Rivian is proving that there's more than one way to get more range out of an electric car.

A new patent filing from the company reveals that its upcoming R1T electric pickup could have even more than 400 miles of range by using an auxiliary battery in the bed of the pickup. 

The filing was revealed on Rivian Forums last week.

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According to a drawing in the patent filing, the battery could sit in the pickup bed like an auxiliary toolbox mounted to the bed rails behind the cab. Another configuration shows a battery in a flat format sitting on the bottom of the truck bed. It's not clear whether Rivian might offer both types of accessory batteries, or if it's just planning for either possibility. The R1T also offers an enclosed pass-through storage compartment under the bed behind the cab and a front trunk.

A unique connector on the front wall of the pickup bed feeds makes the battery an extension of the main, 180-kilowatt-hour battery to extend the range beyond the truck's estimated 400 miles of range.

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The battery will have its own liquid cooling circuit as well, and the drawings show the coolant circuit has its own connector on the front of the truck bed to connect to the truck's cooling system. The patent text, however, says the auxiliary battery has its own coolant circuit. "The auxiliary battery module includes an integrated cooling system for cooling itself during operation of the electric vehicle, including a conduit therein for circulating coolant," it reads.

Rivian has announced that the R1T will be available (at least eventually) with two different battery sizes, the larger 180-kwh pack, that the company estimates will deliver 400 miles of range, and a smaller 135-kwh pack to come later. The patent does not reveal whether Rivian will sell the auxiliary battery with the larger or smaller pack, but Rivian spokesman Michael McHale says the truck doesn't need the auxiliary pack to reach its targeted 400 miles of range. 

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The patent gives no indication how much capacity the auxiliary battery could add.

It is also not clear whether Rivian could develop such a battery for its R1S electric SUV set to follow a year after the pickup. Based on the patent drawings, it doesn't look like either of these battery configurations would fit in the SUV.

The R1T is due out late next year as a 2021 model.