A Best Guide on Lithium Polymer Battery Voltage

Lithium polymer batteries are also known as Li-Po batteries and are widely used in various industries. You should understand lithium-polymer battery voltage, as these cells operate within specified voltage ranges during charge and discharge. 

In this guide, we will discuss in detail the Li-Po battery voltages. It will include details on nominal voltage values, cut-off values, storage levels, and how factors such as load and temperature affect the voltage rating.

Part 1: An Overview of Lithium Polymer Battery Voltage

The rated voltage of a single Li-polymer cell is set to 3.7 V, which is close to its midpoint during discharge. In real time, the operating voltage of the Li-Po cell ranges from 3V to 4.2V (empty to fully charged) under normal operating conditions.

You need to understand the difference between the mentioned/labelled rated voltage and the actual operating voltage range. Let’s understand this with an example. Most of the 3S battery packs are labelled as 11.1 V (3 x 3.7V) nominal voltage, whereas the real cut-off and fully charged voltage range from 9 V to 12.6 V.

Part 2: What Are the Voltage Types of Li-Po Batteries?

The voltage of the lithium polymer cells is classified into various types.

• Nominal Voltage – It is the midpoint voltage of the cell, which is approximately 3.7V per cell for Li-Po batteries. If a 4S battery pack is labelled 14.8 V, you should know that the actual voltage can be higher (fully charged) or lower (empty).
• Resting Voltage – It is the maximum voltage of a cell when fully charged. For a standard Li-Po cell, the resting/open-circuit voltage value is 4.2V per cell. I recommend not to exceed this value unless the cell is rated for higher voltage, such as LiHV 4.35V cells.
• Cut-off Voltage – It is classified as the minimum safe level voltage of a cell. For a Li-Po cell, the cut-off voltage is 3 V. Most battery management systems and electronic speed controllers cut off at this voltage to avoid damage. To improve performance, most users set a slightly higher cut-off limit, such as 3.2 V or 3.3 V.
• Storage Voltage – It represents the recommended voltage for a cell (40 to 60% charged) for long-term storage. It is often set near the nominal/mid-level voltage, around 3.8 V for a Li-Po cell, to avoid battery aging.
• Operating Voltage Range – For a lithium-polymer battery, the operating/rated voltage ranges from 3 V (drained) to 4.2 V (fully charged). During operation, the voltage stays flat from 3.5 to 3.8V and drops drastically at the end (near discharge), which is also known as the voltage knee.

Part 3: Safe Operating Window for the Li-Po Batteries

The safe operating window (SOW), which is also known as the integrity operating window (IOW), for a lithium polymer cell ranges from 3 V to 4.2 V. Keeping your cell within this operating window prevents damage, overheating, cell failure, and improves service life. In a multi-cell series pack, the individual cell voltage should stay between these limits. 

For example, the SOW for a 4S battery pack is 12.6 V (4 x 4.2 V) at full charge and 9 V (4 x 3 V) at cut-off. It is recommended to use a reliable BMS or ESC system that cuts power when any cell hits its minimum or maximum limit.

Part 4: Why the Last 10 to 20% Takes So Long – Understanding CC-CV Charging

The lithium polymer batteries use a CC-CV (constant current/constant voltage) profile during charging. Here is how it works:

• At first, the charger supplier maintains constant current (CC phase) until the Li-Po cell reaches its resting/peak voltage of 4.2 V per cell. 
• After the CC-phase, the CV-phase begins, during which the charger maintains a 4.2 V voltage and gradually reduces the current.

The cell gains its last 10 to 20% during the CV phase, which takes time due to low current supply as it drops to nearly zero. 

For example, a 1000 mAh battery may take approximately 1.5 hours to reach 80% during CC mode. But the last 20% may take around 1.5 hours due to the CV-phase, which is equal to the total charging time in the CC-phase. The total charging time for the battery will be 3 hours (1.5 hours CC and 1.5 hours CV).

Part 5: Temperature, Internal Resistance, and Aging Effects on Voltage Behavior

The voltage behavior of a battery pack is affected by various factors that include temperature, internal resistance, and Cell Aging.

Temperature Effect

The temperature variation affects the voltage behavior of a cell. Let’s check how a cell performs at low and high temperatures.

• At low temperatures, the Li-polymer battery may reach its cut-off voltage before it’s entirely empty.
• At high temperatures, the voltage of the Li-Po battery may increase slightly, which also increases the risk of overheating and degradation.

It is recommended to operate the Li-Po cells between 20 °C and 40 °C.

Internal Resistance (IR) Effect

The IR of the Li-Po cells is inversely proportional to the performance. Cold or aged cells have high internal resistance that causes deeper voltage sags under continuous load.

Aging Effect

The capacity of the Li-Po cell drops as it ages. It causes shorter run cycles, less stable voltage curves, and lower voltage under load. You might see an older battery pack hitting the voltage knee much earlier than a new one, under the same operating conditions.

Part 6: How to Measure the Lithium Polymer Battery Voltage Properly?

You can use a voltmeter to measure the voltage of the Li-polymer battery. For multi-cell battery packs, use a dedicated voltage checker or a balance-port reader that shows the voltage rating for each cell as well as for the whole pack. A multimeter can also be used to measure the voltage at the main terminals.

A Tip – For accurate readings, measure the voltage once the battery has rested for 1 or 2 minutes. 

Part 7: Designing the Voltage Thresholds in Your BMS or Firmware

The role of the BMS or battery control firmware is to ensure the safe voltage limits. The system should stop charging when the cell reaches 4.2 V and cut off discharge when the cell’s voltage decreases to 3 V. For safety, some BMS use 3.3 V as a cut-off voltage. Also, ensure cell balancing in multi-cell packs such as 3S, 4S, and 6S.

Temperature Gating

Charging must be carried out within the recommended temperature range of 0 to 45 °C for Li-Po cells. A well-designed BMS or firmware continuously monitors temperature and prevents charging the cell outside safe operating ranges.

These thresholds help you overcome issues such as overcharging, thermal stress, and deep discharge.

Part 8: Li-Po High-Voltage Variants and Their Applications

In addition to standard Li-Po cells, HV Li-Po cells are designed to be charged to a higher voltage. The high-voltage lithium polymer has a resting/peak voltage of 4.35 V and a nominal voltage of around 3.8 V. The HV variants offer 10% extra capacity, allowing each cell to store more energy than standard variants.

Applications

HV variants of Li-Po cells are mainly used in high-energy-drain applications such as RC devices, drones, power tools, flashlights, and more. 

Part 9: Compliance, Standards & Safety Considerations for Li-Po Batteries

According to the UN 3480 Class 9, the lithium polymer batteries are classified as dangerous goods for airshipping. The standard requires the batteries to pass the UN38.3 safety test before shipping. The Li-Po battery undergoes altitude simulation, thermal exposure, and vibration/shock testing under this test. 

Most manufacturers also provide MSDS reports for their battery packs that detail the hazards. For example, an MSDS report contains details for an electrolyte that is flammable and can cause severe burns or respiratory issues. 

You should follow the manufacturer’s guidelines for proper handling. It includes using BMS for protection, using a correct charger, maintaining the safe charge limit, and avoiding puncturing/shorting the cells. Following these standards and safety guidelines ensures safe usage and legal compliance with international standards.

Part 10: An Engineering Checklist for Li-Polymer Battery Voltage

• Always use a suitable and reliable BMS or firmware system to ensure voltage limits. 
• Charge your Li-Po cells with a CC-CV lithium charger that meets your required voltage (4.2 V/cell for standard battery packs and 4.35 V/cell for HV battery packs).
• If you need to store or ship the battery, make sure that it is at the recommended storage level (3.8 V/cell) and passes the UN38.3 safety test. Don’t leave your lithium polymer batteries unused for an extended period of time.

Conclusion

For better design and usage, it is important to understand the lithium polymer battery voltage. You can improve the performance and lifespan of your Li-Po cell by following CC-CV charging practices, keeping the cell within a 3-4.2 V per-cell window, and using a smart BMS system.

Don’t forget that a well-maintained Li-Po battery offers continuous power and high ROI. You can also reach out to VTCBATT for further assistance or top-notch lithium polymer battery solutions.

Part 11: Frequently Asked Questions

1. Can I charge a Li-Po battery to 4.25 V per cell?

No. A standard Li-Po cell has the rated resting voltage value of 4.2 V per cell. Charging beyond this level can cause cell swelling, overheating, and reduced life span. If your project requires a cell with a 4.25 V charging limit, then consider using HV Li-Po cells. They can handle charging up to 4.35 V per cell.

2. Why does the voltage drop so quickly near the end of a Li-Po cell discharge?

Before going completely empty, the battery hits the “voltage knee.” That’s why you see a sudden drop in the voltage near the end of a Li-Po cell discharge.

3. What is the recommended voltage to store a Li-Po battery?

 The recommended storage voltage limit for a lithium polymer battery cell is 3.8 V. It is close to the mid-level voltage, and storing it at this level reduces chemical aging. 

4. What is li po battery low voltage limit?

   Normally, the li po battery is suggested for low voltage limits at 3.0V, which not only can avoid over-discharge, but also extend the battery cycle life. But there are some situations where the low voltage limit is 2.75V.  

1.  If it is a normal li-po battery cell without PCB, the low voltage limit is 2.7V, which means that the battery is completely drained. Due to PCB power consumption, li-po battery with PCB normally set at 3.0V,instead of 2.75V.
2. Some ICs, such as DW01 from Taiwan Fortune Semiconductor, can support a 2.75V discharge cut-off for li-po battery, but mostly Seiko IC S8261DAI with 3.0V low voltage limits are more widely used.

5.What is li po battery voltage range?

A: The li po battery voltage ranges from 3.0V to 4.2V. 3.0V is the discharge cut-off voltage. 4.2V is the fully charged voltage. Li po battery nominal voltage is 3.7V. But there is a stable voltage platform for discharge around 3.7V-3.8V and charge around 4.1V-4.2V, where the li po battery voltage decreases or increases very slowly.

6.What is li-po battery charge voltage?

A: Normally, the li-po battery charge voltage is 4.2V, where the li-po battery is fully charged.

Below is different li-po battery pack charge voltage table:

7.Is there voltage and current protection for li-po battery chip​?

A: Yes. li-po battery PCB is integrated with a voltage protection chip and a current protection MOSFET. Normally, Seiko IC  S8261DAI and DP8205 MOSFET are widely used in li-po battery.

Seiko IC  S8261DAI can protect the li-po battery from over-discharge at 3.0V and fully charge at 4.2V.

While, single DP8205 MOSFET can support 3-5A, for high current protection, multiple MOSFETs will be used to protect the battery from overcurrent.

8.How long do Lithium polymer batteries take to charge?

A: The charge time depends on the capacity of the lithium polymer batteries and the charging current. Typically, 0.1C,  0.2C, 0.5C,and 1C rated charge currents are used to charge lithium polymer batteries.C means the battery capacity.

Below is the charge time table for different rates of charge time.

9.How do I know if my lithium polymer battery is fully charged?

For an external lithium polymer battery charger or to charge the lithium polymer battery in the products,  normally, there will be a LED indicator light or voltage indicator. When the light turns from red to green, it means that the battery is fully charged. Although the chargers have the same CC-CV charge phase, there are slight differences in the ending current, 0.01C or 0.02C. The smaller the ending current, the fuller the battery is charged.
 After long-term usage, the li-po battery will be aged and take more time to charge.

Use the multimeter to test the battery voltage to identify. If the battery voltage is above 4.10V, it means that the li-po battery is almost fully charged. Even though the fully charged voltage is 4.2V, the battery voltage will drop slightly after lay aside.

10.How many times can you charge a lithium polymer battery?

A: The lithium polymer battery can be charged and discharged for 1000 cycles.

Even after 500 times, the battery can still keep 80% of its original capacity.

11. What is the maximum safe voltage for a Li-Po battery?

A: The maximum safe voltage for a li-po battery is 4.2V fully charged voltage.

Don’t suggest to charge above 4.2V, which will cause the battery to overcharge in risk of heat and explosion.But normally,the protection IC will prevent the overcharge.

12.What is the minimum safe voltage for a Li-Po cell during discharge?

A:The minimum safe voltage of a LiPo cell is 2.75V during discharge.

Don’t suggest discharging to a lower voltage below 2.75V, which will cause the battery to be overdischarged in risk of fire. Furthermore, the li-po cell doesn’t provide with protection IC to prevent over-discharge; it is dangerous to discharge below 2.75V.

13.How does Li-Po battery voltage change during discharge?

A:The voltage change depends on the discharge rate. If it is a high rate of 1C,2C, or 3C, the voltage will drop directly.  For a small current discharge rate, the LiPo battery will drop gradually from 4.2V to 3.8V. When reaching 3.7V to 3.8V, there will be a stable platform for the voltage drop. After 3.7V, the voltage will drop quickly.

14.Why does Li-Po battery voltage drop under load?

A:There are internal resistance of the Li-Po battery and the load.When the current flows through the battery and load, a voltage drop occurs across these resistances.

If the load is high power consumption,the voltage will drop fast.

15.How does temperature affect Li-Po battery voltage?

A: The temperature affects the li-po battery voltage greatly.At 20℃ temperature, the lipo batteries can perform the best in delivering maximum capacity, stable voltage output, and efficient charge&discharge cycles.

Low temperature will make the chemical reaction slow with a lower operating voltage. High temperature will cause the LiPo battery to age with a higher operating voltage.

16.What is the resting voltage of a fully charged Li-Po cell?

A: After disconnecting the fully charged Li-Po cells, their voltage will drop slightly below 4.2V.

After 1-2 hours resting, the Li-Po cells’ voltage will remain around 4.16V. Due to the self-discharge rate,the longer the cell resting, the voltage will drop gradually.Thus,it is suggested to keep the Li-Po cells and batteries charged per 6 months to avoid over-discharge.

17.What voltage should a Li-Po battery be stored at?

A:The Li-Po battery should be stored at around 3.7V-3.8V, about 50%-60% SOC. During this storage,the battery discharge rate is low and loses the least capacity.

18.What is the voltage per cell for a 2S/3S/4S Li-Po battery pack?

A: For 2S/3S/4S Li-Po battery pack,the nominal voltage per cell is 3.7V.The fully charged voltage is 4.2V per cell.The fully discharged voltage is 3.0V per cell.

19.Why does my Li-Po battery show different voltages per cell?

A: After long-time usage,it is common that the Li-Po battery shows different voltages per cell. Because each cell self-discharge rate is different,which cause the cell imbalance.It is better to use cell balance BMS and cell balance charger to average each cell voltage.Cell imbalance may cause the battery run time shorter, quick charge and capacity lose,which affects the battery performance greatly.

20.What happens if a Li-Po battery is over-discharged below the cutoff voltage?

A:Never over-discharge the lipo battery below the cutoff voltage 3.0V per cell ! It will cause irreparable damage and risks of fire and explosion.

21.What happens if a Li-Po battery is overcharged above the max voltage?

  A: If a Li-Po battery is overcharged above the max voltage 4.2V per cell,it will cause the safety risk of fire, explosion and heat.This will make the Li-Po battery damaged permanently.

22.Why does Li-Po voltage bounce back after resting?

A:It’s normal for the Li-Po battery voltage bounce back after resting,which is caused by the chemical reaction.When the battery is discharging,the lithium ions moves from the anode to the cathode and placed unevenly.

After disconnect the load,the lithium ions will be relaxed and placed evenl,which cause the battery voltage naturally recovers and bounced.

23.How do you measure Li-Po battery voltage correctly?

A:First,set the multimer to DC voltage higher than the battery fully charged voltage.

    It’s a common mistake for people to set the AC voltage. Note the symbol difference.

    Second, use the multimeter to connect the + and – of the Li-Po battery. 

    Then the multimeter LCD will show the battery voltage.

24. What causes sudden voltage drops in a Li-Po battery?

A: It may be caused by many factors, such as high discharge current of the load, a low-temperature environment, and battery status. The heavy load will drain the current rapidly, which causes the voltage drop fast. The low temperature will make the battery reaction slow, which causes the voltage drop.

The battery cell imbalance will cause some cells to stop discharging suddenly. When the battery is close to end of discharge, the battery voltage will drop immediately.

25. What is the difference between nominal voltage and full-charge voltage in Li-Po batteries?

A:The Li-po battery’s nominal volate is 3.7V and fully charged voltage is 4.2V.

   Nominal voltage is the average value for operating voltage, capacity calculations and specifications.

    Fully charge voltage is the maximum voltage that the Li-Po battery can reached within safety limits for charging.

26.Can Li-Po voltage be used to estimate remaining battery capacity?

A: Yes.Li-Po voltage can be used to estimate the battery capacity.Below is the battery voltage and capacity table.

    Li-Po Battery Voltage VS Capacity

27.Why does a Li-Po battery swell when voltage drops too low?

  A:When the voltage drop too low,it will cause the Li-Po battery over-discharged,making the battery chemicals and physical structures are seriously damaged inside to swell.

28.What voltage imbalance between cells is considered unsafe?

A:For li-po battery pack should be in great cell consistency in voltage,capacity ,and internal resistance. If each cell voltage is more than 50mV,it will be unsafe to use the battery pack  anymore.

During imbalance,it will cause some cells over-discharged while some other cells are still not fully charged.You need to use balance charger to average the cell voltage or replace the battery pack directly.