HOVERBOARDS ARE CHALLENGING BATTERY SAFETY
What do Chappaqua, NY and Brentwood, CA have in common? Burning hoverboards….this is a new category of battery-powered levitating boards, sort of electrically-powered, self-balanced skateboards. In both cases, the fires and ensuing explosion were attributed to the lithium-ion battery. Amazon has discontinued the sale of such devices. So what is happening?
I will leave the particulars of each incident to investigative authorities. Instead, I will address the potential failure points of such systems — and how their designs, or perhaps lack thereof, put the safety of consumers at risk.
Back in the early 2000s, laptops suffered from many recalls because of battery safety issues. Two major sources of problems were identified: i) the quality of the battery design and manufacturing, and ii) operation of the battery under unsafe conditions. The battery & PC industries, under the weight of costly recalls, reacted and implemented rigid processes to ensure the safety of their products. The major Li+ battery manufacturers, in particular Sony Energy, LG Chem, Samsung SDI, implemented extensive safety design features in the cells themselves, as well as exhaustive test procedures to ensure the safety of the battery under a wide range of conditions — for example, batteries today are designed to withstand a sharp object piercing it (this has become a common test today). In parallel, electronic circuitry with advanced safety features were added alongside. This circuitry ensured that corner conditions, such as excessive voltage or excessive current or even excessive temperature conditions triggered a safe shutdown and isolating the battery from the rest of the electrical systems. The result today is that the vast majority of consumer devices follow stringent safety rules and are by-and-large very safe.
A hoverboard, in contrast, has a much larger lithium-ion battery than any of other consumer electronics. Most smartphones have batteries with capacities in the range of 8 to 15 W.h. Laptops and tablets have capacities in the range of 30 to 80 W.h. A hoverboard’s battery has a capacity of 150 to 200 W.h. This is a lot of energy that can cause serious damage if not operated properly and safely.
There are no known standards yet for hoverboard batteries but judging from the offerings on Internet stores, the battery pack seems to consist of a stack of 11s and 2p (22 cells in total), totaling a nominal voltage of 36V and charge capacity rating of 4,400 mAh. That means that each cell has a rating of 2,200 mAh, and is most likely a cylindrical 18650. These types of cells have become commoditized (they were used in PCs some 10 years ago) and are made in China often by low-tier manufacturers. I estimate the cost to be in the range of $15 to $25 for the entire pack. They sell on eBay for about $75 each. Several of them carry a Samsung label — I would not be surprised if they are all fraudulently made in China.
So how can these batteries be unsafe? Many factors come into consideration. First, 18650 cylindrical cells, especially with lower capacity ratings (2 to 2.5Ah) are virtually all made in China, many in factories that lack the quality control and safety measures that their Japanese and Korean counterparts learned the hard way 10 to 15 years ago. Second, the operation of the hoverboard requires relatively large currents (typically 3 to 6 A). Wirings, solder junctions, and battery contact points all become potential failure points if not designed properly. Thermal design in such tight volumes becomes paramount — leading potentially to fatiguing of the wirings and connectors. In a nutshell, when several such considerations lack in the design and manufacture of the product (as is commonly the case with products coming out of China), the outcome is quite unsafe leading to fires, explosion and possibly loss of life.
So before you run out a buy yourself or your family a hoverboard for the holidays, do yourself a favor, complete your diligence to ensure that the product has gone through stringent qualification tests and procedures.