Electric vehicle have become a common sight on streets today, and many people have started to enjoy the quiet comfort, getting used to charging at night and silently doing errands.
Electric vehicles are new enough to wonder, how long will they last? This is not a simple question, because it depends on chemistries, sizes, operating conditions, and pack configurations.
Most electric vehicle batteries are lithium based. When a lithium battery is charged and discharged once, it is called a cycle. Lithium battery capacity degrades as the cycle number increases. Battery cycle life is measured in cycles, with an industry standard of cycles to 80% capacity often used as a benchmark. What makes lithium batteries last a long time? Let’s turn that upside down. What shortens lithium battery life?
- High temperatures.
- Overcharging or high voltage.
- Deep discharges or low voltage.
- High discharges or charge current.
Overcharging at high temperatures — what does that mean? When a lithium battery is charged, its voltage goes up slowly. When it reaches full charge, battery voltage is highest, and will not go up much more. The max voltage (V) varies with lithium cell chemistry. Chemistries ranging from laptop batteries to power tools using lithium-cobalt blends and blends containing manganese, nickel, and aluminum have terminal voltages around 3.9 to 4.2V. Lithium-titanate batteries charge to 2.85V. Lithium-iron-phosphate batteries charge to about 3.65V.
Lithium battery voltage must be prevented from exceeding this voltage because it not only ruins battery life; it can lead to battery destruction or overheating and fire in some lithium batteries. Battery management systems (BMS) are used to control charging voltage so that the max charging voltage and temperature is never exceeded.
High voltage also leads to another limit, called calendar life. When foreign matter builds up, it prevents the flow of ions at the electrodes. Lithium-ion batteries contain electrodes, conductors through which current enters or leaves the cell. In between the electrodes is an electrolyte, a solution used to conduct current between the cells. Conduction is achieved through exchange of ions between electrodes and through the electrolyte. The chemical interaction within the battery is called a redox reaction.
When a lithium-bearing electrolyte comes in contact with the electrode, it forms a layer. The interface where the exchange of ions happens between the electrode and electrolyte is called the solid electrolyte interface (SEI) and this forms an SEI layer.
Buildup of material blocks the flow of ions at the SEI layer at the end of calendar life.
The SEI layer contributes to internal resistance. As the battery ages, the layer increases and internal resistance increases. At some point, the layer becomes large enough that no ions can pass and the battery life ends. This kind of battery lifetime limit is worsened the longer the cell is kept at maximum voltage and high temperature. The idea here is to avoid maximum voltage and high temperatures for extended periods of time. Battery manufacturers are aware of this, and keep their batteries at states of charge of as low as 40% to maintain battery capacity during storage and shipment.
To increase cell calendar life, overvoltage and high temperatures must be avoided.
At the other end of cell voltage and charge, for maximum cycle life, deep discharge must be avoided.
The charge level in batteries is described in two ways. One description is called state of charge (SoC). If a cell is fully charged, it is said to be at 100% SoC. The other description is depth of discharge (DoD). If a cell is fully discharged, it is said to be at 100% DoD. That means a 100% SoC cell is the same as 0% DoD. SoC works like a fuel gauge.
For maximum battery cycle life, 100% DoD must be avoided. Researchers have found that improvements in cycle life increase non-linearly as depth of discharge is reduced.