12 Jun 19

Get up to speed with DC fast-charging

With battery size increasing and both heavy-duty and long-distance use becoming ever more feasible, DC fast charging is gaining in importance. Here’s what you need to know and what EV pioneers Tesla, BMW and Nissan have to say about it.

Time is probably the most precious resource in a business environment. Salespeople, service technicians, delivery men, ride hailers… they cannot afford to park their EV at an 11-kW AC station for two or three hours during business hours before being able to continue their journey. For long-distance driving, there is no alternative to DC fast-charging – unless you swap your battery pack, but that is only possible in China today, if you own a Nio.

The charging capacity of such DC fast chargers ranges from 20 to 150kW, depending on location and charging standard, but the majority works at 50kW. The higher the capacity, the higher the strain on the battery cells, which degrade overtime. Knowing that the capacity will go up to 400kW, are OEMs covering themselves with warranty restrictions and cautionary paragraphs in user manuals, or can you DC charge at libitum? 

Tesla: stay away from zero

Tesla was unavailable for a telephone interview, but shared with us the sources of the relevant information. The Tesla support page mentions that the peak-charging rate of the battery may decrease slightly after a large number of high-rate charging sessions, such as those at Superchargers. To ensure maximum driving range and battery safety, the battery charge rate is decreased when the battery is too cold, when it is nearly full or when its condition changes with usage and age. These changes in the condition of the battery may increase total Supercharger time by a few minutes over time.

As to the announced V3 charging stations that offer a capacity of up to 250kW, Tesla says that all Tesla models/trims will benefit from the elimination of power sharing in V3’s architecture. The peak rate each vehicle achieves will vary with size of battery pack, State-of-Charge, battery and ambient temperature conditions. 

Basically, the system is clever enough to protect itself against damage and if you want to DC charge frequently, that’s absolutely fine from a warranty point of view. The latter only voids when the damage can clearly be attributed to non-compliance with Tesla’s words of caution in the manual: “Do not use the battery as a stationary power source. Doing so voids the warranty.” A second circumstance under which the customer is held responsible for any damage to the battery is when you let the charge level fall to 0% - something for which the car will try and protect itself – or when you leave the Model S unplugged for an extended period. 

BMW: use DC at will

The German premium carmaker will hit the 100,000-unit sales milestone in the coming months with the i3, which never seized to improve itself. From 22.6kWh in 2013, the battery capacity has now reached a much more comfortable 42.2kWh for some 300 WLTP km of range.
Regarding DC fast-charging, BMW says that the stress put on an individual battery is a combination of DC charging power, battery capacity, charging frequency, charging duration and charging profile. All BMW i3 (max. 50kW DC charging) and the upcoming BMW iX3 (max. 150kW DC charging) can be charged with their maximum nominal DC charging power without negative effects, the carmaker confirmed. 
For all current BMW BEV models the same warranty conditions apply, even if you use the maximum nominal DC charging power all the time. For future DC charging powers above 150kW the individual charging profile becomes more relevant to limit stress for the battery, BMW says. The battery cells will need advanced cooling during charging and a more robust cell chemistry.

As to the charging profile, the charging power over time is controlled by the car. Even if the DC charging station offers 350kW, the car itself limits the power to what the manufacturer considers optimum for the car. This way the charging usually starts with the maximum power the car allows, e.g. 150kW, and after a while it goes down e.g. 120kW to limit stress for the battery. The individual programing of the charging profile is key for the optimum between short charging time and a long battery life. The BMW battery warranty conditions reflect this approach, dixit the carmaker.

Nissan: battery outlives the car

The Japanese OEM has sold over 150,000 all-electric vehicles in Europe and more than 400,000 worldwide since the launch of the first-generation Leaf in 2010, making it the manufacturer with the most extensive experience in e-mobility. It is the only large EV manufacturer that sticks to the Japanese ChaDeMo fast-charging standard (see box out). “The ChaDeMo network is still the biggest DC fast charging network in Europe, with 8,000 charge points”, explains Ana Paola Reginatto, Head of EV at Nissan Europe. 

“Frequent use of DC chargers is not a problem per se, but we recommend a mix of AC and DC. It’s important not to wait until the battery is empty, but charge whenever you can – which is mainly AC at home, at the office or semi-public stations at supermarkets, for instance.”   

“The majority of ChaDeMo chargers today deliver DC at 50 kW, but the latest ones reach 100 kW. Our upgraded Leaf with a 62-kWh battery can charge at this capacity,” she adds. ChaDeMo also allows Nissan to offer bi-directional charging – something that the other OEMs are not yet ready for. 

Bi-directional charging enables you to give power back to the grid and make money off your EV’s battery. That has an impact on the TCO and actually does not put extra strain on the battery, so you do not have to worry about accelerated degradation. “Actually, the battery will outlive the car and can be put to use as a power storage unit.”

Image: BMW's i3 can charge at 50kW, but the upcoming iX3 is able to digest 150kW.

Authored by: Dieter Quartier