The current generation of electric vehicles relies on lithium battery packs with an energy range between 16kWh and 53kWh. A single gallon of gasoline contains more than 36kWh of energy.
For an electric or hybrid electric vehicle (HEV), or any high-power battery system, to compete with an internal combustion engine (ICE) requires squeezing every bit of energy out of the batteries. To accomplish this, each individual cell within the pack has to be carefully monitored and controlled.
High power battery packs consist of a long string of series connected cells. Directly connected to each cell is a battery monitor IC, responsible for accurately measuring each cell voltage. This is no simple task, as the cells are positioned at various points along a very high voltage string that is subject to horrendous electrical spikes and electromagnetic interference (EMI).
>A battery management system (BMS) combines the cell voltage with current, temperature, and operating history, to continuously assess each cell's condition. It's a tough challenge, but with accurate monitoring and control, the driving range, reliability and safety of the battery pack can be maximized.
Batteries in an HEV or EV are expected to last 10 to 15 years, and they are considered to be at their end-of-life when they have lost 80 percent of their original capacity. Battery lifetime and reliability are maximized by restricting the operating state of charge by not allowing them to be fully charged or discharged.
A typical battery pack is operated in a restricted range, such as 20 percent SOC to 80 percent SOC, where SOC is the State-of-Charge. These SOC limits could be adjusted with age and operating conditions, such as high temperature environments. As a result of the limits, battery packs are not utilized to their full capacity. For example, operating a pack with 20 percent SOC to 80 percent SOC limits the usable SOC range to 60 percent.
The challenge for the BMS is to operate each cell as close to the limits as possible, without exceeding them. Amplifying the challenge, Lithium batteries exhibit a flat discharge curve over their operating range. As a result, there is a very small change in cell voltage over the operating range and the battery monitor must make very accurate measurements as part of the SOC calculation.
To illustrate the importance of cell measurement accuracy, consider the simplified lithium battery discharge curve shown below. This curve has a constant 5mV/percent (SOC) slope across the operating region. A battery pack operating within a 20 percent to 80 percent SOC range and a similar discharge characteristic will face a big penalty for poor cell voltage measurement accuracy.
Simplified Battery Discharge Curve
If the battery monitor has a cell voltage measurement error of ±10mV, a measured cell voltage of 3.75V could actually correspond to a real cell voltage between 3.74V and 3.76V., as shown below; This corresponds to an actual SOC range from 76 percent to 80 percent.