Clearing up a kW conundrum

Current E Spark-Renault powertrain

If you caught yesterday’s Formula E news about Jarno Trulli at the sport’s official website, you may have come across a phrase that confused you: “the final race-going 200kw battery”.

In our guide to decoding the terminology behind electric vehicles (which you can read here), we explained that battery capacity is normally described in kilowatt hours (kWh), and the power of electric motors in kilowatts (kW). That’s because kWh is a measure of energy, whereas kW is a measure of power.

Up until now, Formula E organiser FEH has specified that the Spark-Renault battery is rated at 30kWh, and that the electric motor (the motor generator unit, or MGU) develops 200kW. Is the latest information incorrect, then?

No, as our electrical engineering expert Chris Vagg explains – it’s just describing a different facet of performance.

“Think about an electrical powertrain in terms of a conventional, internal combustion engine set-up,” Vagg explains. “You have a petrol tank, an engine, and a fuel pump. The petrol tank and fuel pump are roughly equivalent to the battery; the engine is like the MGU.”

The petrol tank will have a finite capacity, measured in gallons or litres. That’s the energy store for the powertrain. In a similar manner, the battery is an energy store, but measured in kWh.

The ICE engine will have a power output, usually rendered in bhp or PS, which describes the rate at which the engine converts the energy from the petrol into something else – kinetic energy to drive the wheels. That’s exactly like the MGU in an electric car, except that its power is usually defined in kW.

So far, so straightforward.

“The confusion here comes from the rate at which energy is delivered to the motor,” says Vagg. “A battery will have an energy capacity, in kWh – but it will also have a rate at which in can discharge or replenish that energy – and that can be talked about in kW.”

The battery acts a bit like the ICE fuel pump, then, delivering energy to the MGU at a particular rate – and that rate affects how the motor works. “If you have an MGU rated to 200kW, you need a battery capable of supplying energy at that rate,” says Vagg.

Think back to the ICE car: there’s no point having a snarling V12 if the pump can only deliver a tiny stream of petrol. The solution is to fit a fuel pump capable of delivering what the engine asks of it. Similarly, the battery must be able to meet the draw of the MGU.

“In this way, the battery can be said to be rated to 200kW – but it will probably actually be able to supply more than that in order for the motor to be able to hit 200kW,” says Vagg.

Approaching the comparison logically, describing the battery in terms of its discharge capacity doesn’t define the size of the energy store however – in the same way that knowing how hard the petrol pump can work doesn’t tell you how big the petrol tank is.

Has the battery size changed? No. “It’s still 30kWh,” an FEH spokesperson assures us. Thus, the battery can be said to be both 200kW and 30kWh.

Phew. Got all that?

2 Comments

  • Cliff says:

    “Thus, the battery can be said to be both 200kW and 30kWh.”

    But not at the same time. There is always the fine print. The battery pack is rated to deliver 30 kWhrs under specific conditions including a set power level. One condition that is often used as standard is the capacity the pack will deliver if discharged completely in an hour (referred to as the 1C rate).

    So this pack might be rated to deliver 30 kWhrs (capacity) if discharged at 30kW (power). At 30 kW, the pack will be empty in an hour. If we discharge the pack slower, say over two hours, we will get greater capacity. We might be able to run at 16 kW (power) for two hours giving us a total of 32 kWhrs (capacity) from the same pack.

    On the other hand, if we discharge faster at the 200 kW (power) rate, we will get less than 30 kWhrs. How much less? That is the key question that will determine how long the races will be. Any predictions?

    • Ross Ringham says:

      Hi Cliff – good to hear from you! Spot on, as usual. It appears the races will still be no more than an hour, with two mandatory car changes. So each stint is likely to be in the region on 20 minutes. But it also seems that the cars will indeed have different power restrictions, giving each driver a higher and a lower power car. That will really make speculating tough – from di Grassi’s comments today, it sounds like it’s going to be quite the mathematical headache for teams trying to plan strategy.

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