The best thing about the iPhone XR is not its design, processor, camera, or even its fun colors. The best thing is the XR’s battery life, which lets me use the phone all day without ever having to worry about charging it. Many days, I go to bed with 50 percent left in the tank.

So it came as a bit of a surprise to see Apple announce a new Smart Battery Case for the iPhone XR a few weeks ago. The Smart Battery Case, which is also available for the iPhone XS and XS Max, sells for $129, comes in black or white, and is designed to dramatically extend the time you have before you have to plug your phone into a wall charger. Apple says the iPhone XR version of the case extends talk time to 39 hours (from 25), internet use to 22 hours (from 15), and video playback up to 27 hours (from 16). That’s a significant jump from the XR’s already impressive stamina, but since Apple’s battery life estimates can be tough to translate to real-world use, I decided to use the new Smart Battery Case with my iPhone XR for a week to see what it could really do.

The design of the new case is similar to the original model that came out for the iPhone 6S and 7, but it has a longer “hump” on the back that extends to the bottom of the phone. There’s no getting around the fact that the case adds considerable thickness and weight to the phone — even the smaller iPhone XS turns into a veritable chunk when the case is on. The larger iPhone XR and XS Max are legit thicc bois with their respective cases, which makes them a bit harder to handle and easily slip in and out of a pocket.

Can I use the iPhone XS Smart Battery Case with an iPhone X? Yes, it is possible to use the iPhone XS Smart Battery Case on an iPhone X, even though Apple does not explicitly support it. The iPhone X does fit in the XS case, though there’s a little gap around the camera housing and it isn’t quite as snug. Additionally, the speaker and microphone holes on the bottom of the case do not precisely line up with the iPhone X’s hardware, so audio quality for calls might be diminished.

The gummy silicone finish on the case not only makes it tough to slip into a pocket, but it also collects a ton of dust and gets greasy with fingerprints after just a couple of minutes of use. The silicone does provide an excellent grip, but it’s a bit too sticky for my preference. I’d love for Apple to release a leather version of this case, but I’m not confident that will ever happen.

The new cases add some features that weren’t available before: you can fast-charge the case and the phone, provided you have the right cable and charger, or you can drop the phone in its case on a wireless charger to charge both. Apple says the 5W brick that comes with the iPhone will charge the iPhone first, then the case, while a 10W or 12W charger will charge both simultaneously at 5W. An 18W charger will fast-charge the iPhone first before filling up the case, while 30W or higher chargers will fast-charge both the phone and case simultaneously. On a wireless charger, the iPhone will charge first before the case starts to refill its tank.

The cases have a Lightning port on the bottom that supports data and audio pass-through in addition to charging. You can plug Lightning headphones or a headphone dongle into this port or plug your phone into a computer or car system for CarPlay without having to remove the case. It behaves basically the same way as the Lightning port that’s on the phone.

But the reason these things exist is for their battery life, and it is impressive. The Smart Battery Case was able to easily keep my iPhone XR charged at 100 percent for an entire day of rather heavy use. It effectively extends my phone’s usable life to two days from one; once the case is tapped out, I can just remove it and rely on the XR’s internal battery. On one day, I unplugged the phone in its case at 7:30AM, and I didn’t have to plug it into a wall charger until around 7:30PM the next day — roughly 36 hours, or two full practical days of use.

The thing is, I rarely need that kind of battery life because I often have opportunities to charge up throughout the day, whether that’s in my car while I’m using CarPlay, on the wireless charger on my desk, or through a charger on my nightstand while I sleep. It’s nice to not have to think about charging my phone during the day, but I don’t really have to think about it much with the XR as it is, and I don’t know if the Smart Battery Case is worth the $129 for my needs. Plus, the added bulk and weight make the XR less pleasant to use during the day. I’m much more likely to use an external battery pack on the off chance I do need to charge up, as it’s much less expensive, has a larger capacity, and can be used to charge devices other than just my phone.

But if you’re the kind of user who goes hard on your phone for many hours a day and doesn’t have many opportunities to charge up, the Smart Battery Case could be the right tool for you. There are other battery cases on the market, but the Smart Battery Case’s integration with iOS, intelligent charging, and ability to pass data and audio through its Lightning port provide a better experience without a doubt — as long as you don’t mind turning your svelte iPhone into a beefy brick.

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