Battery Safety Part 2 – Temperature is the Enemy

18650/18350 Batteries
18650/18350 Batteries


A lot of things can affect how safe a battery is to use. But one of these things also affects how long your battery will last before you need to replace it. What is it?​


Knowing whether or not your batteries are getting too hot is the most important thing you can do to vape safely. Not letting your batteries overheat also slows down the aging of your batteries and helps them last longer. The higher the battery temperature, the faster it ages and the more internal damage it suffers.

Battery Aging – Internal Resistance

This aging consists of changes to the battery’s “internal resistance” and its “capacity”. The internal resistance is just like the resistance you have in an atomizer’s coil, only much lower. While a coil might have a resistance of 1 ohm, the internal resistance of a battery is usually around 0.02 ohms or less. A big difference! But, even this tiny resistance can heat up your battery when current is being drawn from it. Just like how a coil heats up when you pass current through it.​

As a battery ages, or if it is damaged from getting too hot, the internal resistance starts increasing. And since it is this internal resistance that makes the battery heat up, it gets hotter. This can lead to more damage, which increases the internal resistance even more, which heats up the battery even more….and so on. It’s a never ending cycle that eventually leads to the battery either no longer working, or if the temperature is high enough, to venting of the battery.​

Venting is when the pressure inside an overheating battery causes a “vent”, a preweakened spot on the top of the battery, to open up and release the excess pressure. When the battery temperature rises high enough, extra gas is created. It’s this extra gas that increases the pressure inside the battery. Once this happens the battery is badly damaged and should be disposed of. We’ll cover venting in more detail in a later article.

Battery Aging – Capacity Loss

The other way a battery ages from high temperatures is loss of “capacity”. Capacity is a measure of how long the battery will last when you use it. For vaping you’ll usually see it expressed as the number of “mAh”, milkiampere-hours, it is rated at. If a battery gets too hot and is damaged, its capacity starts to drop. Have you ever had to replace a battery after only a few months because it wasn’t lasting nearly as long before needing charging? It could have been due to overheating of the battery, speeding up its aging and loss of capacity.

Current Rating

Several things can help to increase battery temperature; charging too fast, a nearby warm atomizer, no ventilation in your device’s case, or a hot circuit board (the “chip”). But the biggest thing that brings your battery temperature up is how much current you draw from it.​

One of the ratings you’ll see for a battery is its “continuous” current rating. This is exactly what it sounds like. The current isn’t in pulses, but is drawn from the battery in one long discharge until the battery is empty and it has dropped to a certain voltage, typically between 2.5V and 2.8V.​

This continuous rating is often called the “CDR”, Continuous Discharge Rating. Other names for it include “MCC”, Maximum Continuous Current and “MCD”, Maximum Continuous Discharge. They all mean the same thing though. This rating is set by the manufacturer of the battery and usually corresponds to the current level where the battery temperature reaches about 75°C/167°F. At this temperature the aging of the battery and the internal damage starts quickly increasing. This is why the manufacturer sets the CDR at the current level where this happens. It’s to let us know to not operate the battery above this current level if we want to be safe and have our batteries last a long time.

Keeping a Safety Margin

Vaping doesn’t discharge the cell continuously though. Because we only use the battery a few seconds at a time it will run cooler even if each time the device is fired it draws current equal to the battery’s CDR. But we must have a safety margin when using our batteries! If a device malfunctions then knowing that the battery you have picked will not vent, or worse, is very important. A battery in a malfunctioning device might not destroy itself, and your device, if we can pick the one that runs cooler at high discharge current levels. This can only be done if we know that battery’s continuous discharge rating and we don’t go over it.​

We must have a safety margin when using our batteries!

And this is why it’s important to know how much current you’ll be drawing from your batteries and to choose a battery that will be safe to use and will last a long time. These are topics we will be discussing in upcoming articles.

Since 1992 I have been working as an electrical engineer designing battery related products like chargers, dischargers, and battery management and protection systems. A lot of that work involves researching and testing batteries for my clients. Over time I have acquired a lot of knowledge about how to choose batteries and use them safely. Through these articles I will be sharing what I've learned.