One of the best features on the Apple Watch Series 7 is its newly added fast charging system, which Apple claims is 33 percent and capable of charging the watch to about 80 percent charge in about 45 minutes. But if you want to take advantage of the fast charging, you’ll need to make sure to use the charger that comes with the Series 7, which is a brand new “Apple Watch Magnetic Fast Charger” that’s different from the old model (which doesn’t offer similar charging speeds).

The old USB-C charger and the new model look identical from the outside, but only the new cable can charge the Series 7 at full speed (assuming you’ve got a USB-C charger that can put out the proper wattage, that is — Apple recommends its own 20W USB-C charger, of course).

And while Apple has already stopped selling the old USB-C charger that doesn’t offer fast charging on its site, they’re still available from other retailers like Amazon, so you’ll want to make sure you’re buying the right one if you’re looking to pick up a spare. It also means that if you own an existing Apple Watch charging dock or stand that has a built-in charger, you won’t be able to take advantage of the faster charging available on the Series 7.

Of course, none of this matters if you own an older Apple Watch: even if you’re using the proper fast charging cable, the fast charging technology still needs a Series 7 watch to work. As Apple’s website notes: “Fast charging is only compatible with Apple Watch Series 7. Other models will have regular charge times.”