Hot Swap versus Fast Charge

Hot Swap vs Fast Charge: Swapping battery packs has come and gone, only to return in force with NIO having swapped 20 million packs (up to May 2023). However, how does this compare to fast charging?

Battery Swap

For this comparison we are using the 100kWh NIO ES6 as the default.

Fast Charging

For fast charging perhaps the Porsche Taycan is the ultimate benchmark.

Charge Time

The NIO ES6 has a quoted range of 372 miles with the 95kWh usable battery and that can be swapped in just under 5 minutes at a battery swap station.

  • Average charge power = 1,140kW

The 83.7kWh usable battery in the Porsche Taycan gives a range of 295 miles and charges from 5 to 80% in 22.5 minutes.

  • Average charge power = 167kW

167kW is my estimation of the power assuming 75% capacity added in 22.5 minutes. Data as measured by P3 [3] gives an average power of 227kW.

Structure

A concern with battery swapping is that the connection between the battery and the body is weaker.

Having a battery swapping station network is a huge investment and means the battery and installation need to be kept the same across the vehicle lines and through model years. This is a significant constraint.

Against this, NIO have just announced the 150kWh ES6 with a ground breaking 260Wh/kg battery that fits in the same location as the 100kWh pack.

In the case of fast charging the battery to body structure can be optimised with respect to connections and load paths.

The likelihood of a battery pack breaking is very low and hence it is very unlikely it will ever be removed from the vehicle before being recycled.

This post has been built based on the support and sponsorship from: About:EnergyAVANT Future MobilityQuarto Technical ServicesTAE Power Solutions and The Limiting Factor. 

Design

The battery pack shape is fixed by the need to reduce the number of options and to fix the design of the swap station.

Cell to Pack and Cell to Body is reducing the overall impact of the battery package on the height of the vehicle. This level of integration and optimisation will always be ahead of a battery swap option.

Cost

NIO have a one time fee of ~$4200 for battery swapping and this gives you 4 battery swaps per month [2].

Let’s assume 10 years and you use this to the maximum this gives you:

  • 12 months x 4 per month x 10 years = 480 swaps
  • $4200 / 480 = $8.75/full charge
  • $8.75 / 372 miles = $.023 / mile

The cost of fast charging varies. Example [1] cost $16.80 to charge to 99%.

  • $16.8 / 295 = $0.057 / mile

However, if you only charge from 5 to 85% the fast charge cost is much lower [1] at $4.80

$4.80 / (295 x 0.75) = $0.022 / mile

The cost of your time of 17.5 minutes has not been taken into account. However, I would suggest that on long journeys stopping regularly for a coffee break is a good thing.

Note that you can pay on a yearly basis to access the Porsche fast charge network.

Investment

For this we need to look at the cost of the swapping stations and extra battery packs needed in the system.

  • cost of a swap station ~$500,000
  • up to 21 packs held at a station ~$250,000

Fast charge station cost:

  • up to $140,000 installed

As you would need 4 fast chargers to match the overall speed of the swap station this puts the investment cost in the same ballpark.

Durability

Swapping the battery pack means that it has to be disconnected. In small emobility vehicles swapping batteries has resulted in connectors wearing out. However, a good mechanical design and material selection should allow this to be minimised.

Fast charging a battery pack does age it faster. The high current generates heat in the cells and the onboard cooling system needs to operate to reject this heat.

Network

Battery swapping allows a lower impact on the networks and, therefore, lower power charges, and also some flexibility to charge at times of better price or lower carbon content.

On the other hand, it requires more than one battery per vehicle, with the consequent impact on the amount of minerals needed.

  • capable of 408 swaps/day
  • 80% charge of 100kWh pack
  • => 408 x 80kWh = 32.64MWh
  • over 24 hour day that would require 1.36MW of power supply, ignoring charger efficiency

Most fast charging hubs reduce the peak power to any vehicle if every charger is being used.

Conclusion

There are pros and cons with both options. Nio are demonstrating that battery swapping is viable and like by customers. They have also shown that it doesn’t need to be a design constraint with their latest 150kWh semi-solid state pack fitting the existing envelope.

However, the cost of fast charging is roughly equivalent to the cost of the battery swap. Plus you need to pay up-front for the battery swaps. Overall for me this makes the pay as you go fast charging a better option for the average user as a lot of the time you will charge at home, slower but much cheaper.

Which would you chose: Hot Swap versus Fast Charge?

Hoang Nhat: The swapping concept had quite a few difficult points that need to be overcome:

  1. GD&T
    • it required maintaining the accuracy of battery tray’s hole less than 1mm.
    • It come from requirement of High Voltage/Low Voltage/Coolant connector spec..
  2. Available components
    • At the time i worked on this project, there was no swapping high voltage connector that can withstand >500 Amp. it did not match with our project.
    • And same story with coolant connector.
  3. Expense for construction.

Please let us know your experience with both of the these options. Also, we would like to know details around the different design requirements nigel@batterydesign.net

References

  1. Watch The Porsche Taycan Fast Charge Like A Champ, InsideEVs
  2. NIO slashes EV prices and removes key free battery swap service, electrek
  3. Charging Index Report 07/22 – Comparison of the fast charging capability of various electric vehicles, P3 Group
  4. Take A Closer Look At A Nio EV Battery Swapping Station In China, InsideEVs

Designing swappable batteries

Designing Swappable Batteries

This article reviews and looks at designing swappable batteries. When plug-in charging is replaced with swapping, it leads to some non-obvious differences. Through the prism of practical situations, the readers can understand what is important in designing swappable batteries including the development of its concept, choosing the optimal form factor, and working out external parts and battery management systems (BMS). Meanwhile, swappable and fixed batteries share essentially one inner design, so we aren’t looking at it.

2 thoughts on “Hot Swap versus Fast Charge”

  1. Thank you for the informative comparison between fast charging and swapping. However, in conclusion it was mentioned that while fast charging and swapping cost the same, user has to pay upfront for the swapping. In this context, I would like to point out that in swapping, the battery pack is usually owned by the manufacturer or the swapping centers which reduces the initial cost of the EV by almost 40%. Therefore, the initial cost of ownership of EVs and time saved per charge/swap of 17.5 min make swapping much attractive.

Leave a Comment