Ultra-fast Chargers
Nowhere is ultra-fast charging in bigger demand than with the electric vehicle (EV). Recharging an EV in minutes replicates the convenience of filling 50 liters (13 gallons) of fuel into a tank that delivers 600kWh of energy. Such large energy storage in an electrochemical device is not practical as a battery with such a capacity would weigh 6 tons. Most Li-ion only produces about 150Wh per kg; the energy from fossil fuel is roughly 100 times higher. (See BU-1007: Net Calorific Value).
Charging an EV will always take longer than filling a tank, and the battery will always deliver less energy per weight than fossil fuel. Breaking the rule of law and forcing ultra-fast charging adds stress, even if the battery is designed for such a purpose. We must keep in mind that a battery is sluggish in nature. Like an aging man, its physical condition becomes less ideal with use and age. So is the ability to fast-charge. One assumes that all charge energy goes into the battery, whether charged slowly, rapidly or by ultra-fast method. Batteries are nonlinear devices and most chemistry accepts a fast charge from empty up to about 50% state-of-charge (SoC) with little losses. NiCd does this best and suffers the least amount of strain. Stresses occur in the second half of the charge cycle towards full charge when acceptance becomes labored. An analogy is enjoying the dessert after the hunger is stilled.
Applying an ultra-fast charge when the battery is empty and then tapering off the current when reaching 50% SoC and higher is called step charging. The laptop industry has been applying step charging for many years, so does the EV. The charge currents must harmonize with the battery type as different battery systems have dissimilar requirements in charge acceptance. Battery manufacturers do not publish charge rates as a function of SoC. Much of this is proprietary information.
Research companies claim to achieve benefits with pulse-charging Li-ion instead of applying the regular CCCV charge as described in BU-409: Charging Lithium-ion. The scientific community is skeptical of alternative charging and takes the “wait-and-see” approach.
As our bodies work best at 37ºC (98ºF), so does the transport mechanism improve when a battery is warm. Modern EVs will enable the “pre-charge” feature to prepare the battery temperature for the pending fast-charge while driving. (See also BU-410: Charging at High and Low Temperatures)
Whether you own an EV, e-bike, a drone, a portable device or a hobby gadget, the following conditions must be respected when charging a battery the ultra-fast way:
- The battery must be designed to accept an ultra-fast charge and must be in good condition. Li-ion can be designed for a fast charge of 10-minutes or so but the specific energy of such a cell will be low.
- Ultra-fast charging only applies during the first charge phase. The charge current should be lowered after the battery reaches 70 percent state-of-charge (SoC).
- All cells in the pack must be balanced and have ultra-low resistance. Aging cells often diverge in capacity and resistance, causing a mismatch and undue stress on weaker cells.
- Ultra-fast charging can only be done under moderate temperatures, as low temperature slows the chemical reaction. Unused energy turns into gassing, metal-plating and heat.
An ultra-fast charger can be compared to a high-speed train (Figure 1) traveling at 300km per hour (188 mph). 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.
A well-designed ultra-fast charger evaluates the condition of the “chemical battery” and makes adjustments according to the ability to receive a charge. The charger should also include temperature compensations and other safety features to lower the charge current when certain conditions exist and halt the charge if the battery is under undue stress.
A “smart” battery running on SMBus or other protocols is responsible for the charge current. The system observes the battery condition and lowers or discontinues the charge if an anomaly occurs. Common irregularities are cell imbalance or the need for calibration. Some “smart” batteries stop functioning if the error is not corrected.
The 10-minute Charge
The automotive industry is demanding ultra-fast charging. Research laboratories are responding by heating Li-ion batteries to a temperature that prevents lithium plating while limiting the growth of solid electrolyte interphase (SEI) that occurs at elevated temperatures. The chosen charging temperature is 60ºC (140ºF), heated by heating elements for the duration of the charge and then cooled to about 24ºC (75ºF) with the onboard cooling system of the EV to limit the time the battery dwells at high heat. This enables charging Li-ion at a C-rate of 6C to 80% SoC in 10 minutes.
Charging at 60ºC prevents lithium plating while deterring SEI growth because of the short duration at high temperatures.
A technology called Aligned Graphite® Technology claims to reduce a charge time of 25 minutes to only 15 minutes by organizing the graphite flake on the negative electrode into vertical order. Battrion, a spin-off of the Swiss Federal Institute of Technology (ETH Zurich), says that this orientation reduces the distance lithium travels, enabling very high charge and discharge currents without degradation.
Limitations to ultra-fast charging Li-ion
The maximum charge current a Li-ion can accept is governed by cell design, and not the cathode material, as is commonly assumed. The goal is to avoid lithium-plating on the anode and to keep the temperature under control. A thin anode with high porosity and small graphite particles enables ultra-fast charging because of the large surface area. Power Cells can be charged and discharged at high currents, but the energy density is low. Energy Cells, in comparison, have a thicker anode and lower porosity and the charge rate should 1C or less. Some hybrid Cells in NCA (nickel-cobalt-aluminum) can be charged above 1C with only moderate stress.
Apply the ultra-fast charge only when necessary. A well-designed ultra-fast charger should have the charge-time selection to give the user the option to choose the least stressful charge for the time allotted. Figure 3 compares the cycle life of a typical lithium-ion battery when charged and discharged at 1C, 2C and 3C rates. The longevity can further be prolonged by charging and discharging below 1C; 0.8C is the recommended rate.
Charging and discharging Li-ion above 1C reduces service life. Use a slower charge and discharge if possible. This rule applies to most batteries.
Lithium deposition
Lithium deposition forms if the charge rate exceeds the ability by which lithium can be intercalated into the negative graphite electrode of Li-ion. A film of metallic lithium forms on the negative electrode that spreads uniformly over the host material or gravitates to one region in planar, mossy or dendritic format. The dendritic form is of concern because it may increase self-discharge that in extreme cases can create a short and lead to venting with flame.
Environmental conditions affect the 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-of-charge level
- The buildup is also said to increase as Li-ion cells age due to raised internal resistance.
Consumers demand fast charging at low temperatures and this is especially critical with the electric vehicle. Solutions include special electrolyte additives and solvents, optimal negative to positive electrode ratios, and special cell design.
The question is often asked; “Why do ultra-fast chargers charge a battery to only 70 and 80 percent?” This may be done on purpose to reduce stress, but is also caused naturally by a lag between voltage and state-of-charge that amplifies the faster the battery is being charged. This can be compared to a rubber band lifting a heavyweight. The larger the weight, the wider the lag becomes. The ultra-fast charge forces the voltage to the 4.20V/cell ceiling quickly while the battery is only partially charged. A full charge will occur at a slower pace as part of saturation.
Lithium Titanate may be the exception and allow ultra-fast charging without undue stress. This feature will likely be used in future EVs; however, Li-titanate has lower specific energy than cobalt-blended Li-ion and the battery is expensive. (See BU-205: Types of Lithium-ion)
Nickel-cadmium is another battery chemistry that can be charged in minutes to 70 percent state-of-charge. Like with most batteries, the charge acceptance drops towards full charge and the charge current must be reduced.
All ultra-fast methods need high power. An ultra-fast EV charge station draws the equivalent electrical power of five households. Charging a fleet of EVs could dim a city.
Summary
All batteries perform best at room temperature and with a moderate charge and discharge. Such a sheltered lifestyle does not always reflect real-world situations where a compact pack must be charged quickly and deliver high currents. Such typical applications are drones and remote control devices for hobbyists. Expect a short cycle life when a small pack must give all it has.
If fast charging and high load requirements are prerequisites, the rugged Power Cell is ideal; however, this increases battery size and weight. An analogy is choosing a heavy diesel engine to run a large truck instead of a souped-up engine designed for a sports car. The big diesel will outlive the light engine even if both have the identical horsepower. Going heavier will be more economical in the long run. Table 4 summarizes the charge characteristics of lead, nickel and lithium-based batteries.
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, Li-ion | 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, Li-ion | 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. |
Each chemistry uses a unique charge termination.
Simple Guidelines Regarding Chargers
- If possible, charge at a moderate rate. An ultra-fast charger should provide the option to charge at a regular rate when time allows reducing stress.
- Fast and ultra-fast charge fills the battery only partially; a slower saturation charge completes the charge. Unlike lead-acid, Li-ion does not need the saturation charge but the capacity will be a bit lower.
- Do not apply a fast charge when the battery is cold or hot. Only charge at moderate temperatures. Avoid fast charging an aged or low-performing battery.
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We wsnt to charge 12v 12Ah litheam iron batteries.