Nominal Voltage

If you're delving into the world of batteries, you've likely encountered the term "nominal voltage." Don't be intimidated by its technical-sounding name; it's a fundamental concept that's easy to grasp. And besides, the whole reason it exists in the first place is to make things easier to understand. 

A nominal voltage is basically just the average voltage that the battery will be close to during most of its operation. While the nominal voltage isn't the entire story, it provides enough information to know what devices are compatible with what batteries. So, a 24-volt battery won't always be at 24 volts but a 24-volt inverter does not have to run at exactly 24 volts either. As long as the battery and device you are running have the same nominal voltage, they should work together without issue.

In this article, we will elaborate on what a nominal voltage is, how it can be used, why the concept of a normal voltage is important, and go over nominal voltage for common cell chemistries.

What's Nominal Voltage Anyway?

Nominal voltage is basically a convenient compromise. It's a value that represents the middle ground of a battery's operating voltage over its discharge cycle, without having to plot the entire discharge curve. This single figure is much easier for the average consumer to use when comparing battery-operated systems to the batteries that will be used to operate them. 

An interesting thing to note about battery nominal voltage is that the nominal voltage is the best voltage for storing a battery pack at. 

Basically, it’s just a lot easier to grasp a single voltage rather than a range of voltages. The actual voltage of a battery can fluctuate due to various factors such as its current state of charge, the load it's under, the temperature, and its age. A battery's voltage when fully charged will be higher than its nominal voltage, and as it discharges, the voltage will drop, often dipping well below the nominal value before the battery is considered dead. This fully charged voltage and discharged voltages are the voltage range for instance an 18650 cell has a min voltage of 2.5 and a max voltage of 4.2 assuming its NMC chemistry. 

For instance, a 12-volt 3s NMC lithium-ion battery's resting voltage when fully charged is 12.6 volts. It will drop too well into the 11-volt range as it's being discharged which makes its nominal pack voltage 10.8V. We still refer to it as a 12-volt battery because when using NMC cells, 3 cells in series is as close as you can get to a 12V lead acid battery voltage range. 

While nominal voltage might seem like an annoying oversimplification, it turns out to be a handy little shorthand to understand a battery's operating range without getting bogged down in the details. 

Why's Nominal Voltage Important?

Nominal voltage serves as a dependable reference point. It plays a crucial role in determining how batteries are grouped for a particular device, ensuring that the device receives the correct amount of power. Imagine your gadget requires 12 volts to operate. If you know the nominal voltage of your battery, you can easily calculate how many of them you need to connect in series to achieve that precise voltage level.

First of all, the concept of a nominal voltage simplifies the design process of electrical systems. Engineers and technicians can select components that are compatible with the nominal voltage, thus ensuring that each part functions within its intended voltage range. This reduces the risk of overvoltage or undervoltage, which can lead to inefficiency, potential damage, or even safety hazards.

Also, a nominal voltage is vital for the standardization of electrical components and appliances. It allows manufacturers to produce batteries, devices, and other electrical components that are universally compatible with standard systems. This standardization is what enables consumers to purchase a battery or device off the shelf and confidently expect it to work with their existing setup.

It's Not Just About One Battery

Remember that when you're using multiple batteries together, such as in a DIY portable power station or an electric vehicle, the nominal voltage of the entire battery pack is what matters. This collective voltage depends on the number of batteries you have and how they are connected. 

For example, if you have 10 NMC lithium-ion cell groups in series, its nominal voltage is going to be 36V. 

Why Voltage Varies And Why We Need A Nominal Voltage

Nominal voltage gives us a baseline understanding of a battery's performance under typical conditions. It's like the average temperature of a city over a year – it tells you what to expect, but the actual daily temperature might vary.

When designing a battery pack, the nominal voltage is a key factor to consider, especially when the application requires a specific voltage to operate efficiently. The nominal voltage is determined by the voltage of individual cells and how they are connected. For example, if each NMC (Nickel Manganese Cobalt) lithium-ion cell has a nominal voltage of 3.7V, then connecting them in series will sum their voltages. In a scenario where you need a 48V battery pack, you would need to connect 13 cells in series (13 cells x 3.7V per cell = 48V). This is a common setup for light electric vehicles, such as e-bike battery packs, which often operate at a nominal 48V.

For more demanding applications, such as larger electric vehicles or more robust portable power stations, a higher nominal voltage is often required. To achieve a 72V nominal voltage, you would need to connect 20 NMC lithium-ion cells in series (20 cells x 3.6V per cell = 72V). This higher voltage can provide increased power and efficiency for the system it's designed to run. It's important to note that the battery management system (BMS) is essential in both cases, as it ensures that the battery pack operates within safe parameters, maintaining the longevity and safety of the pack. The BMS will manage the charging and discharging of the cells to prevent overcharging or deep discharging, which can be harmful to the battery's health. Furthermore, the BMS will work to keep the battery in balance to help extend the pack's life.

Charging and Discharging: During charging, the voltage of a battery increases, often exceeding the nominal voltage. Similarly, when discharging, the voltage drops below this level.

Battery Chemistry: Different battery chemistries have different nominal voltages. For example, a standard lead-acid battery typically has a nominal voltage of 2 volts per cell, while a nickel-cadmium (NiCd) cell has a nominal voltage of 1.2 volts.

Real-World Examples of Nominal Voltage in Action

Electric Vehicles (EVs): EVs use large battery packs. Let's say each cell has a nominal voltage of 3.6 volts (common for lithium-ion cells). If an EV requires a 400-volt battery pack, you would need about 112 cells in series (400 volts / 3.6 volts per cell). This series configuration increases the voltage to meet the vehicle's requirements.

Portable Power Stations: These often use lithium-ion (NMC) or lithium-iron phosphate (LiFePO4) batteries. For a power station with a nominal voltage of 24 volts, if using LiFePO4 cells with a nominal voltage of 3.2 volts, you'd need 8 cells in series (24 volts / 3.2 volts per cell). In contrast, using NMC cells requires just 7 cells in series to be considered a 24V battery. This is due to its higher 3.6 volt nominal voltage.

Solar Power Storage: In a DIY solar power system, batteries store the energy collected by solar panels. If the system is designed for 48 volts, and you're using 12-volt AGM (Absorbent Glass Mat) batteries, you would need four of these batteries in series to achieve the required voltage. In this case, the actual voltage is going to be way higher or way lower than 48 volts because 48 volts is just the normal voltage of this particular system. 

Nominal Battery Voltage By Cell Chemistry

Each type of battery chemistry has its own nominal voltage, which is essentially the average voltage a battery maintains during most of its discharge cycle. So, it’s important to consider this when you are working with a given battery.

Keep in mind the numbers below are single-cell nominal voltage. So a battery with 13 cells in series would be 13 times the nominal voltage of one cell to give you a pack nominal voltage.  

Here is a handy list of nominal voltages for the most common cell chemistries:

NMC (Lithium Nickel Manganese Cobalt Oxide): 3.6 volts

LFP (Lithium Iron Phosphate): 3.2 volts

Lead Acid: 2.1 volts

LTO (Lithium Titanate Oxide): 2.4 volts

Sodium Ion: 2.1 volts

LiCoO2 (Lithium Cobalt Oxide): 3.7 volts

NiMH (Nickel-Metal Hydride): 1.2 volts

NiCd (Nickel-Cadmium): 1.2 volts

LiMn2O4 (Lithium Manganese Oxide): 4.0 volts

Li-S (Lithium-Sulfur): 2.15 volts

Conclusion

Nominal voltage is more than just a numerical value; it's an important property of an electrical system and understanding it will ensure the efficient and safe operation of battery-powered devices. By understanding the nominal voltage, you can easily know what other systems your battery can safely work with. 

So, nominal voltage is a fundamental concept that empowers you to make informed decisions about battery configurations. It’s important to remember that the nominal voltage is by no means the entire story.

We hope this article told you everything you needed to know about nominal voltages, thanks for reading!