- FairGreen
- Posts
- Battery Know-what/why (Article 5)
Welcome to article 5 of the Battery Know-what/why Series! The logical follow-up to the topic of cycle metrics is lifetime metrics. What are lifetime metrics? Like all devices, lithium-ion battery (LIB) cells suffer from performance deterioration with increase in age. You have definitely come across a an electronic device with a ‘worn-out‘ battery thus in need of a battery replacement. An experience we are all familiar with is we typically need to recharge our mobile phones more frequently as time progresses. Age can be expressed in the cumulative number of usage years or cycles. LIB cells typically have a lifetime of approximately 10 years or 3000 cycles depending on use characteristics or applications. Why do we care about lifetime metrics? Lifetime metrics reveal battery performance deterioration with age. The most significant lifetime metrics are capacity degradation, state of health (SOH) and roundtrip efficiency (RTE) degradation. The operational timeline of a battery cell is described using terms such as beginning of life (BOL) and end of life (EOL).
Beginning of life (BOL)
Know-what: this refers to the chemical state of a new unused battery. The capacity of a battery to store charge diminishes with age and other factors. The capacity of a battery cell is typically considered ~100% at BOL.
Know-why: the BOL state serves as a baseline for characterizing performance degradation.
Nameplate capacity
Know-what: this is the capacity of the battery cell at BOL and as indicated on the battery cell label.
Know-why: serves as baseline for tracking lifetime performance metrics.
Cycle (capacity) degradation
Know-what: this is a metric characterized by a gradual permanent decrease in the nameplate capacity of a battery cell. Degradation increases with an increase in the number of cycles delivered. Capacity degradation is mainly caused by an increase in internal resistance and loss of electrolyte.
Analogy: imagine that you refill your water tank with water containing sand and the sand settles at the bottom of the tank at the end of the filling process. Imagine you repeat this several times and the volume of sand sediments at the bottom of the tank increases. The increasing volume of sand corresponds to a decreasing volume of water since the tank volume is fixed. Cycle capacity loss in a battery cell is analogous to the decrease in water volume in the tank.
Know-why: determines replacement frequency of batteries thus cost. As such, efforts are made to minimize cycle degradation.
Calendar (capacity) degradation
Know-what: this is another degradation mechanism similar to cycle degradation. The difference between the two is calendar degradation occurs when the cell is at rest (delivering zero power). Calendar degradation is driven by passage of time (age). If one uses their battery sparingly over several years, the battery loses charge storing capability due to age. The combination of cycle and calendar degradation yield total capacity degradation.
Analogy: imagine that you refill your water tank with water containing sand and the sand settles at the bottom of the tank at the end of the filling process. Imagine you repeat this for several years and the volume of sand sediments at the bottom of the tank increases. The increasing volume of sand corresponds to a decreasing volume of water since the tank volume is fixed. Calendar capacity loss in a battery cell is analogous to the decrease in water volume in the tank.
Know-why: determines replacement frequency of batteries thus cost. As such, efforts are made to minimize calendar degradation.
State of health (SOH)
Know-what: refers to the actual capacity of a fully charged battery cell expressed as a percentage of its nameplate capacity at BOL. 100% SOH indicates a battery cell with its full capacity still intact. A battery cell can theoretically reach 0% SOH by losing all its storage capacity. This does not happen in practice.
Analogy: imagine a water tank with an increasing volume of sand sediments at the bottom - SOH is analogous to the instantaneous volume of water in the water tank expressed as a percentage of the volume of water a full tank held before sand began accumulating at the bottom.
Know-why: an indicator of a battery’s end of useful life.
SOC vs SOH: SOH tracks how the capability of a cell to store charge decreases over years while SOC tracks a cell’s remaining capacity since the last charge cycle.
End of life (EOL)
Know-what: refers to the state of a battery that can no longer serve its intended application reliably - basically the end of useful life. In practice, battery cells reach the end of useful life at SOH values around 60% depending on the application. Batteries that have reached EOL for one application can be repurposed in a less demanding application.
Know-why: the EOL state determines battery replacement point.
Roundtrip efficiency (RTE) degradation
Know-what: the ratio of electrical energy out to electrical energy tracks SOH. RTE is directly proportional to SOH thus decreases as SOH decreases.
Know-why: expresses the increase in energy losses as a battery cell ages. Charge/discharge energy losses are minimum at BOL and maximum at EOL.