Rivian: the powertrain details you won’t read about elsewhere

The long-quiet automotive startup Rivian has just revealed at the LA Auto Show a highly innovative new all-electric R1T pickup truck as well as a full-sized R1S SUV using a new underlying powertrain.

While many articles this week have described Rivian’s new vehicles inside and out along with their no-compromise performance and range specifications, the low-level details on the battery pack and the regenerative braking capabilities have largely been missing.

Let’s dive in.

Battery pack

It’s been widely reported that the pack can charge at up to 160 kW using standard CCS DC charging when connected to the latest generation of charging equipment. Rivian says the larger pack can add up to 200 miles of range in about 30 minutes.

Rivian is offering unusually large battery capacity options. Their smallest pack, at 105 kilowatt-hours (kWh) is bigger than Tesla’s largest pack which comes in at 100 kWh. Rivian also offers packs at 135 kWh and 180 kWh allowing, it says, just over 400 miles of range.

They are able to do this because both of their new vehicles have a long wheelbase and this allows for a long pack. The pack is also unusually thick but this is not a problem on a high-riding pickup or SUV.

According to Richard Farquhar, Rivian’s VP of Propulsion, the pack is thick because it contains not one, but two layers of vertical 21700 (also known as 2170) cylindrical cells — the same cell format made by Panasonic at Tesla’s Nevada Gigafactory and used in Tesla’s Model 3 sedan.

Rivian isn’t saying who their cell supplier is although it does not seem likely to be sourcing Gigafactory cells. Samsung also makes 21700 cells and other battery makers are free to use the same size as well instead of the older and more common 18650 format.

According to Teslarati.com, Rivian is cooling the pack with a 7mm thick aluminum cooling plate sandwiched between the two layers of cells. A water and glycol mixture similar to that used to cool conventional gasoline engines flows through the plate to transfer the heat. The website also reports that US government records show that Rivian has recently imported a substantial number of battery cells from LG in South Korea.

Rivian is also not yet ready to say what cathode chemistry is used in their cells. The two most common choices today are NCA and NMC and both have a nominal or mid-point voltage of roughly 3.6V.

Tesla uses NCA cells meaning that the metal in the cathode (or electrically positive side of the cell) consist of Nickel, along with a small amount of Cobalt and Aluminum. Most of Tesla’s competitors are using NMC which consists of Nickel, Manganese, and Cobalt.

In typical cells today, Nickel is the predominant cathode metal and this is especially true of Tesla’s NCA cells.

Looking up towards the radiator and front wheels at the top.

The large flat “skateboard”-style pack is internally made up of multiple modules (shown in yellow). Each module contains a number of cell groups.

Each cell group contains a number of cells wired together in parallel to effectively create a single logical cell with a larger energy capacity.

The cell groups within a module are wired together in series which causes them to be seen electrically together as a multiple of an individual cell group’s nominal 3.6V. For example, in the smaller Rivian packs there are 12 cell groups in each module at 3.6V each which would cause the nominal module voltage to be about 43V.

Finally, the modules themselves are wired together in series. But, regardless of the number of modules, cells, or pack capacity, it always works out to 108 cell groups.

With 108 cell groups at about 3.6v each it multiplies to around 389V nominal (or just under 450V when fully charged). All three pack capacity options operate at the same pack voltage.

The battery packs in the Jaguar I-PACE and Audi e-tron also use 108 cell groups.

See also: Audi e-tron vs Jaguar I-PACE battery pack comparison

Three pack sizes

Rivian offers three pack energy capacities at 105, 135, and 180 kWh. Each variant uses the same cell type so the pack size is determined by the total number of cells and modules.

The two smaller packs are the same physical size and shape. Motor Trend has an article saying the smallest capacity pack has seven modules and the mid-capacity pack has nine. This is wrong, according to Richard Farquhar of Rivian.

Actually, the smallest pack also uses nine modules but each of the cell groups in the modules contain fewer cells in parallel.

The largest capacity pack, at 180 kWh, has 12 modules and thus the pack is physically larger.

This is where the packs for the truck and SUV diverge. The truck uses a longer pack to hold the extra three modules while the SUV stacks the extra modules as an extra layer on top of the pack skateboard. This extra thick section of the pack ends up under the rear seats.

Stacking extra cells under the rear seats is a strategy used in some other cars including the Chevrolet Bolt EV and the various Hyundai and Kia 64 kWh packs. It makes sense since this area is otherwise wasted space.

This is how the pack organization math works for each pack size:

105 kWh: 9 modules, 12 cell groups per module, 56 cells per group, 6,048 cells

135 kWh: 9 modules, 12 cell groups per module, 72 cells per group, 7,776 cells

180 kWh: 12 modules, 9 cell groups per module, 96 cells per group, 10,368 cells

This implies that each cell holds about 17.36 Wh or about 4.8 Ah at 3.6V.

These calculated numbers are consistent with, and implied by, other statements by Rivian executives but have not been specifically verified.

Coolant loops

Update: the description of the coolant loops has been updated.

There is a single overall vehicle water-based liquid coolant system with at least two sub-loops and a grill-mounted radiator and fan, according to Richard Farquhar of Rivian.

A first sub-loop runs through and cools components like the motors and motor inverters. A second sub-loop runs through the battery pack.

Computer-controlled valves and replicated water pumps allow these loops to either operate separately or be joined together in order to exchange heat between the components and possibly the radiator.

For instance, although the battery pack has its own dedicated electric heater it can sometimes be effective and more efficient to share heat from the motors and motor inverters with the battery pack in cooler weather.

The battery pack loop also has a “chiller” connection to the air conditioning system to help cool the pack when it gets too hot. There is also apparently a heat exchange path that allows battery pack heat to help warm the vehicle cabin in addition to or instead of electric resistive heating.

A Rivian patent describes a similar system although it’s unclear how precisely this relates to the actual implementation used in Rivian’s initially announced vehicles.

Full-stop one-pedal regen braking

Like an increasing number of electric cars, the new Rivian vehicles can not only regenerate power back into the battery to help slow down but can quickly bring the vehicle to a full stop and hold it there without use of the traditional brake pedal — even on an incline.

The BMW i3, Chevrolet Bolt EV, 2018 Nissan LEAF, and the new Hyundai Kona Electric can all do this.

On the other hand, so far, Tesla vehicles require use of the brake pedal to quickly come to a full stop or hold the vehicle in place.

Like the Bolt EV and 2018 LEAF, Rivian supports driver configurable options to control the intensity and behavior of regenerative braking that occurs on the accelerator pedal.

Regenerative braking is ineffective at very slow speeds (the last several mph) so car makers that support full-stop one-pedal braking either blend in friction braking (in the 2018 Nissan LEAF) or they use active motor torque using a small amount of power from the battery as the Bolt EV and Rivian do. Similar strategies are used to hold a car on an incline.

Charles Sanderson, Rivian’s VP of Development & Integration, says they can hold the vehicle at up to a steep 20 percent road grade just using active motor torque.

Like almost all other electric and hybrid cars, Rivian also initially uses regenerative braking as the traditional brake pedal is pushed but transition to friction braking as the need for anti-speed increases as the driver pushes down farther on the brake pedal.

Like many other car makers, Rivian is using Bosch’s iBooster brake system. Even Tesla has used this system since they introduced partially-automated driving features into the Model S although Tesla has programmed it to use only friction braking when the driver steps on the pedal, presumably to guarantee the highest level of consistent braking feel.

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Categories: Battery, General

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5 replies

  1. Very interesting, thanks.

    Liked by 1 person

  2. Thank you Jeff. Please let us know if/when you find out if they are using NCA or NMC, and from which cell maker.

    Liked by 1 person

  3. I’m a bit confused. Why would the brake pedal need to do any regenerative braking if it has single pedal driving with the accelerator pedal? ie. just lift off the accelerator fully to have full regen braking?

    Like

    • One answer is that the car will have driver-adjustable regen options and so just lifting off of the accelerator pedal may just glide or may have only modest regen which is much less than what the brake pedal regen will be able to recapture.

      I haven’t driven a Rivian test car so I can’t personally say exactly how the configurable accelerator
      regen levels compare to regen levels obtainable on the brake pedal.

      Like

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