Rechargeable batteries – lithium-ion batteries – are all around us. These potentially flammable batteries power cellphones, computers, tablets, e-bikes, hover boards, and even cars (EVs). Lithium-ion batteries store a charge after plugging them into an electrical outlet. Depending on the quality and size of the battery, it will hold charge and power your device for a prescribed period of time. 

How It Works

The anode releases ions (when battery power is needed). The ions travel through a gel called electrolyte to enter a cathode. A separator prevents the anode from touching the cathode. (More on separators later). 

Recharging? The ions reverse course. The electricity evicts the lithium ions from the cathode back to base camp, the anode. 

So why do lithium-ion batteries lose their charge?  And why does it get worse over time? 

Every time a lithium-ion battery is charged and discharged, tiny changes occur inside it. Over time, these changes reduce the battery's ability to store energy. This can happen for several reasons:  

1. During charging the anode hosts a defense layer around it called a Solid Electrolyte Interphase (SEI). After many charges the defense layer becomes bulkier than before. The thicker it gets, the more ions become trapped and unavailable to carry energy.  
2. If the room is hot, a 100% charged battery deteriorates faster than normal. Heat expands the SEI shield too much too quickly.  
3. The ion give-and-take highway breaks down over time. Each charge brings about 10% expansion/contraction, causing cracks in the channel.  
4. If the room is cold during charging, the ions cannot return to the anode. These lost ions loiter on the anode and create a lithium metal plate. The ions are lost forever, leaving less ions for your charge. Dendrites can form and grow large enough to pierce separator, which could short-circuit the system. (More on short circuits later).   

The Dangers

Dendrites can cause fire, but the reaction is slow. Fortunately, Teslas have a battery management system that alerts the driver to any dendrite formation. But e-bikes generally do not warn you of a growing problem. Fires can happen fast if an EV car crashes. The thin battery separator gets crushed, forcing the anode and cathode to meet. When they touch, a chemical reaction of hundreds of degrees of heat in a split second cause a fire. A nail punched into the battery could cause the same fire. So could dropping the battery from a high height. 

Sometimes the news reports charging devices like an e-bike as the cause of a home fire. Those fires probably happened because electricity continued to enter the battery after full charge. Why? Although current e-bikes have a battery management system, owners sometimes buy an aftermarket mismatched charger. If the charger fails to shut down the charge, the cathode releases oxygen. The ions form those lithium metal plates and short circuits happen. The result is a fire.  

The opposite charge percentage can cause fire as well. For example, if a battery is at zero percent, the anode’s copper current collector could migrate in liquid form to the electrolyte. Then when you recharge that battery, the melted copper in the electrolyte forms a dendrite, and a short circuit can happen. 

Heavy heat surrounding the battery can raise the battery temperature to 260 degrees fahrenheit. At that point, the separator melts, and you better call the fire department. 

Not all lithium batteries are perfect when they leave the factory. Floating copper or iron dust inside the battery can move around and eventually attack the separator, causing a short circuit. his can happen just after purchase or several months down the line. 

Warning Signs

It’s important to look for signs of a damaged lithium-ion battery.  If the battery looks buff or is bulking out, cracking, leaking or has a white/blue/green powder on the terminals, you have a problem.  Look for the following: 

• The battery becomes too hot to handle while it’s charging and the device is not in use 
• The battery drains too fast.  
• The battery takes too long to charge / doesn’t charge.  

Action steps in the event of problem: 

• Unplug from the charger. 
• Relocate the device far from flammables like couches, beds, or carpet.  
• Take the device to an electronic recycler. Don’t use the street trash collection. Garbage trucks compress the garbage, which could cause a fire.  

Signs of immediate danger (call 9-1-1): 

• A burning smell. 
• Hissing, cracking, popping sounds from inside. 
• Smoke (means the fire has started).  

Learn More

• Argonne National Laboratory: Science 101: Batteries 
• U.S. Department of Energy: DOE Explains…Batteries 
• Let’s Talk Science: How does a lithium-ion battery work? 
• ThermoFisher Scientific: The History of the Lithium-Ion Battery

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- Written by Robert Ambrose