Voltage is like a health indicator for lithium batteries, and it is very important to understand it. Mainly, understanding voltage can avoid battery damage and prevent safety risks such as heating and swelling. Voltage is the most direct way to estimate the remaining power SoC. If the voltage drops very quickly, it may also be a sign of a battery problem. By understanding the appropriate voltage range, we can better protect the battery and make it last longer.
The purpose of this guide is to help you understand voltage information, especially voltage charts, so that you can know the differences in voltage between different types of lithium batteries, how to estimate power through voltage, and how to practice to make the battery run safely and efficiently.
The following are key terms related to lithium battery voltage.
Voltage: You can think of it like water pressure in a pipe. Voltage is the force that pushes the current (think of it as water flow) through the pipe. It is measured in volts (V). Voltage is not fixed, it changes as the battery charge changes.
Capacity: Indicates how much power a battery can hold, equivalent to the size of a gas tank. The unit is ampere-hour (Ah) or milliampere-hour (mAh). The larger the capacity, the longer it can theoretically last.
State of Charge (SoC): This is what we often call the percentage of power left. 100% means it is fully charged, and 0% means it is (theoretically) used up. Voltage is a major basis for estimating this percentage.
Nominal voltage: This is the most common voltage on the battery label, such as 3.7V. It represents the average voltage of the battery during most of the discharge process. It is not fixed, and the voltage of the battery is not this number when it is fully charged or almost empty, so it is called nominal. It is mainly used to distinguish batteries of different chemical compositions.
Minimum safe voltage (cut-off voltage / LVC): This is the lowest warning line that the battery can be safely discharged to. For example, it may be 3.0V or 2.8V (different batteries are different!). Once the discharge voltage is lower than the preset value, the battery may be permanently damaged, the capacity will decrease, and it may even become unsafe. The protection board (BMS) on the device usually cuts off the power supply here.
Storage voltage: If you don’t plan to use it for a long time, it is best not to store it fully charged or empty. The recommended long-term dormancy voltage is 40% ~ 60% power, which can minimize battery aging.
Voltage vs. State of Charge (SoC)
When a battery is discharged, the voltage usually drops. Just like draining water from a water tank, the water level (voltage) drops. However, this drop is not a straight line, it has a very special shape, which we call the discharge curve. It is mainly determined by the chemical reaction characteristics inside the battery. Different material combinations (chemical systems) determine the position and shape of the voltage platform.
- At the beginning, the voltage will drop faster.
- Most of the time in between: the voltage will become very stable and drop very slowly. This is the plateau period. Most of the battery’s energy is released in this relatively stable voltage range. It is difficult to accurately determine whether the battery has 60% or 40% of its power left by looking at the voltage alone. Especially for some types of batteries, the voltage change is very small, but the power may have been consumed a lot. So the voltage is just an estimate, not a precise measurement.
- Near the end: the voltage will drop rapidly again until it reaches the minimum safe voltage we mentioned above.
Different types of lithium batteries have completely different voltage curves. The following Figure 1 is a standard lithium-ion/lithium polymer (LiPo) battery discharge curve. Its nominal voltage is 3.6V or 3.7V, the fully charged voltage is 4.2V, and the discharge cut-off voltage is 3V. You see, from 4.0V to 3.6V, the voltage does not change much, but the power may drop from 80% to about 20%.
The following chart 2 is the discharge curve of lithium iron phosphate battery. Its nominal voltage is 3.2V, its discharge plateau is flat, most of the time the voltage is stable between 3.3V and 3.2V, and its cut-off voltage is usually around 2.5V (0%).
These charts allow you to determine the battery type, measure the battery’s current voltage, and find the voltage value you measured on the voltage chart of the corresponding type to find the corresponding SoC percentage.
Why voltage is not a perfect indicator of SoC
Judging how much power is left by looking at the voltage alone is not always accurate. It is only an estimate, and there are several reasons that may interfere:
Remember that middle period where the voltage is very stable? Especially for a standard lithium battery, for example, from 80% to 20%, the voltage may change by a few tenths of a volt. During this process, a slight change in voltage, or a slight error in your measurement, can result in a very different percentage of charge. In this range, the voltage is like a not very sensitive pointer.
Another reason is load. Have you ever noticed that when your phone is running low, the battery percentage drops very quickly as soon as you open the camera or play a game? Or when your drone accelerates, the voltage display drops? This is a voltage sag. When the battery outputs current (powering the device), its voltage temporarily decreases. The higher the current (the more power the device consumes), the more the voltage drops.
The influence of temperature. When the weather is cold, the battery loses power faster. Low temperature will increase the internal resistance of the battery, just like the water in the pipe becomes thicker. This will cause the voltage to drop more easily, especially when using electrical equipment. High temperature may increase the voltage slightly, but it will accelerate battery aging and pose a safety risk, so it should also be avoided.
The impact of battery aging. As the battery ages, its internal resistance increases, and the total amount of electricity (capacity) it can actually store decreases. When an old battery is working, its voltage drops faster and lower than a new battery.
The size of the charge/discharge current. The larger the current you use to discharge, the lower the discharge cut-off voltage will be.
Voltage comparison of different chemical systems
Different types of lithium batteries have completely different voltage characteristics. Your device voltage protection point must be set correctly according to the battery type.
The lithium batteries used in your mobile phone, laptop, most drones, and power banks have a full voltage of about 4.2 volts, an average voltage of 3.7V, and a discharge cut-off voltage of 3V.
Nowadays, many RV power supplies, home energy storage, some power tools and electric vehicles use lithium iron phosphate batteries, with a full charge voltage of 3.6V, an average voltage of 3.2V, and a discharge cut-off voltage of 2.5V.
There are also uncommon lithium titanate batteries, which have a fully charged voltage of 2.8V, an average voltage of 2.4V, and a discharge cut-off voltage of 1.8V.
| Battery Type | Fully Charged Voltage | Nominal Voltage | Minimum Safe Voltage |
|---|---|---|---|
| Standard Lithium-ion / LiPo | 4.2V | 3.6V – 3.7V | 3.0V |
| Lithium Iron Phosphate (LiFePO4) | 3.6V – 3.65V | 3.2V – 3.3V | 2.5V |
| Lithium Titanate (LTO) | 2.8V | 2.4V | 1.8V |
Practical applications of voltage
Now you know the relationship between voltage and power, and you also know that it is not always accurate. So how can we use this knowledge in practice?
To set the low voltage cutoff, you must set it according to your battery type (LiPo or LiFePO4, etc.) and its specification, in order to prevent the battery from over-discharging. It is best to set the actual cutoff voltage higher than the battery’s minimum safe voltage, such as 3.1V or 2.6V.
To estimate the battery life of a device, measure its static voltage. Then, take out a voltage chart for the corresponding battery type and find the approximate remaining percentage corresponding to this voltage.
Safe charging, each battery has its full-charge voltage, such as LiPo is 4.2V, LiFePO4 is 3.65V. Never charge above the maximum voltage, and use a charger that matches your battery chemistry.
You can also use the change in voltage drop to help you quickly determine whether the battery is aging.
Key voltage-related risks
Overdischarge means that the voltage is lower than its safety limit (for example, LiPo is lower than 3.0V, LiFePO4 is lower than 2.5V). This causes permanent and irreversible damage to the battery. The battery capacity will decrease and the performance will deteriorate. More seriously, overdischarge may cause damage to the internal structure of the battery, or even cause internal micro short circuits. The next time such a damaged battery is charged or used, the potential danger will increase. This requires you to set the LVC correctly.
Overcharging means that the charging voltage exceeds the safe upper limit of the battery (such as LiPo exceeds 4.2V, LiFePO4 exceeds 3.65V). This will cause the battery to overheat, bulge, catch fire or even explode.
Do not mix different battery types. Make sure the device and battery type match.
A more accurate SoC estimation method
You may have noticed that it is difficult to accurately determine the percentage of battery power remaining based on voltage alone. Here is a way to estimate SOC more accurately.
The coulomb meter, also known as the current integration method, is like a water meter installed on the battery. When charging, it accurately records how much electricity flows in; when using electricity, it records how much electricity flows out. By recording the in and out in this way, it can know quite accurately how much electricity is left in the battery. Now many smart BMS have built-in coulomb meters.
Today’s high-end mobile phones or electric vehicles will combine all the information such as voltage, current, and temperature, and then use some complex mathematical models or algorithms to calculate a battery percentage (SoC) that is closest to the actual situation.
Manufacturer voltage curve
Just like every person has his or her own characteristics, each lithium battery may have its own precise parameters and requirements. The most reliable and authoritative source of information is the official specification provided by the manufacturer. This specification can be said to be the manual of this battery. It will state the nominal voltage, charging voltage, discharge cut-off voltage, maximum continuous discharge current, peak discharge current, standard charging current, maximum charging current, recommended operating temperature range, storage voltage and conditions, size, weight, cycle life, etc.
So no matter what type of battery you use, be sure to find and read its official specifications carefully.
Conclusion
Voltage is the brain of the battery. Voltage can tell us approximately how much power is left. Different battery types have different voltage ranges and curves.
It can help you choose the right charger, protection board, which can help you set the parameters of the device, and most importantly, it can help you use and store the battery safely.
When you get a lithium battery, you must first find out what type it is (LiPo? LiFePO4?), and then be sure to find its official datasheet! The voltage information written on it is the most accurate, and the datasheet should be the final word.
