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 originally incorrectly assumed they used the P45B. However, the upgrade of the car to the P30B shows that this car used the Molicel INR-18650-P26A.

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 P26A that has a capacity of 2.6Ah.

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 34p would create a pack of 800V and 61.1kWh and 216s 30p would give the 800V nameplate and 60.7kWh

Those are the most obvious options, but there are other options.

The Battery Calculations Workbookbook is a collection of excel sheets that allows you to do estimations such as those shown above and more.

Power

The 216s 30p configuration would be the smarter design as higher voltage is always better. Assuming the cell DCIR is 22mOhms (based on DCIR being 23% better in the P30B) then this could deliver 640kW peak if sticking to the 35A max current as per the specification sheet.

However, the cell resistance will decrease with increasing temperature and hence the pack will be pre-heated for extreme events to reduce the overall resistance and hence enable higher continuous and peak performance.

Pack Weight

• Total cell mass ~ 326kg
  • Pack mass ~479kg (estimated using Pack Mass from Cell Density)
  • Probably 40kg too high for a racecar design and this pack ~400 to 420kg

Packaging

• U-shaped battery pack
• Structural part of the tub – suggests that the enclosure has a dual role

Cell

2021 McMurtry Speirling battery pack uses Molicel 21700 cells, our assumption is that this is the P26A.

• Capacity = 2.6Ah
• Cell Voltage Nominal = 3.6 V
  • Charge maximum = 4.2 V
  • Discharge minimum = 2.5V
• Discharge Current
  • Continuous = 35A (80°C cut-off)
• Temperature
  • Charge = 0°C to 60 °C
  • Discharge = -40°C to 60°C
• Energy Density
  • Volumetric 535 Wh/l
  • Gravimetric 190 Wh/kg
• DCIR ~ 22mΩ

References

1. Molicel B series battery cells unlock faster charging for McMurtry Spéirling PURE electric track car, McMurtry
2. McMurtry Spéirling, Wikipedia
3. McMurtry Automotive
4. What is the McMurtry Spéirling?, Top Gear

See also: Racing Battery Packs, Molicel, Benchmarking Battery Packs, Gen 4 Formula e Battery