Battery and Energy Technologies

Battery Beginners Page

This page provides some basic information to help students with their physics homework as well as to satisfy the curiosity of casual non-technical visitors who have stumbled upon this site by mistake..

Links are provided to pages where more detailed information can be found.

Cells

Energy 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 use "battery" for any electrochemical energy source which can lead to confusion.

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

The C rate is a measure of the battery's current handling capability. It is the constant current charge or discharge rate which the battery can sustain for one hour. The charging C rate is normally less than the discharge C rate.

Battery Power

The 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

Cell or battery 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.

Thus a 12 Volt, 20 AmpHour battery should be able to deliver 20 Amps for 1 hour.

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 battery 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.

• Battery Energy Content

The AmpHour or Coulomb capacity is not a measure of the energy content of the battery. The energy stored in a cell or battery also depends on the voltage and is specified in WattHours or MilliwattHours.

To get the cell or battery energy content, multiply the AmpHour rate by the cell or battery voltage to obtain the WattHours. In the above example the energy in a 12 Volt, 20 Ah battery is given by:

20 AmpHours X 12 Volts = 240 WattHours

When choosing batteries for battery powered applications, the key requirements are the amount of energy which needs to be stored to supply the application and the voltage and current at which it is delivered. The energy content however depends on the battery voltage and this is different for different cell chemistries. Comparing batteries by their AmpHour capacity can lead to misleading conclusions since they may all have the same AmpHour capacity but the energy content may be different as shown in the table below.

• How is it that batteries can contain the same charge in Coulombs, or the same AmpHour capacity, but store different amounts of energy?

 

The answer is that in the higher voltage battery, the charge is stored at a higher potential.

Compare the example with two identical canisters of water, both containing the same quantity of water, but one contains water at atmospheric pressure and the other contains water under high pressure.

 

Series Connections

When a battery is constructed from group of cells connected in series, the battery voltage is the sum of the voltages of the individual cells, but the AmpHour capacity is the same for the chain since the same current passes through all of the cells.

Thus a battery constructed from 10 X 3 Volt X 20 AmpHour cells will have a battery voltage of 30 Volts and an AmpHour capacity of 20 AmpHours. It will have a true capacity of 600 WattHours of energy and will be able to deliver 600 Watts of power.

Note that a single 3.6 Volt 800 mAh Lithium cell stores he same energy (2.88 Wh) as three 1.2 Volt 800mAh NiCads or Nickel Metal Hydride cells.

Parallel Connections

When the same 10 cells are connected in parallel, the battery voltage is the same as the voltage of the single cells, but the Amphour capacity will be the sum of the AmpHour capacity of the cells since the current through the load is the sum of all the currents through the individual cells.

Thus the battery will have a voltage of 3 Volts and an AmpHour capacity of 200 AmpHours. It will still have a true capacity of 600 WattHours of energy and will be able to deliver 600 Watts of power as in the case above.

Looking at this another way; each cell has an energy storage capacity of 60 WattHours. The 10 cells will have a combined capacity of 600 WattHours no matter how they are connected. Similarly the available power will always be 600 Watts. The power is given by the current multiplied by the voltage.

A series configuration thus provides a high voltage but low current and a parallel configuration gives a high current but at low voltage.

Batteries for any current and voltage rating can be made up from combinations of series and parallel connections of small cells.

BUT. Batteries should not be made up from mixed cells. Do not mix cells of different ages, different sizes, different voltages, different chemistries, different capacities, different shapes or different manufacturers.

Mixing cell types within a battery can lead to some cells being overloaded leading to early failure and this could be dangerous.

See also Battery Safety Instructions