Porsche has been working on a new all-electric Mission E which will push the boundaries in a big way on ultra-fast charging. The brand aims to be the first production car that will charge at over 200 kilowatts and in order to achieve that it would also be the first to charge at around 800 volts.
The SAE J1772 standard defines the AC charging plug used in all non-Tesla North American EVs and also likewise defines the DC charging plug, also known as the CCS Type 1 plug, that is being used by Porsche in the Mission E. A different variant of the CCS plug, Type 2, is used in Europe.
Ever since 2012, the J1772 standard has had its upper limits on DC charging set to 200A at 500V. This limits charging power to a theoretical peak of 100 kW although for various reasons almost all actual J1772 DC charging equipment has been designed to support only a maximum of 125A and about 50 kW so far.
Some of the new updates to the international charging standards are still in the final stages of being published but in October of 2017 SAE made 400A at 1,000V the new official DC limit for J1772 and CCS Type 1. This is achieved using the same plug design that was previously limited to 200A at 500V so everything stays compatible.
The new higher amperage limits are enabled by liquid cooling of the charging cable and plug. Although a charger may support higher charging rates, the actual power delivered at charging time is controlled by the car itself.
See Also: J1772 updated to 400A DC at 1000V
Just using the new 400A limit alone is not enough for Porsche to achieve over 200 kW charging. The charging voltage would be limited to around 400V due to the way battery packs in real cars are designed and when 400A and 400V are used to together that’s only a mere 160 kW. That’s why Porsche will be doubling the usual 400V when connected to a DC charger that supports the new higher limit. With twice the voltage the car could, in theory, draw around 320 kW.
A car’s battery pack voltage is a result of how the individual cells are configured into parallel groups and how those groups are connected in series. Cells can be electrically connected in “cell groups” in parallel so as to effectively become bigger cells with more energy storage capacity at the same voltage. Individual lithium-ion cells are typically only about 3.6V but when strung one cell to the next in “series” the voltage adds up. Typically, between 96 and 108 cells are strung together like this to get a total nominal battery pack voltage ranging from around 350 to 400 volts.
The individual battery cells do not electrically “see” this higher-level pack organization. Thus, the cells in the pack do not inherently gain the ability to individually charge faster just because the overall pack voltage has doubled from 400V to 800V due to more cells being strung together in series. The design of the cells typically used in high-capacity battery packs may need to be changed in order to fully take advantage of the higher charging power enabled by higher pack voltages. That’s a subject for another article.
Older DC chargers
Many and probably most of the new-generation of DC chargers will be capable of putting out the higher voltage levels even when configured for more modest overall power output. For example, ChargePoint’s new line of charger hardware supports configurations with output limits as low as 63 kW and 156A. That is only modestly higher than the 50 kW and 125A limits of older products yet the new equipment supports voltages up to 1000V mostly because it shares internal components with higher-end charger products.
The older charger designs with a 500V limit have already been installed at thousands of locations and that generation of equipment may continue to sell for a few years at discounted prices to charging providers for use in so-called community charging near residences and businesses or in rural areas with limited utility power and low traffic volume.
Porsche has said that the Mission E can charge at older stations limited to 500V. This means the battery pack has to somehow adapt to handle the correct charging voltage.
One way is to add some additional “DC to DC” circuitry inside the car between the DC charger and the battery pack that can boost the incoming voltage from around 400V to around 800V but building the circuits to do this at over 100 kW might be too expensive or bulky.
Another alternative is to build two 400V sub-packs that are connected together in parallel as one pack at chargers that are limited to 500V but connected in series for 800V at chargers that can handle the higher voltage limits. This dynamic reconfiguration of the internal pack connections can be done using giant relays called “contactors”.
Contactors are already used in battery packs in order to disconnect the pack from the charging connector pins for safety reasons when not charging. Contactors are also used to disconnect the pack from the rest of the car when it is turned off or after a major collision.
Porsche hasn’t disclosed how the Mission E will implement support for charging at 400 volts versus 800 volts.
The engineers also have to decide which voltage level the rest of the car uses. It’s entirely possible to quick charge the battery at 800V but actually use 400V when driving.
The advantage of using 400V is that more “off the shelf” parts are available from suppliers. Existing high voltage parts like cabin heaters and the like can also be reused from earlier 400V vehicles like the VW e-Golf and some existing cable connectors can be reused that have not been certified for 800V.
The internal wiring and components inside the motor may also be somewhat easier to electrically insulate at 400V although the motors used in hybrids from Toyota, Honda, Hyundai and other companies already use battery power boosted up to 600 to 700 volts in order to improve their torque density.
As far as the Mission E, a Porsche spokesman has been quoted as implying that the engineering team actually decided to design all of the car’s components to work at 800V.
Either way, there may need to be a modest DC to DC voltage converter circuit to feed some high voltage components like the cabin heater when the car is charging at the “wrong” voltage.
The extra bits of circuitry or extra contactors necessary to handle both voltage ranges add cost. Most electric cars in the near future will be using battery cells tuned for high energy density, modest charging rates and low cost. So, only a select few luxury and sports cars may be enabled for charging above 500V for now.
A few other car makers, primarily Nissan, use a Japanese DC charging plug called CHAdeMO. That standard is being updated to support 1,000V charging later this year but no vehicle designs have been announced yet that will take advantage of it.