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March 9, 2020


Electric Vehicles
Fast Charging
Nadim Maluf

Is it true that electric vehicles (EV) need 800-V battery packs for ultrafast charging? Is it true that the Porsche Taycan uses 800-V packs to enable ultrafast charging? Why did GM announce that its new battery platform will support 800 V? Let’s find out.

Let’s start with two essential ingredients required for ultrafast charging:

1. A charging station that is capable of providing a lot of electric power;

2. The ability for the battery to accept the extra charging power without being damaged or degraded.

Charging stations are like AC adapters we use to charge our smartphones. Of course, they provide a lot more power. Standard fast-charging stations in the Tesla network are rated to provide 72 kW; upgraded stations are capable of providing up to 150 kW. Electrify America’s network is adding stations that can charge at up to 350 kW — that’s sufficient to run about 50 average residential homes.

When the vehicle is charging, its on-board computer communicates with the charging station. In essence, the vehicle tells the station, among other things, how much power it needs to charge the battery. It is the vehicle, not the charging station, that sets the charging power, and consequently the charging time — as long as the charging power is lower than the maximum power available at the charging station. The reason is that the vehicle must balance the charging power vs. the likelihood of battery degradation during charging.

When we speak of battery in an EV, we usually refer to a “pack.” A battery pack is made of hundreds or even thousands of individual smaller battery cells that are assembled together to increase the amount of stored electrical energy. A single battery cell may store anywhere from 17 W.h (e.g., Panasonic 21700 cell used in a Tesla) up to 300 W.h (e.g., cells used in some of the German-made EVs).

The standard 50-kWh pack in the base Tesla Model 3 includes a total of 2,976 cells, organized into 96 modules, each module consisting of 31 individual cells.  A BMW i3 pack, by contrast, has a total capacity of 43 kWh and uses 96 modules, each module consisting of two cells. The cells in the BMW packs are substantially larger than the ones used by Tesla.

So we notice that the number “96” appears regularly in the pack design. That’s because there are 96 modules electrically connected “in series”: in other words, the electrical wiring of the pack is such that the voltage of the entire pack is the sum of the voltages of each individual cell or module. For the vast majority of lithium-ion cells used in automotive, the module voltage is on average 3.7 V, but can range from 3.1 V when the cell is depleted to  4.2 V when the cell is fully charged.

We can now calculate the maximum voltage of the pack: 96 multiplied by 4.2 V equals 403 V. This is nominally the pack voltage for the vast majority of EVs on the market including all Tesla models, Chevy Bolt, Nissan Leaf, BMW i3, and many others.

Let’s now calculate the maximum charging current that a 400-V battery pack will request at an ultrafast-charging station. Porsche says that its Taycan can accept up to 270 kW at its dedicated Turbo Charging stations, so we will use this figure.

You may recall from basic physics that power equals voltage multiplied by current. Therefore, the charging current is power (270 kW) divided by voltage (403 V) equals 670 A !!! By any standard, this is a very large current that will require a thick copper cable harness, adding significant weight and cost. Porsche says the extra harness weight amounts to 66 pounds (30 kg) — quite a bit when compared to the pack’s overall weight of 1,389 pounds.

So what if we raise the voltage to 800 V, up from 400 V? You guessed correctly: the charging current drops by half…thus saving significant weight and cost.

That is exactly what the Porsche Taycan’s battery design does. Its packs contains a total of 396 individual cells made by LG Chem, mechanically arranged into 33 modules of 12 cells each. Electrically, the 396 cells are divided into two sets connected in parallel. In each set, there are 198 cells connected in series. The maximum voltage of the pack is 835 V (that’s 198 multiplied by 4.2 V).  Each LG Chem cell is rated at 64.6 A.h, equivalent to an energy capacity of 236 W.h. When the energy from all 396 cells is added, we calculate a total pack maximum energy capacity of 93.4 kWh — just a little less than the maximum pack energy capacity of the Tesla Model S (100 kWh).

So will it be 400-V or 800-V for future EV packs? You decide!

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