Honestly, the Whoop Battery Pack 4.0 is kind of a weird piece of tech when you first look at it. It’s this tiny, pebble-shaped thing that costs a chunk of change, and if you lose it, your expensive fitness tracker basically becomes a high-end rubber bracelet in about four days. I’ve seen so many people treat these things like disposable lighters, but there is actually some pretty sophisticated engineering under the hood that can be finicky if you don't know the quirks.
If you're wondering why your charger isn't "lighting up" or if you can actually wear it in the pool (spoiler: yes, but with a catch), you’re in the right place. Most people just slide it on and hope for the best. But given that this little guy is the lifeblood of your data, it's worth knowing how to actually keep it alive.
The "Invisible" Tech: How the Whoop Battery Pack 4.0 Actually Works
The biggest change from the old 3.0 version to the Whoop Battery Pack 4.0 wasn't just the size. It was the move to NFC wireless charging.
See, the old version used those tiny copper pins. They’d get sweaty, they’d corrode, and eventually, they’d just stop making a connection. The 4.0 pack ditched that for a custom solution developed with a company called NuCurrent. This is basically a high-power NFC (Near Field Communication) setup. It delivers about 1.2W of power, which is actually more than double what standard NFC charging usually does.
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What this means for you? No exposed metal bits. That’s why it’s waterproof.
The internal capacity is roughly 638 mAh. To put that in perspective, your iPhone 15 has a battery more than five times that size, but for a tiny sensor on your wrist, it's plenty. It takes about 2 to 2.5 hours to juice up the pack itself via USB-C, and then another 2 to 2.5 hours to transfer that power to your Whoop strap.
It’s slow. It’s intentional. Rapid charging generates heat, and heat is the absolute enemy of tiny lithium-ion batteries worn against human skin.
Is the Whoop Battery Pack 4.0 Really Waterproof?
This is the question that gets people into trouble. The short answer is yes. The long answer is "up to a point."
The Whoop Battery Pack 4.0 carries an IP68 rating. Technically, that means it is dustproof and water-resistant up to 1 meter (about 3.3 feet) for roughly 2 hours.
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- Can you shower with it? Totally.
- Can you swim laps with it? Maybe, but I wouldn't.
- Can you dive into a 10-meter pool? Absolutely not.
The Whoop 4.0 sensor (the part on your wrist) is rated for 10 meters. The battery pack is only rated for 1 meter. If you go deep-sea diving with the charger attached, you're going to have a very expensive, very dead piece of plastic. If it gets wet, especially in salt water or a chlorinated pool, just rinse it with fresh water and let it dry completely before you plug it back into a USB cable.
Corrosion doesn't happen on the charging face anymore, but it can still ruin the USB-C port if you aren't careful.
Troubleshooting the "Dead" Battery Pack
We've all been there. You slide the pack onto your strap and... nothing. No lights, no "charging" bolt in the app. Total silence.
Before you assume it’s broken and contact support, there are a few things that usually fix it.
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The Double-Tap Trick
Sometimes the pack is just "asleep." Double-tapping the top of the battery pack should trigger the LED indicator.
- Solid Green: You're good (50-100%).
- Solid Yellow: Getting low (20-49%).
- Solid Red: It's struggling (under 20%).
- Blinking Red: It's basically dead (under 5%).
The Firmware Fix
Believe it or not, your charger has its own "brain" and needs firmware updates just like your phone. If your Whoop Battery Pack 4.0 isn't charging your strap, it might just be a software glitch.
Whoop actually has a standalone Battery Pack Updater tool for Mac and PC. You plug the pack into your computer using the original USB-C cable (don't use a cheap gas station cable, they often don't transfer data properly) and run the utility. I’ve seen this revive dozens of "dead" packs that were just stuck in a boot loop.
Resetting the Sensor
If the pack is green but the Whoop app says it isn't charging, the problem might be the sensor on your wrist.
- Open the Whoop app.
- Go to Device Settings.
- Hit the Advanced tab.
- Select Reboot Device.
This won't wipe your data. It just kicks the hardware into gear.
The Secret to Battery Longevity
Lithium-ion batteries hate being empty. If you let your Whoop Battery Pack 4.0 sit at 0% in a drawer for three months, there is a very high chance the chemistry inside will degrade to the point where it won't hold a charge anymore.
Try to keep it topped up. Even if your Whoop strap is at 80%, there’s no harm in throwing the pack on for 20 minutes to keep everything moving.
Also, charging "simultaneously" is a thing. You can actually plug the battery pack into the wall while it is attached to your wrist. It’s awkward as hell because you’re literally tethered to a wall outlet by a three-foot cord, but it works. It will charge the pack and the strap at the same time.
Does it matter which way it slides on?
Yes. Sorta. It only fits one way securely. The "Whoop" logo should be facing up and readable when you slide it over the sensor. If you force it on backward, you aren't just being a rebel; you're potentially stressing the plastic rails that keep the connection tight.
Actionable Steps to Keep Your Whoop 4.0 Powered
Stop treating the battery pack like a passive accessory and start treating it like a tool. Here is exactly what you should do to make sure you never miss a minute of strain or recovery data:
- Update the firmware today: Download the Battery Pack Updater on your laptop and check for updates. It takes five minutes and fixes 90% of charging "bugs."
- Dry it before you dock it: If you showered with the pack on, slide it off and pat the underside dry. Moisture trapped between the pack and the sensor can occasionally interfere with the NFC induction.
- Use the original cable: I know, USB-C is supposed to be universal. But the Whoop-provided cable is specifically gauged for this power draw. If you're having issues, go back to the black cable that came in the box.
- Check the app status: Don't just trust the LED. Once you slide the pack on, open the app and wait for the "Charging" animation. Sometimes the pack isn't seated perfectly on the rails, even if it looks right.
The Whoop Battery Pack 4.0 is a rugged little beast, but it isn't invincible. Treat it with a little more respect than a standard AA battery, keep the firmware fresh, and it'll easily last as long as your membership.
Next Steps:
Check your current battery pack firmware version by connecting it to the Whoop Battery Pack Updater tool on your computer. If you haven't done this since you bought it, you're likely running on outdated software that could be draining your battery life prematurely. Also, take a moment to inspect the USB-C port on the pack for any lint or debris—a quick blast of compressed air can often solve "unresponsive" charging issues. Finally, if you're a regular swimmer, make sure to rinse the pack in fresh water after every session to prevent salt or chlorine buildup from damaging the internal seals.