7-Day Battery vs Blood Oxygen: Which Smartwatch Wins?

If you think a week-long battery life is a fair trade for dropping blood oxygen tracking, you're wrong—but not for the reasons you expect. I just spent seven days wearing two smartwatches on opposite wrists: one configured to squeeze out maximum battery life, the other prioritizing health metrics like SpO2. The experiment changed my mind about what “essential” really means in a wearable.

The Contenders

To make this a fair test, I needed two watches that represent the most common trade-off people face today. Not every smartwatch can give you both a 7-day battery and reliable blood oxygen monitoring. So I picked one from each camp.

• The Battery-First Watch: Garmin Forerunner 55. This is Garmin’s entry-level running watch, rated for up to 14 days in smartwatch mode. To recreate the “battery priority” scenario, I disabled the wrist-based pulse oximeter and other power-hungry features. That left a lean, focused sports watch that promises to go the distance without a charger.

• The Health-First Watch: Apple Watch Series 9. Apple’s flagship wearable has a medically validated blood oxygen sensor and a suite of health tools. But its 18-hour battery life means it’s practically glued to a charger every night. This is the “health at all costs” choice.

For context, I also kept an eye on two other popular options: the Amazfit GTR 4 (claims a 14-day battery and has SpO2, but its accuracy is hit-or-miss) and the Withings ScanWatch 2 (30-day battery and SpO2, but it’s more of a hybrid). These will show up later in the value discussion.

What We Tested

I wore both watches for a full seven days, 24/7, doing everything I normally do: sleeping, office work, gym sessions, two runs, and a 3-mile hike. I logged battery drain daily, checked how often I had to charge, and tracked sleep/activity data from both. For the Apple Watch, I used the Blood Oxygen app twice a day—once in the morning and once before bed—and also let it run automatic sleep tracking. On the Garmin, I kept everything standard but turned off the pulse ox and advanced training metrics to simulate someone who prioritizes battery longevity.

I also used a fingertip pulse oximeter as a control to compare SpO2 readings from the Apple Watch against a medical-grade reference. This gave me a sense of which watch’s blood oxygen data you can actually trust.

Design & Build

These two watches are night and day when it comes to construction.

The Garmin Forerunner 55 uses a lightweight fiber-reinforced polymer case (about 37 grams) and a 1.04-inch 208×208 transflective display. It’s comfortable enough to forget you’re wearing it, and the silicone band breathes well during workouts. The screen is always-on, which is great outdoors, but it’s low-res and not a touchscreen. You’ll navigate with buttons—fine for runners, less intuitive for navigating smartwatch UI.

The Apple Watch Series 9 is a slab of aluminum and glass, heavier at around 39 grams, but it feels denser on the wrist. The 1.9-inch Retina OLED display is bright, sharp, and responsive. It’s more like a mini phone than a fitness tracker. However, the square case and flat back can irritate some wrists during sleep. The band system is excellent, though.

If design were the only factor, the Apple Watch wins on screen and polish. But the Garmin wins on durability and all-day comfort. For a week-long test where you never want to take the watch off, comfort starts to matter more than specs.

Performance

Now for the numbers that matter.

Battery Life

• Garmin Forerunner 55: After 7 days, the battery sat at 15%. That’s with notifications on, daily 30-minute GPS gym runs, sleep tracking, and ambient light sensor enabled. I never plugged it in once. Exactly the “set it and forget it” experience.
• Apple Watch Series 9: It needed a charge every 18–20 hours. That turned into 1.5 hours of charging every day—over 10 hours of the week spent with the watch tethered to a cable. It’s not the end of the world, but it’s a constant interruption, especially when you use sleep tracking (you have to juggle charging times).

Blood Oxygen Monitoring

I used the Apple Watch’s Blood Oxygen app to take spot measurements twice a day. The readings typically fell between 96% and 100%, and compared to my fingertip pulse oximeter, they were within 1–2%—accurate enough for general wellness insights. The Garmin, with its pulse ox disabled, gave me zero blood oxygen data. I tried enabling it for one day, and it drained the battery another 20%, dropping the 7-day estimate to 5 days. For someone who doesn’t need SpO2, that’s a decent sacrifice.

But for anyone with sleep apnea, respiratory issues, or altitude training, losing blood oxygen tracking is a real loss. The Apple Watch’s SpO2 feature caught a few dips during sleep that I later discussed with my doctor. That’s something the battery-first Garmin simply can’t provide.

What You Give Up

Interestingly, the Garmin still tracked heart rate, sleep stages, and stress—so you’re not losing all health features. But with pulse ox off, you’re flying blind on overnight oxygen saturation. That could matter more than you think, especially if you snore heavily or live at altitude.

The Apple Watch also gives you ECG, fall detection, and crash detection—safety features that have nothing to do with blood oxygen but add tremendous value. But they all come at the cost of that fragile battery life.

Price & Value

The price gap is significant. The Garmin Forerunner 55 retails for about $199. The Apple Watch Series 9 starts at $399. So the battery-first watch is half the price—and you don’t need to buy a separate charging puck for travel.

But value isn’t just upfront cost. Apple Watch’s health sensors have a longer track record of validation, and the ecosystem is unmatched if you own an iPhone. Garmin’s sleep and activity tracking are also excellent, but the lack of SpO2 in this configuration limits its appeal to serious health users.

If you’re hoping for a deal, check our Best Time to Buy Smartwatches (2026 Guide) to see when prices drop. For a more general overview, the 2025 Guide is also helpful.

Who Should Buy Which

Buy the battery-first watch (Garmin Forerunner 55 or similar) if:

• You hate charging devices and want a smartwatch that just works for a week or more.
• Your primary training is running, cycling, or other sports where GPS and heart rate matter more than SpO2.
• You’re on a budget and don’t want to spend $400+ on a wearable.
• You don’t have any medical conditions that require tracking blood oxygen.

Buy the health-first watch (Apple Watch Series 9 or similar) if:

• You need reliable blood oxygen monitoring for sleep apnea, asthma, or altitude sickness.
• You value ECG, fall detection, and other advanced health features.
• You don’t mind charging your watch every day (or even twice a day if you use GPS a lot).
• You’re already deep into the Apple ecosystem.

There’s also a middle ground: the Amazfit GTR 4 claims up to 14 days of battery and includes a blood oxygen sensor. In my experience, its SpO2 accuracy is inconsistent—sometimes it aligns with a pulse oximeter, sometimes it’s off by 4–5%, which is useless for medical decisions. So you do get battery life and SpO2, but you’re compromising on accuracy. The Withings ScanWatch 2 is another example: a 30-day battery and SpO2, but it’s not a full-featured smartwatch—no touchscreen, limited notifications.

Final Verdict

After a week of real-world use, I’m convinced that 7-day battery life is a phenomenal quality-of-life feature. Not worrying about a charger changed how I use a smartwatch. But it’s not worth giving up blood oxygen monitoring if you need it for health reasons. For most people, though, blood oxygen is a “nice-to-have,” not a “must-have.” So if I had to choose one watch for myself, I’d pick the battery-first Garmin—it’s cheaper, lighter, and doesn’t nag me about battery levels.

That said, if you have even a hint of sleep apnea or spend time at high altitude, the Apple Watch Series 9 is the safer, more responsible choice. Blood oxygen data could be the difference between catching a problem early or missing it entirely. And that’s worth the hassle of daily charging.

Bottom Line

The 7-day battery is a massive convenience win, but blood oxygen monitoring is a health safety net you shouldn’t undervalue. Our test shows that if you’re healthy and active, a battery-first watch like the Garmin Forerunner 55 is the better everyday companion. If you have a real health concern, the Apple Watch Series 9 is worth the trade-off—just be ready to babysit the charger. For everyone else, consider a hybrid like the Withings ScanWatch 2 if you want both without breaking the bank, but understand its limitations.

At the end of the day, the right choice comes down to your health needs, not just battery numbers. For me, the week with the Garmin was freeing, but the data from the Apple Watch made me pay attention. Which one should you buy? Prioritize your health first, and your convenience second.

Frequently Asked Questions

What is a good smartwatch battery life for health tracking?

A good smartwatch battery life for health tracking depends on your needs. For continuous sleep and activity monitoring, look for at least 5–7 days per charge. Watches like the Garmin Forerunner 55 last up to 14 days in smartwatch mode, but enabling SpO2 monitoring reduces that to about 5 days. Apple Watch offers only 18 hours, requiring daily charging.

How does blood oxygen monitoring affect smartwatch battery life?

Blood oxygen (SpO2) monitoring drains battery because it uses optical sensors that emit light through the skin. On most smartwatches, enabling continuous or frequent SpO2 measurement can reduce battery life by 20–50%. For example, the Garmin Forerunner 55 drops from 14 days to about 5 days with pulse ox enabled. Apple Watch uses on-demand spot checks, which is less draining but still impacts daily battery.

Why is blood oxygen tracking useful on a smartwatch?

Blood oxygen tracking measures the percentage of oxygen in your blood, helping you monitor respiratory health, detect sleep apnea signs, and assess fitness improvement. During exercise or high-altitude activity, SpO2 can indicate how well your body adapts. While not a medical device, it provides valuable trends. The Apple Watch Series 9 offers medically validated readings within 1–2% accuracy of medical-grade pulse oximeters.

Who should choose a long battery life smartwatch over blood oxygen monitoring?

Choose a long-battery smartwatch if you prioritize uninterrupted sleep tracking, outdoor adventures, or forget to charge daily. Runners and hikers often prefer Garmin's 14-day battery. If you don't need continuous SpO2 data or have a medical condition that requires regular monitoring, the convenience of a 7-day battery outweighs having blood oxygen. The Garmin Forerunner 55 is ideal for those focusing on activity, not health metrics.