Short answer: In Tom’s Guide’s 5G web-browsing test at 150 nits, the iPhone 18 Pro Max lasted 4 hours 11 minutes longer than the iPhone Duo, with the Duo running on its inner display. The Pro Max also pulled ahead in charging by the 30-minute mark. Apple’s own Duo ratings need context, because its video-playback figures change depending on which screen you use.
Why the battery numbers are hard to compare
A battery rating only means something next to the test that produced it. Apple splits the Duo’s numbers by screen, the Pro line has its own ratings, and the one independent head-to-head we found used a specific brightness and a specific task. Here is what each source measured.
| Test or rating | iPhone Duo | iPhone 18 Pro | iPhone 18 Pro Max |
|---|---|---|---|
| Apple: local video playback | 44 hrs (outer), 31 hrs (inner) | 36 hrs | 45 hrs |
| Apple: typical use | Up to 24 hrs* | Up to 24 hrs | Up to 30 hrs |
| Tom’s Guide: 5G web browsing, 150 nits | 13 hrs 20 min (inner screen) | Not tested | 17 hrs 31 min |
| Tom’s Guide: charge after 15 min | 42% | Not tested | 44% |
| Tom’s Guide: charge after 30 min | 66% | Not tested | 77% |
| Apple: time to 50% | About 20 min | About 15 min | About 15 min |
*Apple’s 24-hour typical-use estimate for the Duo assumes equal use of the inner and outer displays.
What the Duo’s 44 hours means
That figure is for local video playback on the outer screen. If you bought a foldable to read and watch on the big inner display, the more relevant Apple video-playback figure is 31 hours. Averaging the two won’t give you your real day, since your split between screens is your own. And neither number tells you how the phone holds up for browsing, gaming, or taking photos.
iPhone 18 Pro vs. Pro Max
Apple rates the Pro Max higher on both measures: up to 30 hours of typical use versus 24 for the Pro, and 45 hours of video playback versus 36. If you’re leaning toward the smaller Pro to save size and weight, the trade-off is shorter rated battery life. Faster charging can cut your downtime, but it doesn’t change how long the phone lasts between charges.
The head-to-head test
Tom’s Guide adapted its battery test so the browser filled the Duo’s inner display, then ran its standard 5G web-browsing test at 150 nits on both phones. The Duo lasted 13 hours 20 minutes. The 18 Pro Max lasted 17 hours 31 minutes, a gap of 4 hours 11 minutes.
That’s a useful guide if you spend a lot of time browsing on mobile data. It doesn’t tell us how the two phones compare for navigation, video recording, or games, and the 18 Pro wasn’t part of this round.
Two batteries, but not twice the runtime
Apple confirms the Duo has a dual-battery design. The capacity numbers come from Tom’s Guide, not Apple: 4,883 mAh combined for the Duo and 5,567 mAh for the Pro Max. That means the Pro Max has roughly 14% more listed capacity, and it lasted about 31% longer in the browsing test.
So the battery runtime gap is bigger than the capacity gap. Screen size, display hardware, and the rest of the system all play a part, but the test didn’t break down where the power went, so we can’t say how much of the difference comes from the larger inner display.
Charging speed: close early, wider later
After 15 minutes, the two phones were nearly level, 42% for the Duo and 44% for the Pro Max. By 30 minutes, the Pro Max had pulled ahead, 77% to 66%. Charging slows as a battery fills, so two readings early in the curve can’t tell you how long a full charge takes.
Apple’s own figures are about 20 minutes to 50% for the Duo and about 15 minutes for the Pro line, under Apple’s stated charging conditions. Those are manufacturer ratings. The 15- and 30-minute results come from Tom’s Guide’s testing, so we’ve kept the two sets apart.
Before you buy a new charger
A higher wattage on the box doesn’t automatically mean a faster charge. Apple says the Duo can reach up to 50% in around 20 minutes with a 60W-capable adapter or higher and a USB-C charging cable. For the 18 Pro and Pro Max, Apple’s roughly 15-minute claim calls for a 60W-capable adapter or higher with Adjustable Voltage Supply (AVS).
If you’re shopping, check the charger’s supported USB-C output profiles, the cable rating, and how power is shared when more than one port is in use. If the charger you already own meets those requirements, you may not need to replace it. We haven’t tested any specific third-party charger against the numbers above.
Protection: choosing a case
A case isn’t a battery topic, but it changes how you carry and charge these phones every day, so it’s worth settling before you buy.
A foldable adds a hinge and an inner display that a conventional slab phone doesn’t have, so case design matters more. When you shop for an iPhone Duo case, look at how it protects the hinge and the exposed edges, and whether it adds more bulk than you’re comfortable carrying. For the inner display itself, follow Apple’s care guidance rather than assuming a standard glass screen protector is appropriate.
The Pro Max is large and heavy, and a phone that size is easy to drop. An iPhone 18 Pro Max case with raised edges and solid corner protection is a sensible starting point. On either phone, if you use magnetic or wireless charging, make sure the case is designed for the charging setup you use, since case construction can affect magnetic alignment and charging performance. Check each listing for model-specific compatibility rather than assuming a case made for an earlier iPhone carries over.
Which one fits your day
If you’re often away from an outlet: The Pro Max has the stronger result in the browsing test. That doesn’t guarantee it lasts through every heavy day, but it’s the best head-to-head data available.
If you can top up at lunch: Look at the 15- and 30-minute charging numbers. Battery life tells you how often you’ll plug in, and charging speed tells you how long you’ll wait once you do.
If you’ll mostly use the Duo’s inner screen: Plan around Apple’s 31-hour video figure and Tom’s Guide’s 13-hour-20-minute browsing result, not the outer-screen number.
Before your next purchase, note when your battery usually runs out and what you’re doing at the time, then match that to the closest test you can find. It will tell you whether you need a different phone, a better charging setup, or nothing at all.
