When it comes to vaping, there are two types of devices. We have mods with internal cells that cannot be changed and we have mods with swappable batteries. Each category has its pros and cons, and in the end it all depends on personal preference and day to day use.
If your vape works with regular 18650, you're probably wondering when to replace mod batteries. A decent 18650 cell should hold its charge up to 300 – 500 cycles. But if you don't always use it properly it might give you some indicators that it needs to be changed.
DAMAGED BATTERIES ARE VERY DANGEROUS
First and foremost if the wrap around your cell has been deteriorated, it's best to stay away from it. Also if the outer casing shows signs of mechanical damage, scratches or leaks then you're dealing with a potential safety hazard. Immediately stop using these batteries and dispose of them at your local recycling center.
Batteries that are in good condition also need to be replaced at a certain time interval. We usually swap them after 6 months of day to day use. This is not a general rule, but it's something most vapers agree on. It's also the common lifespan of most 18650 cells on the market today.
INDICATORS YOU NEED TO REPLACE YOUR BATTERY
If you're vaping at roughly the same wattage and the battery lasts half as long as when you purchased it, it's probably time to swap it for a new one. Then, if your mod constantly shows "low battery" after slightly increasing the wattage or the battery gets way hotter when in use or recharging you know you need to change it.
Also if you're using an external charger and the battery doesn't go up to 4.2V it's another sign that it needs to be replaced. Good external chargers are excellent for preserving battery life and that's why we always recommend them.
HOW TO INCREASE THE LIFESPAN OF YOUR VAPE BATTERIES
Of course, there a couple of things you can do to increase the lifespan of your good 18650. The most important is to prevent them from overheating. Heat is the main enemy in case of all cells and once it goes over 112F/45C they will start aging at a faster rate. That's why we don't recommend chain vaping or using very low atomizer resistances because it takes batteries to the edge. Also it's not a good idea to use them when the temperature is below -20C/ -4F due to inefficient internal chemical reactions.
Another thing is to never let your batteries discharge below 20%. Most regulated mods these days stop working when the cell reaches 3.3V but it's a good idea to stop higher than that. This will ensure the battery stays between functional parameters for a longer period of time.