Facts about 18650 battery voltage - to better use and longer battery life

1.What is the battery voltage?

To talk about the battery voltage, the potential difference between the anode and cathode of the battery must be measured. The greater the potential difference, the greater will be the voltage of the battery and vice versa.

When we further understand the electric potential, it is the difference in charge between the two poles of a cell, one with a negative charge and the other with a positive charge. 

When we discuss the voltage for battery pack, several cells are connected in series to make a battery pack and all cells have individual voltages which add up to make a nominal voltage of a battery pack i.e. 48v, 24v, 12v battery etc.

2.What is the common 18650 battery voltage?

Voltage depends on the type and chemistry of the battery, as all 18650 batteries are Li-ion batteries that have a voltage range between 3.2v to 3.7v. .

And 18650 li-ion batteries are highly advanced with a BMS system which cuts off the overcharging and discharging of the battery,therefore, the battery voltage will not be too low or too high.

3.Why does the 18650 battery voltage change during charge and discharge?

As you know the charging of the battery involves the supply of energy to the battery，and expenditure of energy is involved to discharge the battery.The voltage rises as the amount of electricity rises and falls as the amount of electricity falls.

In general, a battery will have a charge cutoff voltage and a discharge cutoff voltage, which are the upper and lower limits of the battery voltage, meaning that the voltage rises through charging from the low point to the maximum point, that is, the battery works between these two points.

Let’s know about voltage changes while charging and discharging. Consider your 18650 battery voltage is 4.2v when fully charged. When you connect a device or a bulb at this voltage point, energy is consumed by the bulb and charge is lost to some extent. The bulb is continuously on and the battery is also being discharged with the passage of time and the voltage is also decreasing due to decreasing in charge. Eventually, the bulb is dimmed when the battery is discharged till the BMS cut off the discharging. So the voltage also decreased to its lowest range.

As a lithium-ion battery, when the 18650 battery is charged, it experiences a process of constant current and constant voltage. In the previous stage of charging, the current is large and constant, and the voltage rises until the charging cut-off voltage. At this time, the voltage is constant, and the charging current slowly decreases, until the battery is fully charged.

4.Is there a relationship between 18650 battery voltage and SOC?

To understand the relationship between voltage and SOC let's use the voltage vs SOC chart in the 18650 batteries.

The voltage of lithium-ion batteries changes concerning the SOC (state of charge) of the battery which varies from 100% to 0% i.e. fully charged state to fully discharged.It is seen that a fully charged battery has maximum voltage while the drained one has the lowest range of voltage.

Although the voltage can be affected by factors like temperature and current etc. Voltage can be used to determine the state of charge of the battery. Now voltage translation is used vs the state of charge of the battery for an explanation.

Here the voltage is maximum at 100% and lowest at 0%. And voltage changes on both ends of the state of charge.

5.What voltage is too low for the 18650 battery?

LiFePo4 18650 batteries with a maximum voltage of 3.65V and a minimum voltage of 2v. If the voltage is below this range, it can adversely affect its health.

These types of batteries are provided with a BMS system which prevents the battery to discharge below the lowest value of the voltage. It turns the sleep mode on to make the battery safer.

6.How to measure 18650 battery voltage?

• First disconnect the 18650 battery from the circuit.
• Set the dial of the multimeter at a higher range than the specific voltage of the cell.ie 4.2v for the 3.7v cell.
• Connect the red probe to the positive terminal of the cell and the black probe to the negative terminal of the cell.
• Take the reading on the Led display if the reading is higher than are same to a maximum value then the battery is fit.
• If the battery is showing a reading below than specific value then the battery is overdischarged or faulty and needs to be charged or repair

7.Can I charge 18650 with a 5V charger? What is the best charge voltage for a 18650 battery?

You must notice the compatibility of both the battery and charger. The voltage range of the charger must be within the voltage range of the battery. A charger must be provided with a safety that would not allow the voltage to be exceeded the cut off voltage of the battery.So you must see the specification of the battery and charge the batteries with the appropriate charging current to their recommended value of voltage with their compatible charger.

Be careful of the cell chemistry.  Different types of batteries need to use different chargers due to different charging modes, which is determined by their chemical properties. Therefore, as a lithium-ion battery, the 18650 battery must be charged with a dedicated CC-CV charger.

Some 18650 batteries can be charged to 4.2v but not all.18650 lfp battery cannot bear a charge of more than 3.65V and NMC 18650 can be charged to 4.2v. Therefore it is necessary to get information from the manufacturer.

8.What happens if the 18650 battery voltage exceeds 4.2v?

Although the 18650 li-ion batteries are protected with auto shut to prevent overcharging or discharging.

Even so, if you use the battery in the wrong way to charge more than 4.2 volts, it may has safety hazard.

Make sure the specific charger is of good quality and is designed not to exceed the rated voltage. A poorly built battery can show uncertain results on overcharging it. So keep your batteries on notice when are being charged and do not let them exceed 4.2v.

9.How to prevent the 18650 battery voltage from exceeding the normal range?

Your battery exceeds the spike of voltage, now it’s time to think about your battery safety.

When you are using 18650 rechargeable battery, a safety management system is required to ensure your battery’s safety which otherwise can get premature worn out.

This shield is provided with a cut-off system which saves the battery from exceeding the normal value of the voltage

10.What factors affect the voltage of the 18650 battery?

The voltage is affected by some parameters:

• The chemistry of the cell and chemical reactions involved in the battery system determines the voltage of the battery

• The concentration of reactants in the battery increases the forward reactions which increase voltage but also cause voltage drop due to the rise of resistance.

• The increased temperature also increases resistance in the circuit so voltage decreased. While the current increases with increased temperature.

The surface area of electrodes also matters in an increase of voltage.

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