Energy

EV Batteries

In the early 1900s, the electric vehicle was reserved for dignitaries the likes of Thomas Edison, John D. Rockefeller, Jr. and Clara Ford, the wife of Henry Ford. They chose this transportation for its quiet ride over the vibrating and polluting internal combustion engine. Environmentally conscious drivers are rediscovering the EV with a choice of many attractive products.

The EV culture is developing distinct philosophies, each satisfying a unique user group. This is visible with vehicle sizes and the associated batteries. The subcompact EV comes with a battery that has 12–18kWh, the mid-sized family sedan has a 22–32kWh pack, and the luxury models by Tesla stand alone with an oversized battery boasting 60–85kWh to provide extended driving range and achieve high performance. Table 1 lists the most common EVs.

Model Battery Charge Times
Toyota Prius
PHEV
4.4kWh Li-ion, 18km (11 miles) all-electric range 3h at 115VAC 15A;
1.5h at 230VAC 15A
Chevy Volt
PHEV
16kWh, Li-manganese/NMC, liquid cooled, 181kg (400 lb), all electric range 64km (40 miles) 10h at 115VAC, 15A;
4h at 230VAC, 15A
Mitsubishi iMiEV 16kWh; 88 cells, 4-cell modules; Li-ion; 109Wh/kg; 330V, range 128km (80 miles) 13h at 115VAC 15A;
7h at 230VAC 15A
Smart
Fortwo ED
16.5kWh; 18650 Li-ion, driving range 136km (85 miles) 8h at 115VAC, 15A;
3.5h at 230VAC, 15A
BMW i3
Curb 1,200kg
(2,645 lb)
22kWh (18.8kWh usable), LMO/NMC, large 60A prismatic cells, battery weighs 204kg (450 lb) driving range of 130–160km (80–100 miles) ~4h at 230VAC, 30A;
50kW Supercharger; 80% in 30 min
Nissan Leaf* 30kWh; Li-manganese, 192 cells; air cooled; 272kg (600 lb), driving range up to 250km (156 miles) 8h at 230VAC, 15A;
4h at 230VAC, 30A
Tesla S*
Curb 2,100kg (4,630 lb)
70 and 90kWh, 18650 NCA cells of 3.4Ah; liquid cooled; 90kWh pack has 7,616 cells; battery weighs 540kg (1,200 lb); S 85 has up to 424km range (265 mi) 9h with 10kW charger; 120kW Supercharger, 80% charge in 30 min

Table 1: Electric vehicles with battery type, range and charge time. 
*  In 2015/16 Tesla S 85 increased the battery from 85kWh to 90kWh; Nissan Leaf from 25kWh to 30kWh.

The makers of Nissan Leaf, BMW i3 and other EVs use the proven lithium-manganese (LMO)battery with a NMC blend, packaged in a prismatic cell. (NMC stands for nickel, manganese, cobalt.) Tesla uses NCA (nickel, cobalt, aluminum) in the 18650 cell that delivers an impressive specific energy of 3.4Ah per cell or 248Wh/kg. To protect the delicate Li-ion from over-loading at highway speed, Tesla over-sizes the pack by a magnitude of three to four fold compared to other EVs.

The large 90kWh battery of the Tesla S Model (2015) provides an unparalleled driving range of 424km (265 miles), but the battery weighs 540kg (1,200 lb), and this increases the energy consumption to 238Wh/km (380Wh/mile), one of the highest among EVs. (See BU-1005: Fuel Cell Vehicle.)

In comparison, the BMW i3 is one of the lightest EVs and has a low energy consumption of 160Wh/km (260Wh/mile). The car uses an LMO/NMC battery that offers a moderate specific energy of 120Wh/kg but is very rugged. The mid-sized 22kWh pack provides a driving range of 130–160km (80–100 miles). To compensate for the shorter range, the i3 offers REX, an optional gasoline engine that is fitted on the back. Table 2 compares the battery size and energy consumption of common EVs. The range is under normal non-optimized driving conditions.

EV make Battery Range km (mi) Wh/km (mi) Energy cost/km (mi)
BMW i3 22kWh 135km (85) 165 (260) $0.033 ($0.052)
GM Spark 21kWh 120km  (75) 175 (280) $0.035 ($0.056
Fiat 500e 24kWh 135km (85) 180 (290) $0.036 ($0.058)
Honda Fit 20kWh 112km (70) 180 (290) $0.036 ($0.058)
Nissan Leaf 30kWh 160km (100) 190 (300) $0.038 ($0.06)
Mitsubishi MiEV 16kWh 85km (55) 190 (300) $0.038 ($0.06)
Ford Focus 23kWh 110km (75) 200 (320) $0.04 ($0.066)
Smart ED 16.5kWh 90km (55) 200 (320) $0.04 ($0.066)
Mercedes B 28kWh (31.5)* 136km (85) 205 (330) $0.04 ($0.066)
Tesla S 60 60kWh 275km (170) 220 (350) $0.044 ($0.07)
Tesla S 85 90kWh 360km (225) 240 (380) $0.048 ($0.076)

Table 2: Estimated energy consumption and cost per km/mile of common EVs. Energy cost only includes the consumed electricity at $0.20/kWh; service items are excluded.
* Driving range limited to 28kWh; manual switch to 31.5kWh gives extra 16km (10 mile) spare

Clarification: The driving ranges in Tables 1 and 2 differ. This is less of an error than applying different driving conditions. Discrepancies also occur in topping charge, depth of discharge and fuel-gauging.

The cost of an EV battery has come down to about $350/kWh, but Tesla managed to lower the price to $250/kWh using the 18650, a popular cell of which 2.5 billion were made in 2013. The 18650 in the current Tesla models is an unlikely choice as the cell was designed for portable devices such as laptops. Available since the early 1990s, the 18650 cell is readily available at a low cost. The cylindrical cell-design further offers superior stability over the prismatic and pouch cell, but the advantage may not hold forever as prismatic and pouch cells are improving. Large Li-ion cells are relatively new and have the potential for higher capacities and lower pack-cost as fewer cells are needed.

All EV makers must provide an 8-year warranty or a mileage limit on their batteries. Tesla believes in their battery and offers 8 years with unlimited mileage. Figure 3 illustrates the battery that forms the chassis of the Tesla S Model. The Model S 85 contains 7,616 type 18650 cells in serial and parallel configuration. The smaller S-60 has 5,376 cells.

Figure 3: Battery in a Tesla S Model chassis. The 85kWh battery has 7,616 18650 cells in parallel/serial configuration. At $250 per kWh, the cost is lower than other Li-ion designs.
Source: Tesla Motors

EV manufacturers calculate the driving range under the best conditions and according to reports, the distances traveled in the real-world can be 30–37 percent less than advertised. This may be due to the extra electrical loads such as headlights, windshield wipers, as well as cabin heating and cooling. Aggressive driving in a hilly countryside lowers the driving range further.

Cold temperature also reduces the driving range. What battery users may also overlook is the difficulty of charging when cold. Most Li-ion cannot be charged below freezing. To protect EV batteries, some packs include a heating blanket to warm the battery during cold temperature charging. A BMS may also administer a lower charge current when the battery is cold. Fast charging when cold promotes dendrite growth in Li-ion that can compromise battery safety. (See BU-410: Charging at High and Low Temperature)

EV owners want ultra-fast charging and technologies are available but these should be used sparingly as fast charging stresses the battery. If at all possible, do not exceed a charge rate of 1C. (See BU-402: What is C-rate?) Avoid full charges that take less than 90 minutes. Ultra-fast charging is ideal for EV drivers on the run and this is fine for occasional use. Some EVs keep a record of stressful battery events and this data could be used to nullify a warranty claim. (See BU-401a: Fast and Ultrafast Chargers)

Estimating SoC has always been a challenge, and the SoC accuracy of a battery is not at the same level as dispensing liquid fuel. EV engineers at an SAE meeting in Detroit were surprised to learn that the SoC on some new BMS were off by 15 percent. This is hidden to the user; spare capacity makes up for a shortfall.

EV makers must further account for capacity fade in a clever and non-alarming way to the motorist. This is solved by oversizing the battery and only showing the driving range. A new battery is typically charged to 80 percent and discharged to 30 percent. As the battery fades, the bandwidth may expand to keep the same driving range. Once the full capacity range is needed, the entire cycle is applied. This will cause stress to the aging battery and shorten the driving ranges visibly. Figure 4 illustrates three SoH ranges of an EV fuel gauge.

Figure 4: Driving range as a function of battery performance. A new EV battery only charges to about 80% and discharges to 30%. As the battery ages, more of the usable battery bandwidth is demanded, which will result in increased stress and enhanced aging.

Economics

On the surface, driving on electricity is cheaper than burning fossil fuel; however, low fuel prices, uncertainty about battery longevity, unfamiliarity with battery abuse tolerances and high replacement costs are factors that reduce buyer incentives to switch from a proven propulsion system to the electric drivetrain. The EV will always have shorter driving ranges than vehicles with ICE because oversizing the battery has a diminishing return. When the size is increased, batteries simply get too heavy, negatively affecting travel economics and driving range. (See BU-1005: Fuel Cell Vehicle, Figure 1.)

Technology Roadmaps as part of the International Energy Agency (IEA) compares energy consumption and cost of gasoline versus electric propulsion;

An EV requires between 150Wh and 250Wh per kilometer depending on vehicle weight, speed and terrain. At an assumed consumption of 200Wh/km and electricity price of $0.20 per kWh, the energy cost to drive an EV translates to $0.04 per km. This compares to $0.06 per km for a similar-size gasoline-powered car and $0.05 per km for diesel. Price estimations exclude equipment costs, service and the eventual replacement of the product.

Battery endurance and cost will govern the success of the EV. A consumer market will likely develop for a light EV with a battery providing 160km (100 miles) driving range or less. This will be a subcompact commuter car owned by a driver who adheres to a tightly regimented driving routine and follows a disciplined recharging regime. According to research, 90 percent of commuting involves less than 30km. The EV market will also include high-end models for the ecology-minded wealthy wanting to reduce greenhouse gases.

Driving an EV only delivers optimal environmental benefit when charging with renewable resources. Burning coal and fossil fuel to generate electricity, as is done in many countries, does not reduce greenhouse gases. In the US, 50 percent of electricity is generated by burning coal, 20 percent by natural gas and 20 percent by nuclear energy. Renewable energy by hydro is 8 percent and solar/wind energy is only 2 percent.

Going electric also begs the question, “Who will pay for the roads in the absence of fuel tax?” Governments spend billions on road maintenance and expansions; the EV, and in part the PHEV, can use the infrastructure for free. This is unfair for folks using public transport as they pay double: first paying income tax to support the road infrastructures and second in purchasing the train fare.

The high cost of the EV against the lure of cheap and readily available fossil fuel will slow the transition to clean driving. Government subsidies may be needed to make “green” cars affordable to the masses, but many argue that such handouts should be directed towards better public transportation, systems that had been ignored in North America since the 1950s.

Guidelines for EV Batteries
  • Life span. Most EV batteries are guaranteed for 8 years or 160,000km (100,000 miles). Hot climates accelerate capacity loss; insufficient information is available about how batteries age under different climates and usage patterns.
  • Safety. Concerns arise if the battery is misused and is kept beyond its designated age. Similar fears occurred 150 years ago when steam boilers exploded and gasoline tanks burst. A carefully designed BMS assures that the battery operates within a safe working range.
  • Cost. This presents a major drawback as the battery carries the cost of a small car powered by an ICE. BMS, battery cooling, heating and the eight-year warranty add to the cost.
  • Performance. Unlike an ICE that works over a wide temperature range, batteries are sensitive to heat and cold and require climate control. Heat reduces the life, and cold lowers the performance temporarily. The battery also heats and cools the cabin.
  • Specific energy. In terms of calorific value per weight, a battery generates only 1 percent of what fossil fuel produces. One kilogram (1.4 liter, 0.37 gallons) of gasoline yields roughly 12kWh of energy, whereas a 1kg battery delivers about 150Wh. However, the electric motor is 90 percent efficient while a modern ICE comes in at about 25 percent.
  • Specific power. The electric propulsion system has better torque with the same horsepower than the ICE. This is reflected in excellent acceleration

Source: Battery University