For all car buyers, regardless conventional car or electric car (EV), convenience is a prerequisite. For EVs the charging of the battery is an important part of the convenience.

Even though for most the daily use of an EV there is enough electric energy stored in the battery, range has always been an issue. One solution is a series hybrid vehicle (plug-in electric vehicle, PEV) with a small reciprocating engine as an onboard battery charger. For pure EVs (battery only) the solution for range has been to equip the car with a large battery. Tesla has been a leader in this respect. Model S base model has a 60-kWh battery providing 200 miles’ range. There is also a 90 kWh option providing 300 miles. Nevertheless, for long distance driving that range may not be enough. More energy than what can be stored in the battery is needed. Tesla early recognized the importance of developing a proprietary network of fast chargers, called super chargers. Access to their super charger network, which until last year was free, has been a selling point. The power of the super charger has been increased to 145 kW. It can charge a 90-kWh battery to 50 % of its capacity in 20 minutes.

Four German automakers, BMW, Mercedes, VW and Audi, in 2016 announced the roll-out of “ultra fast” chargers for EVs in Europe. These chargers will deliver 300 kW of power. At a first glance that high amount of power may give the impression it will provide a very fast charge. However, it is not a given, since lithium ion batteries have limitations how fast they can be charged. In fact, fast charging is more about lithium ion chemistries than the power of the charger.

Designing an optimal lithium ion battery depends on the application and is a trade-off between several characteristics such as energy density, power (discharge rate), safety, cost, life span, charge rate, etc. In laptops, notebooks and cellphones energy density is prioritized. Consequently, lithium cobalt oxide (LiCoO2), which has high energy density, tends to be thepreferred chemistry for this type of application. The downside is slow/long charging times. It can take 3 hours to recharge a depleted battery, but since it is predominantly done during the night it is in most cases an acceptable trade-off.

For the EV car makers optimizing the battery is an integral part of the design of the car, since the battery system is key to performance (desired range, acceleration, etc.), while substantially impacting weight, space and cost. The choice of the optimal lithium ion chemistry is very important. The most common basic chemistries for EVs are:

  • Lithium Manganese Oxide (LiMn2O4), LMO, has good a balance of energy, power, safety and cost. BMW and Nissan use LMO for the i3e batteries and Leaf batteries respectively.
  • Lithium Nickel Manganese Cobalt Oxide (LiNiMn CoO2), NMC, can be modified to be optimized for energy or for power. NMC is popular among EV battery makers. NMC and LMO can be combined, which is also the case for GM/LG battery packs for the Volt.
  • Lithium Iron Phosphate (LiFePO4), LFP, has not the energy density like LMO or NMC, but is very strong in terms of safety and life span. LFP batteries are used by BYD and other Chinese car builders.
  • Lithium Nickel Cobalt Aluminum Oxide (LiNiCoAlO2), NCA, has high energy density but relatively high cost and less safety margin than e g LFP. NCA is the basic lithium chemistry used by Tesla/Panasonic. (Elon Musk prefers to call it a nickel-graphite battery referring to the main elements for the cathode and anode respectively.)
  • Lithium Titanate (Li4Ti5O12), LTO, is superior in terms of charge rates, life span and safety, but has low energy density and high cost.

Charge rates are expressed in C. 1C means one hour to fully charge a battery. 2C means 30 minutes to charge to fully charge a battery. 0.5C means 2 hours. The only lithium chemistry which can be charge truly fast without reducing life span and without pushing the safety margins is LTO. It has a 5C charge rate, which means a LTO battery can be fully charged in 12 minutes.

The basic chemistry has a major impact on the charge rates. LMO, NMC, LFP and NCA are all around 1C charge rate. It is important to notice that the basic chemistries can be significantly modified by adding other elements to the formulas and that there is much more to an optimal battery design like the format of the battery cells, the topology of the battery pack, the battery management system, etc.

Proterra, a leading EV bus maker, purpose builds its buses and offers different lithium ion chemistries for different requirements. One of their bus designs, intended for short routes, has a 100 kWh LTO battery pack, which the company claims can be fully charged in 5-13 minutes.

EV passenger car makers seem to stick with one lithium ion chemistry to optimize desired performance and to achieve economy of scale. None of the major manufacturers use LTO because of the low energy density and high cost. So, with chemistries like LMO, NMC, LFP and NCA, the only option to speed up the charging is to design the battery large enough to allow a fast charge for a part to of the battery. Thus, Tesla Model S with the 90-kWh battery can charge 50 % of it in 20 minutes. It corresponds to a charge rate of 1.5C. Nissan claims to be able to charge 80 % of the Leaf’s battery in 30 minutes, which corresponds to 1.6C. BMW states i3e will be fully charged in 1 hour and 15 minutes, but charging 80 % of the battery can be done in 40 minutes, which corresponds to 1.2C.

It triggers the question what is fast? For an EV driver wanting to fast charge the battery with the same convenience as a conventional car driver at the gas pump 5 minutes is ultimately what a customer would expect. 5 minutes to charge 50 % of battery would require 6C. 10 minutes may be OK, but that is probably the upper limit for what can be considered comparably convenient. Even so, 10 minutes to charge 50 % of a battery would require a lithium ion chemistry capable of 3C.

The problem is that, without compromising on energy density and cost, there are no such lithium ion chemistries. Lithium ion batteries will continue to improve, but it would require step-function improvements to achieve commercial batteries capable of 3C. Reaching 2C, enabling charging 50 % of the battery in 15 minutes or 1/3 of the battery in 10 minutes, in the next 2-4 years would be a big step forward.

In recognizing the challenges for fast charging of EVs one should keep in mind that for the dominant use of EVs fast charging will not be an issue. On the contrary, charging the EVs at home or at the workplace is/will be convenient and, in most cases, allow enough time to fully charge. In fact, the more time available to charge the battery the more opportunities for smart charging as well as for the EV becoming an integral part of a smart home and/or for providing grid services.