Question: Charge to 100% and drive for 500 metres and the SOC drops to 97%. Drive for a few more km e.g 5 km at 7%. Now, the dashboard shows 90%. Shutdown the vehicle and turn the vehicle ON after few hours.
Answer: Some of the reasons for the above can be explained as below.
1. Measurement and calibration of voltage vs SoC.
2. Voltage ‘Bounce back’,
A commonly used term that is used for a phenomenon for Li-ion cells. During high current draws, the voltage across terminals drops lower than the open circuit voltage, or the voltage for lower current draws.
This happens on account of internal vs external resistance changing constantly.
This graph kind of shows that effect. (Source)
Which means it is extremely difficult to measure the instantaneous SoC and you cannot rely on the voltage alone. Based on the average current drawn, the value of the displayed SoC has to be fine tuned.
3. Sudden drop in voltage for the initial draw.
This again can be one of the factors that you see because of which the SoC suddenly drops for the initial few kms. Especially if the acceleration is high as soon as the vehicle starts.
4. The last thing that may be happening can be
Once the vehicle gets fully charged, the charging point cuts off power and the battery goes into the idle state. After which the charge goes down by a certain value. After a while, it restarts charging back to 100, and the cycle continues. It may so happen that you remove the charger during the idle stage when the battery is discharged by a small amount and as soon as you start moving the SoC drops.