2021 McMurtry Speirling

The 2021 McMurtry Speirling is perhaps even more impressive an electric vehicle than the Rimac Nevera.

Note: this is a very limited benchmark with just a few known parameters. This does though show the approach used to look at what the pack configuration might be.

• Goodwood Festival of Speed hill climb record, completing the 1.87-kilometre (1.16 mi) course in 39.08 seconds
• 0-60mph in 1.40s
• 0-100mph in 2.63s
• 0-145mph in 4.98s
• ¼ mile (400m) in 7.97s

We don’t know too much about this battery pack, we do know that it uses a Molicel, we assume they use the P45B and that has been analysed by About:Energy.

Energy content of the cell and energy lost to heat versus different continuous discharge powers.

Even at a 50% state of charge (SOC), the P45B delivers a remarkable power output of 168 W for 10 seconds, while at 90% SOC, a ridiculous 184 W is potentially accessible from the cell. What is more crucial is that the P45B has a relatively flat resistance value within its operational window, allowing for reliable power delivery even in the lower SOC region, while limiting excessive heat generation and mitigating the risk of the cell.

Exploring Molicel’s new power cell, Yashraj Tripathy, About:Energy

Pack Configuration

Using the numbers we have, let’s look at the possible configurations.

It is stated that this is an 800V pack. A quick look at the cell nominal voltage of 3.6V suggests that it might not be based on the nominal voltage as that would result in a very high maximum charge voltage of >940V.

Maybe, as we have seen before, the 800V pack claim is based on the maximum cell charge voltage of 4.2V

An 800V pack can then be claimed with 192 cells in series. A useful number as it is divisible by 16 and 12. Hence making it convenient to construct 12s modules or at least groups of cells.

If we assume 192s for now we can then take a look at the number of cells required to produce a 60kWh pack using the Molicel P45B that has a capacity of 4.5Ah.

Here we now see the pack size in kWh shown against the number of cells in series and parallel. Shown in green are the resultant total pack energy from 60 to 70kWh.

192s 20p would create a pack of 800V and 62.2kWh

Power

Using these numbers along with the cell parameters it appears that at 50% SoC the pack can just deliver the 746kW at a pack current of around 1500A. With a 20p configuration each cell would be delivering 75A, well beyond the continuous current of the specification sheet, but above the minimum cell voltage of 2.5V

Using the numbers from the About:Energy analysis the battery can deliver 192s x 20p x 120W = 460.8kW continuous and a total of just over 38kWh.

Pack Weight

• Total cell mass ~ 269kg
  • Pack mass ~410kg (estimated using Pack Mass from Cell Density)
  • Probably 40kg too high for a racecar design and this pack ~350 to 380kg

We ignored the 75Ah as this appears to just be a “800V and 60kWh, hence must be 75Ah”. At the moment this is an educated guess, as more data is released on this remarkable car we will update our benchmarking data.

Packaging

• U-shaped battery pack
• Structural part of the tub

Cell

The pack uses Molicel 21700 cells, our assumption is that this is the P45B.

• Capacity = 4.5Ah
• Cell Voltage Nominal = 3.6 V
  • Charge maximum = 4.2 V
  • Discharge minimum = 2.5V
• Charge current
  • Standard = 4.5A
  • Maximum = 13.5 A (70°C cutoff)
• Discharge Current
  • Continuous = 45A (80°C cut-off)
• Temperature
  • Charge = 0°C to 60 °C
  • Discharge = -40°C to 60°C
• Energy Density
  • Volumetric 643 Wh/l
  • Gravimetric 242 Wh/kg
• DCIR = 15mΩ

References

1. Exploring Molicel’s new power cell, Yashraj Tripathy, About:Energy
2. McMurtry Spéirling, Wikipedia
3. McMurtry Automotive
4. What is the McMurtry Spéirling?, Top Gear