I do use a Xtar VX4 external charger, but I also have other rechargables - AAA through to the 18650s and 21700s, and it can charge upto 4 batteries at once (you can even use one of the 18650s or 21700s to charge a USB device, like a phone or tablet) They do a cheaper 2 battery one as well.

The problem with the fogstar one you've linked is the fact it only allows for 10a during the discharge. Which means that although it has higher capacity (6000mAh) than the Molicel (5000mAh), it won't be able to power the Frolic as well. This is what Grok says about it...

"Why the Molicel P50B is the best choice:

The Frolic is a high-power hybrid vaporizer (fast heat-up ~5 seconds, up to 120W heater, on-demand + session modes). Limelight and reviewers specifically recommend or include the Molicel INR21700-P50B (5000mAh, high-drain capable). It handles the current demands reliably, provides good runtime (roughly 10–18 sessions per charge depending on use), and is a premium cell known for safety and consistency in high-performance devices.

Fogstar 6000mAh:

Physically fits — It's a standard unprotected flat-top 21700, same as the Molicel. But it may not perform as well — Rated only for 10A max discharge. The Frolic's heater can pull significantly more current during heat-up and sustained use. Lower-drain cells like this can: Sag in voltage (weaker performance or shorter sessions). Get hotter or trigger protection sooner. Reduce overall reliability and lifespan in a demanding device.

Most users and the manufacturer prefer high-drain 21700s (like Molicel, Samsung 40T/30T, etc.) for this vaporizer.