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Lithium-Ion Battery Glossary: 17 Need-to-Know Terms

Get to grips with the basics of lithium-ion batteries in this helpful glossary, including all those need-to-know terms that might have otherwise left you scratching your head.

Batteries of any kind can quickly get technical. You’ve got enough on your plate without having to learn the ins-and-outs of how lithium-ion (Li-ion) batteries work, but a basic understanding is important for making sure you’re using them safely and responsibly. 

Read on for an alphabetised list of the most commonly used terminologies talked about by Li-ion battery suppliers, battery pack designers, and OEMs — the words and phrases you’re most likely to encounter when researching, discussing, or buying Li-ion batteries.

Click to download your copy of our four-step risk assessment checklist for Li-ion batteries.

Glossary of lithium-ion battery terminologies


Capacity can mean three different things, depending on whether you’re talking about theoretical capacity, actual capacity, or rated capacity.  

The theoretical capacity is the highest possible value calculated. If you’re comparing different batteries, you might see “specific capacity” mentioned — this is the theoretical capacity per unit volume or mass. 

Actual capacity is the power output of the battery under specific conditions, and it is always less than the theoretical capacity. 

Lastly, the rated capacity is the minimum amount of power that a battery should discharge according to relevant standards.

A battery’s capacity is an important factor to consider. This capacity is represented by the symbol C and is measured in units of ampere hour (Ah) or milliampere hour (mAh). 


As battery technology advances, it's important to understand the language used to describe battery performance. One of the most critical measurements is the C-rate, which allows you to gauge the speed at which a battery can be charged or discharged. For instance, a C-rate of 1C means that a battery can be charged from 0-100% in just one hour.

Many manufacturers of Li-ion batteries recommend charging at 0.8C or less to prolong battery life. That said, most batteries can take a higher charge C-rate without suffering.

This simple unit of measurement will help you to make more informed decisions about the batteries you use in your operations. If you're looking for a high-performance battery, make sure to pay attention to the recommended C-rate and choose the one that suits your needs.


Discharge can be thought of as the primary function of a battery. It describes the process of the battery being used to power something — in slightly more technical terms, it’s delivering a current to a circuit by converting chemical energy into electrical energy.

Discharge voltage

Discharge voltage (also known as working voltage) is the amount of battery voltage available at any given point while the battery is discharging. You’ve installed the battery and the instrument/machine is running; this is the voltage being produced at the time.

Li-ion batteries run out of power just like any other battery. In discharge voltage terms, this is shown by the voltage gradually decreasing as it discharges. The rate of this decrease depends on multiple factors such as the device being powered and the battery itself.

Cut-off discharge voltage

The cut-off discharge voltage represents the lowest level at which the battery is no longer suitable for further use. Knowing the cut-off discharge voltage of your batteries will help you to maintain them better as well as manage your operations more effectively. 

One battery is not the same as another. Because different battery types and discharge conditions require varying levels of capacity and lifespan, it therefore follows that the specified cut-off discharge voltage for your batteries will likely differ between types.

Depth of discharge (DoD)

The depth of discharge (DoD), or discharge depth, directly impacts the charging life of a rechargeable battery. Specifically, the deeper the discharge depth, the shorter the charging life. For a long charging life, which would benefit operations with long shift cycles, few breaks, or continued machine use, deep discharge should be avoided as far as possible.


The electrolyte is the medium within a cell that enables ions to move from one side to the other and back again, in this way facilitating the battery’s ability to charge (and recharge).

Internal pressure

The pressure that builds up inside a battery is known as the internal pressure. This is a result of the gas produced during the charging and discharging cycle. The material of the battery, its manufacturing process, its structure, and other factors all play a role in affecting the gas produced and, therefore, the internal pressure. 

Monitoring the internal pressure of a battery is important for ensuring the safe and efficient functioning of the battery and preventing any hazards that may arise due to excess pressure.

Internal resistance

The resistance within a battery to the flow of the current is called the internal resistance. Because there’s a lot of movement going on inside a battery (for example, the flow of ions, changing electrolyte concentration, and inconstant temperatures) internal resistance isn’t fixed.


This term describes the escape of the electrolyte to the outer surface of the battery. This is very dangerous, as it can easily cause chemical reactions that generate heat. 

This can damage other cells, leading to a chain reaction capable of starting fires or triggering explosions (see “Thermal runaway”, below).

Lithium ion

Lithium ion is an emerging technology with applications across single-use and rechargeable batteries. Compared to nickel-based batteries it offers greater performance and double the energy density, giving it a wide range of applications across industries.

Read more: Lithium-Ion: What Is It and How Does It Work? 


Overcharging is when you fail to remove a battery from its charging point even after it has fully charged. The result is to force more current into the cell than it can handle, in other words after all its active material has already been converted into energy reserves. Long term, this will damage the battery and can result in overheating and increased risk of fire.

Rapid charge

Rapid charge is the ability to charge a battery to its full capacity within 2.5–6 hours. As we referenced earlier for “C-rate”, the advised charge rate of a Li-ion battery is between 0.5C and 1C, but the complete charge time is typically about 2–3 hours, highlighting the speed at which these batteries can be comfortably recharged and made ready for use.

Rechargeable batteries/cells

See "secondary batteries” below.

Secondary batteries

Secondary batteries are those batteries that can be recharged by reversing the electromagnetic process going on inside them. Commonly called rechargeable batteries. (As opposed to primary batteries, which are one-use.)

Thermal fuse

The risks surrounding Li-ion batteries are well publicised. To minimise the risks, these batteries are fitted with thermal fuses. This component helps protect against overcurrents as a result of frequent overcharging, which is a common cause of thermal runaway (see below), thereby making your batteries — and your operations — safer.

Thermal runaway

One of the most common terms used in association with the risks surrounding Li-ion batteries is thermal runaway. This phenomenon occurs when a Li-ion battery is overheated, usually as a result of overcharging, although other triggers can damage the integrity of your battery, too. Chemical reactions inside the battery release all the energy stored very quickly, causing the temperature to spike (to around 400 degrees Celsius) within milliseconds. A fire usually follows, but sometimes the battery will outright explode. 

The heat of the reaction and the temperatures involved make the resulting fires very difficult to extinguish, and because batteries are rarely used in isolation, other batteries in the area will often become damaged and catch fire or explode too.

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Learn more about Li-ion batteries with our checklist

Batteries can quickly get technical, but we hope the list of terms and phrases above helps you to make sense of the technology powering your operations and how to use it safely.

If you’d like more prescriptive advice for how to keep your operations safe against the risks associated with Li-ion batteries, we’re currently giving away a risk assessment checklist. Download your free copy by clicking the image below to access an easy-to-follow framework made up of four key steps for keeping your batteries and your operations safe.

To download your copy of our four-step risk assessment checklist for lithium-ion batteries, click here.