The battery life of smartwatches varies significantly more than with other technology products like smartphones, tablets and laptops.

Their relatively small batteries have to juggle power and efficiency as 4G, GPS, Bluetooth and Wi-Fi fight for energy with heart rate monitors, accelerometers, barometers, and a display expected to constantly show the time.

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Generally speaking, more functionality equates to shorter battery life. As such, feature-packed smartwatches from Apple and Samsung must be placed on their charging docks daily, while hybrid watches like those from Withings can last 25 days.

The Wyze Watch is a newcomer to the market, a $20 device that promises a nine-day battery life. Primarily a smart home company, Wyze has yet to provide more details on the battery life, but the watch's lack of energy-sapping GPS, 4G, and music streaming will play a vital role here. We'll be keen to review.

As an example of how much GPS and 4G can affect a smartwatch battery, look at Apple's claims for the Watch Series 6.

The company states 18 hours of battery life is possible during a normal day, which includes 90-time checks, 90 notifications, 45 minutes of app use, and a 60-minute workout with music playback over Bluetooth.

But if you take a call over the Watch's 4G connection, the battery will die after just 90 minutes, and recording outdoor exercise with GPS and 4G active will kill the watch in five hours. (We've seen similar results with the Apple Watch in our testing.)

You should consider all this when buying a new smartwatch, and while your personal usage stats will vary, it is interesting to see just how much variation there is in the manufacturer's claims. Here we have gathered up six smartwatches whose claimed battery life (while used as a proper smartwatch and not in a battery-saver mode) ranges from 18 hours to 21 days.

Garmin Fenix 6X Pro: 21 days

The Garmin Fenix 6X Pro has a claimed 21-day battery lifeGarmin

The current king of smartwatch battery life is the Garmin Fenix 6X Pro, which claims to have 21 days of use.

The Fenix 6X Pro is a very large watch with a 51mm case, likely an acquired taste for many. But that large case means a big battery and tough build quality with waterproofing to 100 meters. As well as acting like a smartwatch and fitness tracker, the watch has ski maps for 2,000 resorts worldwide, animated workout guides, an expedition mode, advanced run tracking, and data for 41,000 golf courses.

Garmin claims 21 days of smartwatch use or 60 hours of GPS use. That falls to 15 hours when music is played simultaneously, while a special, energy-sipping 'Expedition GPS Mode' can last up to 46 days, and the battery-saver mode is good for a claimed 80 days.

Garmin Fenix 6X Pro, Premium Multisport GPS Watch, features Mapping, Music, Grade-Adjusted Pace Guidance and Pulse Ox Sensors, Black (Renewed)

List Price: $349.00

New From: $339.00in Stock

Garmin Vivoactive 4S: Seven days

The Garmin Vivoactive 4S has a claimed seven-day battery lifeGarmin

Another option from Garmin this fitness-focused smartwatch mixes advanced exercise tracking and health data collection with smartwatch features like calls, notifications, contactless payments, and music streaming. The Vivoactive 4S also has a heart rate monitor, an oxygen saturation sensor, hydration tracking, sleep monitoring, and an app for checking your stress levels.

Exercise features include the Garmin Coach app for helping you with 5K, 10K, and half-marathon runs, and there is also a set of on-screen workout animations.

Despite the many features, Garmin says the Vivoactive 4S has a battery life of seven days in "smartwatch mode," — meaning that should be achievable when using the watch as normal and not restricting it to a state where it only shows the time. Garmin says up to five hours of battery life is possible with GPS enabled while streaming music.

Garmin vivoactive 4S, Smaller-Sized GPS Smartwatch, Features Music, Body Energy Monitoring, Animated Workouts, Pulse Ox Sensors and More, Silver with Gray Band (Renewed)

Garmin vívoactive 4S, Smaller-Sized GPS Smartwatch, Features Music, Body Energy Monitoring, Animated Workouts, Pulse Ox Sensors and More, Silver with Gray Band (Renewed)

List Price: $209.99

New From: $209.99in Stock

Fitbit Versa 3: Six days

The Fitbit Versa 3 has a claimed six-day battery lifeFitbit

Another fitness smartwatch with strong battery life is the Versa 3 by Fitbit. This watch has all of the exercise and fitness tracking features you would expect from Fitbit, plus built-in GPS for recording outdoor runs, music playback, sleep tracking, and a choice of Amazon Alexa or Google Assistant.

It also has a 24/7 heart rate monitor and can calculate your oxygen saturation level. The watch can handle calls and texts via your smartphone, includes Fitbit Pay for contactless credit card payments, an always-on display, and there are three color options to pick from.

Fitbit says the Versa 3 has a battery life of six days, during which time it will track your movement all day and night. The company says how battery life "varies with use and other factors; up to 12 hours with continuous GPS."

Fitbit Versa 3 Health & Fitness Smartwatch with GPS, 24/7 Heart Rate, Alexa Built-in, 6+ Days Battery, Black/Black, One Size (S & L Bands Included)

List Price: $199.95

New From: $199.95in Stock

Mobvoi TicWatch 3 Pro: Three days

The TicWatch 3 Pro and its dual-display system has a three-day battery lifeMobvoi

Falling a little behind the Fitbit is the TicWatch 3 Pro by Mobvoi. This smartwatch runs Google's Wear OS software (whereas Garmin and Fitbit use their own, potentially improving efficiency), and works much like any other, with fitness, sleep and activity tracking.

There is also a heart rate monitor, blood saturation tracking, IP68 waterproofing, making it safe to swim with, integrated GPS, and an altimeter.

What's clever here is how the TicWatch 3 Pro has two displays. First, there is a regular AMOLED display, similar to the screens of other smartwatches; above that sits a low-power display that is transparent when not in use and looks like a Kindle ebook reader. This means the low-power, the greyscale display can be used to show the time constantly, then the color display beneath shines into life when you want to do more.

It's a clever setup that means the TicWatch can last 45 days when using only the low-power screen. When using both, Mobvoi claims up to three days of battery life.

TicWatch Pro 3 GPS Smart Watch Men's Wear OS Watch Qualcomm Snapdragon Wear 4100 Platform Health Fitness Monitoring 3-45 Days Battery Life Built-in GPS NFC Heart Rate Sleep Tracking IP68 Waterproof

List Price: $299.99

New From: $299.99in Stock

Samsung Galaxy Watch Active 2: Two-and-a-half days

The Galaxy Watch Active 2 has a claimed 2.5-day battery lifeSamsung

Samsung's latest smartwatch, the Galaxy Watch Active 2, has a sleek design, the company's own Tizen operating system, and is a worthy rival to the Apple Watch. Available in 40mm and 44mm case size options, the watch can be fitted to any strap thanks to industry-standard lug bars and is available in pink and black.

There is an NFC for contactless payments, Bluetooth for music playback, constant heart rate tracking, IP68 water resistance, and a suite of exercise and fitness apps to log every exercise session.

Samsung is a little vague on battery life, stating that 60 hours (2.5 days) is possible with "typical usage", and up to 131 hours (5.5 days) can be achieved with "low usage." The company also says: "Actual battery life varies by network environment, features and apps used, frequency of calls and messages, number of times charged, and many other factors."

SAMSUNG Galaxy Watch Active 2 (40mm, GPS, Bluetooth) Smart Watch with Advanced Health Monitoring, Fitness Tracking, and Long Lasting Battery - Rose Gold (US Version) (Renewed)

List Price: $94.50

New From: $94.50in Stock

Used From: $98.00in Stock

Apple Watch Series 6: 18 hours

The Watch Series 6 offers 18 hours of battery lifeApple

As we mentioned earlier, the Apple Watch doesn't have a particularly long battery life compared to the claims of others. However, Apple goes into more detail than most regarding what an average 18-hour day with a Watch might look like, as explained earlier.

The Watch Series 6 is as feature-packed as smartwatches get, with Wi-Fi, Bluetooth, GPS and optional 4G, plus activity tracking, heart rate, and sleep. It can also take an ECG, monitor blood oxygen saturation, make an emergency call if you fall over and don't get up, and it can be worn while swimming.

It does a lot but requires daily (or nightly) charging. This is mostly okay when used during the day, but if you want to track your sleep with an Apple Watch, you'll have to work out a routine where its battery is topped up at some point during each day.

Apple Watch Series 6 (GPS + Cellular, 44mm) - Space Gray Aluminum Case with Black Sport Band (Renewed)

List Price: $193.99

New From: $208.00in Stock

Used From: $194.04in Stock