Why read this review?
I’ve been wearing a Whoop for the past 3 years and am constantly looking for ways to maximize my technology’s impact on my life. I have owned the Whoop 4.0, as well as Whoop 3.0.
What is the Whoop?
The Whoop 4.0 is a health and fitness tracker that is designed to be worn 24/7 to gain insight into our body. The system is composed of a physical tracking device (composed of a battery, processor, and 5 sensors), a slide over battery charger, the SuperKnit band, an smart phone application, and a processing backend.
The big idea behind the Whoop is to gain real time and trending looks at recovery (how ready your body is to work), sleep (quality & quantity), and strain (how hard you’ve worked). When you layer in daily tracking of your habits (through the easy to use Journal feature), you can understand the impact of various habits on your body (e.g., alcohol right before bed has a super negative impact on my recovery).
While the Whoop doesn’t physically make you healthier, it’s been my experience that the fact that you’re measuring habits will cause you to make healthier decisions. This is more broadly known as the Hawthorne Effect (HBR article)- the effect of observation on improving behavior / outcomes.
How big is the battery on the Whoop 4.0?
The Whoop 4.0 system actually has two batteries – one on the device itself and a larger one on the battery charger. This is because the Whoop is designed so that you do not have to take it off to charge.
- Whoop 4.0 fitness tracker – 192 mAH (3.77V, 0.724 Wh)
- Whoop 4.0 Battery charger – 638 mAH (3.85V, 2.46Wh)
Battery size is measured in mAH or milliampere-hours, which is a measure of electrical storage capacity. I think an easy way to think about mAH is that it’s like describing how many gallons (or liters for our non-US friends) that gas tank on a car holds.
Check out this article on wireless travel chargers, for a slightly deeper dive on mHA and how to think about it.
How long does the Whoop battery last?
Whoop claims that the Whoop 4.0 battery will last ~5 days on a single charge. In my testing (with a Whoop 4.0 that’s ~2 yrs old), I can get at least 4.5 days of operation on a single charge. Considering how quickly smart phone batteries degrade (hence need for wireless batteries), this is quite impressive.
I recently tested my Whoop battery by recording the charge level every 6 hours from fully charged until I was at 10%. As you can see in the accompanying graph, the battery level decreased at an impressively smooth decline over the 4.25 days of testing. I charged my Whoop when it got to 10% as I was planning to use it’s haptic alarm (one of my favorite features) the next morning and didn’t want to sleep in.
How do I tell what level of charge my Whoop has?
There are two primary ways of determining the battery level of your Whoop 4.0 sensor.
- In the app – Go into the app and look in the top right corner. It will show your battery level down to a specific percentage (%).
- From the sensor’s LED – double tap the Whoop sensor and the color of the indicator light will tell you the approximate battery charge
| LED COLOR | Battery Level |
| Green | 50-100% |
| Yellow | 20-49% |
| Red | 5-20% |
| Flashing red | <5% |
How do you charge the Whoop 4.0 battery?
The Whoop was designed to charge while you wear it. To charge the Whoop, slide the fully charged Battery Charger over the device and then tap the battery twice. You’ll know it’s charging when the Battery Charger indicator light is illuminated.
Interestingly, the Whoop uses wireless charging from the Battery Charger to the device.
Once the Whoop 4.0 is fully charged, you should get a notification on your Whoop app.
How do you charge the Whoop Battery Charger?
The Battery Charger (the small battery that slides over the Whoop 4.0) is charged with a USB-C plug from any outlet or device. It takes <1 hr to fully charge the battery and the update in 2022 made it more reliable to keep a charge on the Battery Charger (due to the need to tap feature). You can use the USB-C cable that came with your Whoop or any USB-C cable (including my favorite Amazon Essentials ones).
Whoop 4.0 Fitness Tracker
Fitness tracker that provides real time feedback on strain, recovery and sleep to maximize your day
- 5 sensors to measure multiple bodily functions
- Waterproof design
- Bluetooth connectivity to your smartphone
- Slip over battery charger design
- Powerful analytics to uncover impact of habits
Why is the battery on the Whoop 4.0 Battery Charger larger than the one on the device?
Whoop doesn’t explicitly address why the battery charger has a larger electrical capacity than the Whoop 4.0 device. My sense is that the increased battery size is to power the wireless charging mechanism.
Get extra Whoop 4.0 Battery Chargers on Amazon (and they are HSA-eligible).
Can you get multiple Whoop charges from the Battery Charger?
Based on the difference in storage capacity, it’s theoretically possible to get at least a partial second charge from the Battery Charger. In reality (primarily based on the multi-day battery life of the Whoop sensor), it’s very hard to get any charge beyond the first charge from the Battery Charger (on a single charge)
Can you charge the Whoop 4.0 wirelessly?
I’ve been able to get my Whoop Battery Charger to charge wirelessly, but it’s not an official capability.
