Converting Power Into Confidence

For drivers who are considering making the switch from traditional ICE (internal combustion engine) vehicles to electric vehicles (or EVs), the transition involves learning a whole new electric vocabulary around volts, amps and kilowatts. EV battery charging is not the same as filling up with petrol. There’s no single price at the pump. Many different factors come into play, which is why it is essential to understand EV charging curves, especially when using public or Level 3, DC fast charging stations.

Level 3, fast charging stations, deliver 200-920 volts of direct current (DC). Because the conversion of the electric current from AC (from the grid) to DC (for storage in the battery) takes place within the charging station, EV charging can be very quick, much faster than at home. Public charging stations now provide a range of charge from 50 kW to 350 kW; at 350 kW an EV can be charged in less than 15 minutes.

So what is a charging curve?

When charging an EV battery, the speed or rate at which the battery charges changes. This is illustrated by a charging curve, which shows the different charging speed from 0 to 100 percent of charge. The speed of charge is not constant. Essentially, the charging speed is faster when the battery is nearly empty and slower when it’s nearly full. Filling up a water bottle is frequently used as an analogy; when the tap is first turned on, the flow of water quickly fills up the bottle, so then the tap is turned down to control the flow, before eventually closing, to prevent the contents of the bottle overflowing.

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Why does the charging curve matter?

Knowing the charging curve of your EV gives you a better understanding of pricing for EV charging stations. Although tariff structures differ between countries, networks and charge point operators, the rate or speed of charge is the main pricing feature. Public charging stations are more expensive than home charging, and the faster the charging rate, the higher the price per kWh.

One example is the Shell Recharge Network which offers subscribers different charging prices according to the rate of charge; Fast (7-22kW) £0.45/kWh, Rapid (50kW) £0.55/kWh and Ultra-rapid (150kW & above) £0.65/kWh and sets different prices for access to other networks such as Ionity (350 kW). Their price for ultra-rapid charging is 44% higher per kWh than fast charging.

There are many elements which influence the charging rate to the extent that each EV model has a different charging curve. The most important factor to consider is the battery state-of-charge (SoC).

Let’s consider the peak charging rate of two EV models; the Audi e-tron GT which has a DC peak charging rate of 260 kW and the Volkswagen iD4 with a rate of 135 kW. The ‘peak’ rate represents the highest point of the charging curve for each model.

The Audi e-tron will charge at its maximum rate of 260 kW from 5% to 50% of its battery, and then gradually reduce the rate of charge so that at 75% of charge the rate is around 100 kW, slowing down to 50vkW when the battery is at 95%.

The Volkswagen iD4 will quickly ramp up to its peak charge at 135 kW, then slow down to 100 kW from 30% to 50% of charge, and then drop down again to 80 kW through to 80% of charge. With a battery capacity of 82 kWh capacity, testing has shown that it takes about 38 minutes to charge the iD4 to 80 percent, but 65 minutes to reach 100%. This means the battery takes almost as much time to charge from 0 to 80 percent as from 80 to 100 percent.

At the other end of the spectrum, waiting until the battery is close to zero before charging up is not recommended either, since the time it takes for the battery reach peak charging places unnecessary strain on the battery. Indeed, this is why most EV manufacturers list charging times between 20% and 80%. Not only does it help increase charging speeds, but it also protects the battery.

Understanding the charging curve of your EV allows you to optimize charging times at public charging stations. So for long journeys, it makes more sense to charge the vehicle to 75 or 80 percent and resume the journey, than to charge to full at each stop. There’s really no need to charge the EV to 100%.

What other factors influence the charging curve?  

It is important to remember that a battery is a system which stores chemical energy and converts it into electrical energy thanks to an electro-chemical reaction. This reaction is influenced by temperature and time, as well as the latest vehicle software updates. Each EV manufacturer uses different battery chemistries and cooling technologies, and each will have its own set of optimal operating and charging instructions.

At low temperatures, it takes longer for batteries to reach their peak charging rate; but high temperatures can damage the battery. This is another reason why charging rates automatically slow down when the battery is around 80 percent, to protect it from overheating. Manufacturers recommend maintaining the charge above 20 and below 80 percent to protect the battery and maximize the battery lifespan.

Watt & Well 25 kW Modular Power Unit

The MPU is a 25 kW EV charging module, able to convert power from AC tri-phase grid to DC EV battery. It features a modular design capable of parallel operation and can be used as a part of a high-power, fast-charging system such as 150 and 300 kW EVSE (Electric Vehicle Supply Equipment systems). This DC power module is compatible with smart charging and can dynamically apply limitations on its grid current consumption.

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