and shut the input of the pack off when the battery reaches its maximum voltage or maximum input current. Typically, these BMS devices are also designed to "balance" the battery pack by keeping each cell at the same voltage relative to all the others, thereby ensuring that no one cell will ever fall above or below its threshold of safety.
For reference, here's a chart of the minimum and maximum safe voltages for various lithium-ion packs, as seen at Riderguide.
Our "Competitor" 14s2p pack
As mentioned above, we make our 52v nominal "Competitor" battery pack here in the U.S. using the reliable and high performance Molicel P42a cell. Each of these cells is spaced 2.2mm from any neighboring cell, spot-welded with .3mm thick pure nickel. We also use an input and output fuse. Instead of the plastic found on the batteries coming out of China, we use a 6061-T6 aluminum case with 7075-T6 endcaps that is highly resistant to punctures, but has seems throughout that allow venting in case of cell eruption.
As for our BMS, a 52v nominal battery should never be discharged below 42v or charged above 58.8v. So our BMS will not allow our riders to continue throttling their bikes when the battery approaches 42v, and will resist charging above 58.8v. It also balances the cells and limits the charge and discharge current. However, as discussed below, we also decided to take the additional step of potting our batteries in thermal epoxy.
When precautions fail
Each of the aforementioned standards is often cited as a best practice for battery manufacturers. Batteries would be much safer if manufacturers unwaveringly adhered to them. Still, we consider these standards necessary for safety, but not sufficient. After all, not every BMS will function perfectly. And even when the BMS is functioning well, the materials that encase battery packs (especially plastic!) aren't puncture or water proof. So although battery fires from the more responsible manufacturers are rare, they do occur.
Our primary concern when designing our pack began with a simple question: what happens when these standard precautions fail? What happens when, for example, the battery is punctured in a bike wreck, the BMS fails when connected to the wrong charger, the cells short out by becoming submerged in rain water, or the battery is simply left near a furnace or baking in the sun all day by a negligent owner? The answer was terrifying.
Even battery packs utilizing all of the aforementioned precautions can suffer from thermal runaway and quickly cause a large fire when they fail. This is because the standard safety precautions are aimed primarily at mitigating battery failure, but not so much at mitigating battery fire during those rare cases in which battery failure occurs. Especially for high-C Molicel packs like ours, the spacing between cells just isn't enough, and plastic cases won't hold up.
Thermal epoxy is a semi-rigid material that is thermally conductive but electrically non-conductive. It is virtually water-proof when cured. In its non-cured, liquid state, it looks sort of like a thick gel. Here's a quick clip of it settling into one of our battery packs after being poured into it. When thermal epoxy is poured into a battery case in its non-cured liquid state, it fills all the gaps between and around the cells, and then cures. Very few battery manufacturers encapsulate their packs in this material (with the exception of Luna Cycle). But they should.
Tests have shown that encapsulating battery packs with material of this kind nearly eliminates the possibility of penetration by liquids, increases resistance to punctures, shock absorption, and heat dissipation, and most importantly drastically reduces the chances of thermal runaway and fire propagation.
We chose to use the Epic Resins material S7253. The data sheet is here. Epic Resins S7253 is formulated with a combination of both mineral and non-halogenated flame retardants with impressive thermal conductivity. A study by Electric Goddess (pdf here) showed that when a pack utilizing this material between each cell is ignited, the only cells that propagated were those cells immediately surrounding the initiation cell or a maximum of six cells in total. This is an incredible reduction in thermal runaway. When such a pack is also encapsulated inside this material and encased in aluminum like our Competitor pack, the chances of an uncontainable fire become vanishingly small.
Potting batteries in thermal epoxy adds a little bit of cost and effort to the already complicated battery manufacturing process. But we think it's worth it to help avoid catastrophe. It's the additional step that mitigates runaway battery fires.