Today, producers of electric vehicles are being asked to provide increasingly challenging performance levels: longer range, fast recharghing and lower maintenance costs.

Charging Process of Lithium ion batteries:
A typical charging process of a single lithium battery cell is shown in Fig. below. It consists of two stages. During the first one a constant current (CC stage) is applied to the cell, causing a voltage increase from 3.0 V, associated with a depleted state, up to 4.2 V which corresponds to the nominal cell voltage.

At this point the cell voltage is controlled to remain constant (CV stage) until the current reaches a value close to zero.

Characteristics of Fast Chargers and Ultra Fast Chargers:
Fast Chargers and Ultra Fast Chargers are different in parameters like Voltage rating, Power rating and Charging time.

They also differ in their C rates. Ultra fast chargers require higher C rates and hence higher value of currents as depicted in table below.

Type Chemistry C-rate Time Temperatures Charge termination
Slow charger NiCd
Lead acid
0.1C 14h 0ºC to 45ºC
(32ºF to 113ºF)
Continuous low charge or fixed timer. Subject to overcharge. Remove battery when charged.
Rapid charger NiCd, NiMH,
0.3-0.5C 3-6h 10ºC to 45ºC
(50ºF to 113ºF)
Senses battery by voltage, current, temperature and time-out timer.
Fast charger NiCd, NiMH,
1C 1h+ 10ºC to 45ºC
(50ºF to 113ºF)
Same as a rapid charger with faster service.
Ultra-fast charger Li-ion, NiCd, NiMH 1-10C 10-60 minutes 10ºC to 45ºC
(50ºF to 113ºF)
Applies ultra-fast charge to 70% SoC; limited to specialty batteries.



Table: Charger Characteristics: Each chemistry uses a unique charge termination



An ultra-fast charger can be compared to a high-speed train traveling at 300km per hour. Increasing power is relatively simple. It’s the track that governs the permissible speed of a train and not the machinery. In the same manner, the condition of the battery dictates the charging speed. Hence, fast charging is more about lithium ion chemistries than the power of the charger. Lithium cobalt oxide (LiCoO2), which has high energy density, tends to be the preferred chemistry for this type of application.


Limitations of Ultra Fast Charging:


1. The battery charging time can be reduced increasing the value of the current employed at the CC stage, but high currents can reduce the time of life of the cell or cause permanent damage as depicted in figure. The longevity can further be prolonged by charging and discharging below 1C; 0.8C is the recommended rate.



Fig: Cycle performance of Li ion with 1C, 2C and 3C charge and discharge


2. To operate at power of around 150 KW or higher during ultra fast charging, current existing Li ion battery has to weigh around 1000 kg as most Li-ion battery only produces about 150Wh per kg.This is not practical to have a battery with such heavy weight. Remedy is to choose the high voltage inorder to reduce the battery weight during fast and ultra fast charging.

3. Stresses occur in the second half of the charge cycle towards top charge when acceptance of lithium ions in the anode of Li-ion becomes labored. This can cause rupture.

4. It can cause deposition of lithium as follows:

  • Lithium deposit grows when Li-ion is ultra-fast charged at low temperature.
  • Deposition develops if Li-ion is ultra-fast charged beyond a given state-ofcharge level.
  • The buildup is also said to increase as Li-ion cells age due to raised internal resistance.

How to make effective Ultra Fast Chargers?

1. Resistance of the cells must be lowered by using thin materials, increasing the amount of current collectors, and increasing the electrolyte concentration and reducing its viscosity with solvents. 2. In EVs, the Pulse-Charging technique is commonly used as fast charging method for lithium-ion battery pack. Pulsed chargers feed the charge current to the battery in pulses. During the charging process, a short rest period of 20 to 30 milliseconds, between pulses allow the chemical actions in the battery to be stabilized by equalizing the reaction throughout the bulk of the electrode before recommencing the charge. This method can also reduce unwanted chemical reactions at the electrode surface such as gas formation, crystal growth and passivation. At this stage, it is possible to charge the battery at high current flow which makes charging faster for battery without any damage of gas release.

3. The recommended ultra-fast charger should have three settings: Overnight Charge (0.5C); Fast Charge (0.8–1C) and Ultra-fast Charge (above 1C). This allows the user to limit ultra-fast charging to only when needed and at a suitable temperature.


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