Ultra-Fast Charging: Respecting the Limits of a Battery when Feeding

Content Contributed by Isidor Buchmann, CEO & Founder, Cadex Electronics, Inc.

Consumers demand faster charging times. Leading in this movement is the electric vehicle (EV) industry that strives for charge times similar to filling up a vehicle at a gas station. Pumping 50 liters (13 gallons) of fuel into a tank holds a calorific value of 600 kWh. The fill-up is quick. An EV battery, in comparison, only stores between 50 to 100 kWh of energy and charging takes a long time.

Charging an EV will always take longer than filling a tank with fuel; the battery will always deliver less energy per weight than fossil fuel. A modern Li-ion for EVs produces up to 250Wh per kg; energy from fossil fuel is 13,000Wh/kg, 50 times higher. The advantages of the electric drive are high energy efficiency and clean power. These are valid reasons to switch to electric propulsion.

Ultra-fast charging is a necessity for inter-city travel but it has two drawbacks. One is the expensive power feed of up to 120 kW per station that equals the power needs of five households. A less mentioned disadvantage is the stress induced on the battery when ultra-fast charging.

Most EVs can be charged with three charging systems.

  1. Level 1 is a cord-set that connects to a regular household outlet of 115 VAC, 15 A (230 VAC, ~6 A in Europe). The single-phase hookup produces about 1.5 kW; charge time is 7 to 30 hours depending on battery size. Level 1 meets overnight charging requirements for e-bikes, scooters, electric wheelchairs, and PHEVs with batteries not exceeding 12 kWh.
  2. Level 2 is a 230 VAC, 30 A two-pole hookup resembling a household dryer to charge a mid-sized EV in four to five hours. This is the most common charging station for EVs, producing about 7 kW to feed the 6.6 kW on-board EV charger.
  3. Level 3 is the DC Fast Charger delivering 400 to 600 VDC, up to 300 A. It bypasses the on-board charger and feeds power directly to the battery. Level 3 chargers deliver up to 120 kW to fill a Li-ion battery to 80 percent state-of-charge in about 30 minutes.

The battery is an electrochemical device that can only absorb a given amount of energy. Charging Li-ion occurs by intercalation of lithium ions and electrons on the electrodes. Trying to push more energy into Li-ion than it can effectively absorb creates an over-feed condition. Metallic lithium builds up on the anode, resulting in lithium plating that forms dendrites which compromises safety and shortens battery life. The same symptoms also occur when charging Li-ion at cold temperatures when the intercalation is slowed.

Charge acceptance is governed by cell design, and Li-ion comes in two versions. The Power Cells with its large surface area permits high load currents and fast-charging. This cell is used for power tools and less for EVs because of low specific energy. The more common Energy Cell has a high specific energy (capacity) but its current handling is limited; it also requires longer charging times that the Power Cell. The EV battery is a hybrid gravitating towards the Energy Cell for high capacity and long range.

To achieve fast charging and long driving ranges, the EV battery is being oversized, and the Tesla S 85 is such an example. Supercharging its 90 kWh battery dumps about 90 kW into the battery. This represents a charge C-rate of 1C for a time. As the battery fills, the C-rate falls to a more comfortable 0.8C, and then goes further down, avoiding harmful battery stress that is related to ultra-fast charging.

People ask, “Why does an ultra-fast charger only charge a battery to 70 and 80 percent?” The simple answer is, charge acceptance is best in mid-range; battery stresses are also reduced in mid-range.

When putting the battery on charge, the voltage shoots up. This behavior is similar to lifting a weight with a rubber band in which the weight, or charge, lags behind. Depending on charge times, Li-ion is about 70 percent charged when reaching 4.20 V/cell, a voltage threshold that is common with Li-ion. At this phase, the current begins to taper and charge acceptance slows.

Ultra-fast charging Li-ion must meet these conditions to minimize stress and maintain safety:

  1. The battery must be designed to accept ultra-fast charge.
  2. The battery must be in good condition. Aging slows charge acceptance.
  3. Ultra-fast charging only works to 70 percent state-of-charge (SoC); topping charge takes longer.
  4. All cells must have low resistance and be well balanced in capacity. Weak cells are exposed to more stress than strong ones. This worsens condition of the weak cells further.
  5. Charge at a moderate temperature. Low temperature slows the intercalation of lithium-ions, causing an energy over-supply. Unabsorbed energy turns into gas buildup, heat and lithium plating. Some large batteries include heating and cooling systems to protect the battery.

Increasing the charge current is simple, assessing how much energy a battery can absorb is more difficult. An analogy is a high-speed train traveling at 300 km per hour (188 mph) on a good track. Powerful motors are easy to build, but it’s ultimately the track that governs the speed. In the same manner, the condition of the battery dictates charging times.

A well designed ultra-fast charger evaluates the battery condition to match the charge current with the abortion rate. The charger should also adjust to temperature and observe cell balance. Furthermore, 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. While such a charger may not yet exist, basic battery knowledge and common sense should prevail when charging batteries in an unconventional way.

It is best not to fully charge Li-ion. Every reduction in peak voltage of 0.10 V/cell is said to double the cycle life. (See How to Prolong Lithium-based Batteries.) This is why EVs only operate the battery between 30 and 80 percent SoC when new. The BMS widens the SoC bandwidth as the battery ages to maintain the desired driving range. Equally important are cool temperatures and moderate charge rates.

Moving Away from the Internal Combustion Engine to Batteries
European governments have set an end-date for the internal combustion engine (ICE) in cars. It took 100 years to build the fossil fuel infrastructure, and it may take equally long to switch to electric propulsion. Private enterprises built gas stations; in many regions today tax payers subsidize charging stations and the purchase of an EV. Commuters biking to work or taking transit see this handout as dismay because they pay double.

An important issue that is being overlooked by rule makers is assuring the suitability of a battery in an EV. The Tesla S 85 battery weighs 540kg (1,200 lb). In comparison, a gasoline engine is 130 to 350 kg (300 to 800 lbs). Furthermore, the battery is the weakest component in most devices; its longevity is often lower than the host it powers. Take a battery-operated drill that is wonderful when new but the battery is the first to go. Replacement packs, if available, are expensive and a good drill is often discarded prematurely.

As the EV replaces cars with combustion engines, we ask: “Will the EV hold its value? Will the EV eventually match the price and driving range of a regular car? How will the battery perform when the 8-year warranty expires? Will replacing the pack be economical or will the car be discarded similar to an old mobile phone or electric tooth brush when the battery goes? What environmental problems will develop disposing of large EV batteries?

Batteries for the EV are of higher quality than those in consumer products and experts say that the EV battery will outlive the car. Lab tests have proven this to be true but the real test comes when the shiny new EVs age. Civilized driving, moderate temperatures, and good charging practices help prolong battery life.

EV buyers will, however, cringe when learning that a replacement battery carries the price of a compact car with an internal combustion engine. Regulatory officials should assure that replacement batteries are available at a reasonable cost, lest the EV becomes a disposable item alike a cordless drill or smartphone when the battery dies.

Companies have sprung up that test EV batteries past retirement for reuse in secondary applications. (See Giving Batteries a Second Life). Typical uses are energy storage for solar panels in residences. Not enough information is available on the viability and safety of these batteries in a second life.

Battery users have a strong desire to prolong the life of a battery and here are recommendations of what the battery custodian can do:

  • Charge at a moderate rate. Ultra-fast charging causes stress. (NiCd is the only battery that can accept ultra-fast charge with minimal stress.)
  • If possible, do not fill Li-ion to 100 percent state-of-charge. (Only lead acid requires a fully saturated charge to prevent sulfation.) Most chargers charge the battery fully.
  • Prevent elevated temperatures. Keeping Li-ion at full charge and elevated temperature causes more stress than cycling under normal conditions.
  • An ultra-fast charge fills the battery only partially; saturation charge completes the charge at a slower pace. Go easy on the saturation as Li-ion does not need a full charge.
  • Do not apply fast charge when the battery is hot, cold, has mismatched cells or is faded.

Gas Evolution during Unwanted Lithium Plating in Li-Ion Cells with EC-based or EC-free Electrolytes. By Q. Q. Liu, D. J. Xiong, R. Petibon, C. Y. Du and J. R. Dahn. http://jes.ecsdl.org/content/163/14/A3010.full

IsidorBCadexIsidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behavior of rechargeable batteries in practical, everyday applications, has written award-winning articles including the best-selling book “Batteries in a Portable World,” now in its fourth edition. Cadex specializes in the design and manufacturing of battery chargers, analyzers and monitoring devices. For more information on batteries, visit www.batteryuniversity.com; product information is on www.cadex.com.