The process of adding energy to a battery cell or cells in a pack and so moving the State of Charge from a low value towards 100%. This simple process is actually far from simple and covers the electrochemical, thermal, electrical, infrastructure, standardisation and all the way through to electricity generation.

Approaches to charging:

  • Constant Current – Constant Voltage (CC-CV)
  • Constant Power – Constant Voltage (CP-CV)
  • Multistage Constant Current – Constant Voltage (MCC-CV)
  • Pulse charging
  • Boost charging with a CC-CV-CC-CV scheme
  • Variable Current Profile (VCP)
  • Stepped Constant Current – Constant Voltage (SCC-CV)
fast charging symbol

Key Parameters in Fast Charging

When looking at the key parameters in fast charging a battery pack it is worth looking at the complete system. Also, it is good to look from the cell at atomic scale through the thermal system design to the charging algorithms and handshakes.

The drive for a 10 min fast charge to reach 80% state of charge is tough against the other pressures of reducing cost and shrinking the pack. In most cases this fast charge is the worst case in terms of power requirements for the battery pack.

Stepped Fast Charge Limits

We often see stepped fast charge limits as shown in this BMW iX3 graph.

Often the result of a limited test regime applied by the cell supplier (in this case CATL) to establish the maximum charge current for the cell.

These limits are them applied by the BMS team and without more data they have to drop back to safe charge current limits between these points.

Types of EV Charging Connectors

Alternating current (AC) Chargers – This type of Chargers are good for Home, work and Public use. They are mostly slow to mid speed charging. AC chargers are split into Type 1 and Type 2.

Direct Current (DC) Chargers – These type of chargers are mostly found in Public environment as they need special additional equipment, network connections etc. They are mostly used for Fast Charging. These are called CHAdemo or Combo Type 1 or 2 (CCS connector).

Current from the Grid (AC) goes through EVSE (Charging Station Either Level 1 or 2) through a connector to the On Board Charger. The Onboard Charger rectifies or converts the current into DC and sends it via the Battery Management System (BMS) to the Battery Pack. The Alternating Current AC can be either Phase 1 or 3 depending on the Chargers.


Vehicle to Grid (V2G) allows the battery in an electric vehicle to put energy back into the electricity grid. Essentially the battery has a bi-directional connection. This allows:

  • energy to be sold back to the grid
  • support for local brown-outs
  • ability to power your house off-grid


Vehicle to Vehicle (V2V) allows the battery pack on one vehicle to be used to charge another electric vehicle.

Hot Swap versus Fast Charge

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.