We all know the sinking feeling as our phone battery blinks red and there’s not a phone charger or outlet in sight.

Optivolt Labs, a startup founded by University of Illinois Urbana-Champaign students, wants to put an end to your low battery panic with a solar-powered battery built into a smartphone case.

It’s called Particle. Essentially, it’s a dual smartphone case. There’s an inner case that clips around the phone, similar to a typical phone case. Then there’s an outer case that includes a solar panel, battery, circuitry and Apple Lightning Pin. The inner case slides onto the outer case when a user needs to charge their phone. When a user wants to charge the solar panel battery, they just take off the outer case and let it lay in sunlight or attach it to a bag with a carabiner. LED lights turn on when the battery is charging and to indicate the amount of charge. It will weigh about 100 grams, or 3.5 oz (typical smartphone cases weigh around 17 grams). When fully charged, their battery offers two full iPhone battery charges.

While there are several other companies that already have this concept out on the market, Optivolt differentiates in charge time: In lab tests, they were able to charge the Particle solar battery in just 4.8 hours, about 10 times faster than their competition, founders say.

Rohit Kalyanpur (Credit: Optivolt)

“We’re trying to redefine how people wirelessly charge [their] phones,” said cofounder Rohit Kalyanpur.

Of course that 4.8 hours only comes in optimal conditions, and there are a lot of variables–weather conditions, shade, hours of sunlight in a day–that can go into charging a solar battery. Cofounders Kalyanpur and Paul Couston said it will likely take users between 7 and 12 hours to fully charge their battery when the device first goes on the market. They’re currently in the process of filing patents on the tech.

The idea for Particle traces back to high school for Kalyanpur, a sophomore studying computer engineering at UIUC. After a summer studying engineering at Johns Hopkins University when he was a sophomore, he wanted to find a way to better charge his smartphone on the go. He saw opportunity in solar panels, but realized that people weren’t likely to carry around a clunky panel just to get an additional smartphone charge. Since then, he’s been experimenting with ways to shrink down the battery, while decreasing the battery charging time.

Paul Couston (Credit: Optivolt)

Couston, who studies industrial design at UIUC with experience in lean manufacturing and sustainable design, teamed up with Kalyanpur to create a case that people would actually want to put on their phone.

Currently Optivolt is accepting pre-orders for Particle. The device will cost $119 retail, but they’re currently offering the case for an early discounted price of $72. Currently the Particle only works with iPhones. Founders expect to ship the devices in Spring 2018.

Optivolt is currently in UIUC’s iVenture Accelerator, and they’re currently raising a seed round to get the Particle to production.