# How does Formula E measure remaining battery energy?

My understanding is that the remaining energy from a battery can be approximated using several different methods e.g. measuring the battery voltage. However, I am yet to find a method that is completely accurate.

While some inaccuracy shouldn't generally be an issue when checking your phone's battery percentage, surely a sport like Formula E would need to use the most accurate method to ensure fairness?

For example, in the 2014 Beijing ePrix, 3 drivers were given 57 second penalties for exceeding the maximum energy usage limit of 28kWh. Given a transgression like this can result in such a significant penalty, you would expect that the method for measuring this usage must be highly accurate.

Therefore, I wondered what method they use in Formula E to measure the remaining battery energy?

I see nothing that suggests that the battery itself is measured. Instead, I would presume that the energy flow is measured directly (rather like the power meter into a house).

From the 2017-2018 Formula E regulations:

7.6

The amount of energy that can be delivered to the MGUs by the RESS is limited to 28 kWh. Braking regeneration will be added to this value based on a factor of 0.75 to take account of losses (1kW regen = 0.75 kW released). Measurements will be taken on each DC branch of each power bus. Energy and power going out of the DCDC converter and into the 12V circuit will be subtracted from the RESS energy and power limitation up to a power of 1.2 kW.

This will be permanently monitored by the FIA. Sensors specified by manufacturers and approved by the FIA, 1 specific set for the FIA. The FIA datalogger will monitor :

Sensors directly connected to the FIA logger (sensors defined by the manufacturer, approved by the FIA, and homologated)

• DC voltage on each power bus
• DC current on each branch of each power bus
• DC current on output of DCDC converter (HV to 12V)
• Accelerator pedal position
• Front and rear wheel speed (left and right)

That leads me to believe that the total power consumption is determined by examining the logged values of the current used, not by examining the battery directly.

• Thanks for this answer. Yes, that makes sense, as I guess this is a far more accurate and fair way to calculate energy usage than trying to measure directly from the battery, given the possible inaccuracies associated with this. Apr 10, 2019 at 19:55