Eventually, every pen dies. Here are the clear end-of-life signs: the battery drains overnight, even when off. The pen gets hot to the touch during charging or use. You notice a burnt taste that won't go away, even with a fresh cartridge. Or the device physically cracks or the mouthpiece breaks off. When these happen, it's time to replace.

Don't be tempted to keep using a pen with a cracked body or a battery that swells. That's a safety risk. Lithium batteries can fail dangerously if damaged. Trust me, you don't want to deal with a swollen battery. Replace it.