Comparison of e-car charging performance: will ISO 12906 bring clarity?

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The higher the charging capacity of an electric car, the better its potential suitability for traveling. That is clear. What is less clear is how the performance of the traction batteries can be compared. Although the manufacturers' factory specifications are a guide, they are not standardized. ISO 12906 could provide a remedy. It is a standard, not a law. ISO 12906 is intended to enable transparency and comparability – and make it easier for potential buyers to choose an electric car and the corresponding battery size. What is most important: the peak power when charging, the time it takes to charge from 10 to 80 percent, or the range that can be recharged within a defined period of time?

Things look bad without preconditioning

The starting point for the charging performance of an electric car is the factory specification of the charging time within a certain window. This usually refers to charging from 10 to 80 percent. Some manufacturers vary the lower limit. Regardless of which measurement a manufacturer refers to, a key question in heise Autos' practical tests is whether the promised charging time is reproducibly maintained, even in cold or hot conditions. This requires automatic or manual preconditioning. Automatic means that the traction battery is heated or cooled specifically for the calculated charging stop during route guidance in the navigation system.

This now works excellently for many manufacturers and not at all for some. Tesla Model Y and BYD Seal show just how big the difference can be between a perfect battery system with preconditioning and a less sophisticated battery system. Both have cold-sensitive LFP cells (for lithium iron phosphate). A piquant fact: BYD supplies the cells for the battery system in the Tesla Model Y. Tesla does not provide any information on the charging performance of the electric cars, apart from “up to 275 km in 15 minutes”. The measurement of a Model Y test car in winter showed 20 minutes for the charging stroke from 10 to 80 percent, which is an excellent value. BYD states 37 minutes for the same charging stroke for the Seal, i.e., almost twice as long – and also narrowly misses this statement in a check in summer 2024. That's how big the differences are.

What ISO 12906 specifies

Electric car drivers who only occasionally travel long distances should only opt for a vehicle with automatic or manual preconditioning. Volkswagen, for example, offers a choice between automatic and manual mode: in the latter case, the display shows how high the charging power is at the current level of preconditioning and how high it can still get. ISO 12906 stipulates that preconditioning should be used during the measurement if it is available. However, the standard also makes it easy for other electric cars because the vehicles are left in the laboratory for twelve hours at 23 degrees before they are measured.

Four parameters are measured for ISO 12906:

• Range gain in kilometers after 10 minutes
• Maximum charging power
• Charging time in minutes for the stroke from 10 to 80 percent
• charging efficiency

To measure the range gain in kilometers after ten minutes, the electric car is driven down to a battery charge level (abbreviated SOC for State Of Charge) of nine percent or less and then charged as quickly as possible. The electrical energy recharged in ten minutes (ISO 12906 also recommends measuring points after five and after 20 minutes) is now set in relation to the WLTP power consumption. This also includes charging losses.

Unfortunately, the average value of the WLTP is used here; it would have been more realistic to select the “Extra High” section from the four sub-cycles of the WLTP. Here, too, the average speed is only 92 km/h. Nevertheless, up to 131.3 km/h is reached for a short time; the “Extra High” sub-cycle in the WLTP therefore comes closest to highway driving. Electric cars with a steep charging curve can play to their strengths when it comes to the range gain after ten minutes because they start with a very low SOC. Steep means that the maximum charging power in kilowatts is very high for a short time, even if it drops significantly over time.

Maximum vs. curve

The maximum charging power is the next parameter specified in ISO 12906. For a VW ID.3 (test) with 59 kWh energy content, this is now 165 kW. A Hyundai Ioniq 5 with 84 kWh achieves 260 kW. The maximum charging power says nothing about the curve. A steeply sloping charging curve leads to drivers trying to drive the electric car down to the SOC range of the highest power. Tesla is an example of this. A charging curve that is more like a plateau creates more flexibility, but is usually not associated with a particularly high peak power. This is the case with the Audi Q8 e-tron, for example.