“The data suggest an inverse power-law dependence of the cycle life on the DoD, such that a four-fold lifetime gain is
achieved going from 100% to 50% DoD,” DeVries, Nguyen, and Op Het Veld write.
Here is a Cycle Life versus Depth of Discharge curve:
Cycle life improves faster than DoD reduces, so that the total charge transferred is greater at lower depth of discharge.
This is significant, because it means that a larger battery used at less than full discharge can be more economic and last longer than a smaller capacity battery used at full depth of discharge.
If a battery pack is designed to have capacity providing long range, it is likely that daily charging will be at low depth of discharge. The impact of this on electric vehicle design is important. It means that the route to long range, high capacity, may also result in lower depth of discharge and longer life for a given battery chemistry. Further, if maximum charging is intentionally limited during most operation, battery calendar life may be extended.
Finally, batteries are characterized by C rates. Simply put, a battery will be specified in amp-hours. An amp-hour is the amount of current the battery can pull in one hour.
The C rate is defined in units of C, where 1C means the battery can be charged in one hour. If the battery is charged at 2C, it means the battery may be charged in half an hour. High-C-rate batteries can be charged or discharged very fast and produce a lot of power. Low-C-rate batteries have lower power.
For lithium batteries of a given chemistry type, modifications can be made to raise or lower C. The tradeoff is higher energy capacity in kWh for lower C, or power. In a battery pack, more cells in parallel lower the peak current in each cell and allow each cell to operate at a lower C rate. In an electric vehicle application, the desired peak battery pack current can be reached with either a pack with more parallel cells (thus, larger energy capacity) or fewer parallel cells and a higher C rate. With parallel cells, a low-C battery can stay within its C limit.
The impact of this on electric vehicles is that a battery pack sized for long range can have lower C rate and higher energy capacity.
Exceeding C rates results in anode changes that degrade performance. Proper electrode operation depends on the electrode surface structure. That structure is changed if the C rate is exceeded.
A number of benefits appear when an electric vehicle battery is sized for long range. A larger-capacity battery results in a lower average depth of discharge and consequently longer cycle life and lower peak charge/discharge rate. If maximum charge is limited to 80% under everyday driving conditions, maximum voltage is avoided. If the battery pack is also thermally controlled, both maximum voltage and high temperatures are avoided. In this way, controlled conditions can increase battery life substantially.