**Cell Chemistries - How Batteries Work**

CellsEnergy cells are the smallest individual electrochemical unit which delivers a voltage which depends on the cell chemistry. Examples are cylindrical alkaline cells used in toys and small electronic devices.

They may be

**primary (single use) cells**or they may be

**secondary (rechargeable) cells**.

Strictly speaking, a cell should not be called a battery since a battery is a group of cells but many people (including me at times) use the word "battery" to describe any electrochemical energy source, even if it is a single cell, and this can lead to confusion.

Energy cells provide a DC or Direct Current (unidirectional) source of electricity.

**Batteries**

Batteries and Battery Packs are made up from groups of cells, sometimes designed into a single block as in 12 Volt automotive batteries which are made up from six 2 Volt cells connected in series and integrated into a single unit. Or they may be individual cells wired together in a separate case.

**Cell Voltage**

The cell voltage depends on the combination of active chemicals used in the cell. For commonly available cells the voltage can range from 1.2 Volts for Nickel based cells to over 3 Volts for Lithium based cells.

**Battery Current**

The actual current delivered by the cell or battery at any particular instant depends on the load.

Ignoring the effect of the battery's internal resistance, the current drawn by the load is given by

**I = E ÷ R**

Where

**I**is the current (Amps),

**E**is the battery or cell voltage (Volts) and

**R**is the load resistance (Ohms). This relationship is known as

**Ohm's Law**

Thus for a 2 Volt battery supplying a 2 Ohm load, the current will be 1 Amp.

The

**C rate**is a measure of the battery's current handling capability. It is NOT the maximum current which the battery can deliver which may be specified by the manufacturer as several times the `c rate. It is the constant current charge or discharge rate which the battery can sustain for one hour. Thus a 12 Volt, 20 AmpHour battery should be able to deliver 20 Amps for 1 hour or 2 Amps for 10 hours. If the battery is dischaged at the 10 C rate, it will be completly discharged in 6 minutes.

**Internal Impedance**

The chemicals and current carrying conductors in practical batteries have a small internal resistance which impedes the current flow through the battery. In the diagram below this is shown as resistance

**r**between the battery terminals.

With no load resistance

and there will be a voltage drop across the internal resistance. Also known as the Ohmic loss, the voltage drop

The consequence is that the available voltage at the battery terminals is reduced to

Thus for a 2 Volt battery with an internal resistance of 100 milliOhms feeding a load of 2 Ohms, the operating voltage at the battery terminals will be only 1.9 Volts and the current through the load will be 0.95 Amps.

It appears as if a small voltage

For most batteries the internal resistance

Charge, Energy and Power

Battery PowerThe power which a cell or battery can deliver is normally specified as the power associated with drawing current at the C rate. The actual power delivered however depends on the load resistance as above and is given by:

Where

Thus for a 20 AmpHour 12 Volt battery the power provided is given by;

The power dissipated in the load appears as heat and is given by:

This equation also represents the process known as

Battery Capacity and Energy Content (They are not the same)

**R**on the battery, the open circuit voltage at the battery terminals will be**E**Volts. In this case however, when a load resistance R is connected across the battery, the current flowing will be**I = E ÷ (R + r)**and there will be a voltage drop across the internal resistance. Also known as the Ohmic loss, the voltage drop

**e**is given by**e = I r = E r ÷ (R + r)**The consequence is that the available voltage at the battery terminals is reduced to

**(E - e) = E - ( E r ÷ (R + r)) or E (1- r / (R+r))**Thus for a 2 Volt battery with an internal resistance of 100 milliOhms feeding a load of 2 Ohms, the operating voltage at the battery terminals will be only 1.9 Volts and the current through the load will be 0.95 Amps.

It appears as if a small voltage

**e**is being applied inside the battery in the opposite direction to the battery voltage. Note that**e**is dependent on the magnitude of the current flowing.For most batteries the internal resistance

**r**is very small, only a few milliOhms, so its effect can be neglected, but for high power batteries the effect of internal resistance can be quite significant causing the battery to heat up due to Joule heating (see below) as well as an equivalent reduction in available power. See more about the effects of internal impedance.Charge, Energy and Power

**Charge:**The unit of Electric Charge is the Coulomb. One Coulomb is equal to the charge transferred by a current of one ampere in one second.**Energy and Work Done:**Energy is the capacity to do Work. Energy and Work Done are both measued in Joules or WattHours.1 Joule = 1 WattSecond. See also Glossary (Joule) Energy purchased from the electricity utility, (in this case Alternating Current or AC) is commonly measured in "Units" where 1 Unit = 1 KiloWattHour or 1000 WattHours.**Power:**Power is the Rate of doing Work. It is measured in Watts. 1 Watt = 1 Joule per Second.Battery PowerThe power which a cell or battery can deliver is normally specified as the power associated with drawing current at the C rate. The actual power delivered however depends on the load resistance as above and is given by:

**P = E**X**I**Where

**P**is the power delivered (Watts)Thus for a 20 AmpHour 12 Volt battery the power provided is given by;

**20 Amps X 12 Volts = 240 Watts**The power dissipated in the load appears as heat and is given by:

**P= I2R**This equation also represents the process known as

**Joule Heating**Battery Capacity and Energy Content (They are not the same)

**Battery Capacity (C Rate)**As noted above, the cell or battery current handling capacity is normally specified in AmpHours or MilliampHours and represents the current in Amps or Milliamps which can be sustained by the battery for one hour. This is known as the

**"C" Rate**of the battery, but this measure of charge capacity is confusingly used as an indication of the battery's energy storage capacity, without taking account of the cell or battery voltage.

The rate at which charge is transferred into or out of a cell or battery is simply the current

**I.**

The amount of charge transferred by the current is measured in Coulombs and is given by

**Q = I**X

**t**

Where

**Q**is the quantity of charge transferred and

**t**is the time in seconds that the current flows.

The quantity of charge in a fully charged cell, its Coulomb capacity, is therefore given by the AmpHour capacity multiplied by 3600, (the number of seconds in an hour)

**no matter what the battery voltage is**. Thus a fully charged 20 AmpHour capacity battery contains, or can deliver a charge of:

**20 AmpHours X 3600 Seconds = 72,000 Coulombs**

AmpHours and Coulombs are thus equivalent measures of a battery's charge capacity.

The actual current which flows into the load depends on the battery voltage and this is different for different cell chemistries as shown in the following table. Note that although all the batteries may contain the same amount of charge, when connected to a similar load (2 Ohms in this example) the higher the cell voltage, the more current which flows and the quicker the battery is discharged.