Batteries for benefits : Which is the Best Battery?

What do you look for in a good battery?

The following table shows some important characteristics of batteries. An ideal battery would satisfy all of these. However, thus far, no such miracle battery has been invented and there is always a give and take. For example, to obtain high energy density, service life has to be sacrificed. Therefore, it is important to know which battery has the characteristics that make it applicable for use in a certain electrical appliance.

Parameter

What is it?

Unit

In Batteries

Voltage & Resistance

Electromotive force generated by the potential difference between the anode and the cathode.

Theoretical voltage of a cell:

Eo cathode - Eo anode = Eo cell

The higher the voltage of a cell, the more powerful the battery.

Volts (V)

Let us model the battery using Tevenin's equivalent.

Source: http://sub.allaboutcircuits.com/images/00236.png

Voltage is a force. When current flows through a resistance, some of this force is used to overcome the resistance and hence voltage drops. Every cell contains some internal resistance Rthevenin.

Voltage drop can be calculated using the formula

V = IR, where:

V is voltage in V

I is current in A

R is resistance in Ω

Knowing the amount of current flowing and the internal resistance of the cell would enable you to calculate the voltage drop across the cell. The lower the internal resistance, the lower the voltage drop (or the wasted voltage) and the better the battery.

Discharge Curve

A battery is in the process of being discharged when it is connected to a circuit and current flows through a load. This causes the voltage of the battery to drop with time as the capacity of the battery (stored chemical energy) is used up.

-

An ideal battery would have an almost flat discharge curve, meaning that the voltage remains constant until the capacity of the battery is almost entirely used up, where it will taper off.

Source:http://www.ae5x.com/blog/wp-content/uploads/2009/07/compare-smaller.gif

Battery with red discharge curve is most ideal - curve is flattest and longest.

Battery with blue discharge curve is least ideal - curve is steep and short.

Capacity

The capacity of a battery is a measurement of how much stored chemical energy there is within the battery.

It is defined as how long a battery sustain a certain current before it is completely discharged:

Capacity = Power x time

Power = VI

Therefore, Energy = VIT

Ampere-hr (Ah)

or

Watt-hr (Wh)

The higher the capacity of a battery, the better the battery.

However, in real-life, every battery has a minimum discharge rate. When this maximum discharge rate is exceeded, the capacity of the battery will be reduced.

For example, a battery has:

Capacity: 1000mAh (milliampere-hour)

Maximum discharge: 250mA

Time sustained at 250mA

= 1000mAh/250mA

= 4hr

BUT.

Time sustained at 500mA is less than 1000mAh/500mA = 2hr, because the cell is already overworked. So you get less work done out of the battery.

Energy Density

Energy density is the amount of energy contained within a certain unit volume or mass of the battery.

For the same amount of energy stored, a battery with higher energy density is more compact and requires less raw materials.

Watt-h per g (Wh/g)

Or

Watt-h per cm3

(Wh/cm3)

The higher the energy density of the battery, the better the battery.

Assuming these two batteries provide the same power:

Low energy density:

Source: http://img.diytrade.com/cdimg/748276/7846985/0/1231745788/sealed_lead_acid_battery_12V65Ah.jpg

High energy density:



Source: http://www.clker.com/cliparts/0/b/9/b/1194989707579933886battery_01.svg.hi.png

Temperature Dependence

The temperature of the battery affects its performance.

At low temperatures, the electrolyte might freeze, impeding the movement of ions, whereas at high temperatures, chemicals might decompose or have enough energy to undergo other unwanted chemical reactions.

A good battery should be resistant to temperature changes, particularly at low temperatures.

Why low temperatures?

Generally, moderately high temperatures are preferable to moderately low temperatures.

From the graph, the voltage of batteries at higher temperature are higher than lower temperatures. The voltage drop is also slower.

Source: http://www.doitpoms.ac.uk/tlplib/batteries/battery_characteristics.php

Service Life/Shelf life

Service life only applies to rechargeable batteries. It is defined as the number of recharging cycles before the its capacity drops to less than 80%.

Shelf life refers to how long a battery can be inactive before self-discharge (leakage) causes battery capacity to drop to less than 80%.

Unit of time

The longer the shelf/service life of a battery, the better the battery.

However, sometimes poor handling and other factors could cause the battery to die early, such as extreme temperatures, shock, short circuiting etc.

Charge/discharge cycle

Charge/discharge cycles is a general term that refers to the ease at which a battery can be charged.

(Applies only to rechargeable batteries.)

In general, a battery with good charge/discharge cycle should:

·         Require low voltage to charge

·         Require short time to charge

·         Be safe to charge/discharge

Cost

How expensive the battery is to buy and to maintain.

$

THE CHEAPER THE BETTER.

Ability to deep discharge

Ability to deep discharge refers to how much of the battery's capacity can be discharged without damaging the battery. Most batteries should not be discharged fully, which means they cannot be used to their full capacity. Constantly doing so will shorten the service life of the battery.

The greater the ability of a battery to deep discharge, the better the battery.

What are the types of chemical batteries available today?

Since the battery was first invented in 1886, batteries have revolutionalized the electrical industry. Over the years, new batteries have emerged to meet consumer and industrial needs, such as the invention of rechargeable batteries. Many old batteries have since been replaced and have fallen out of use. The following chart shows some of the most commonly used batteries today.

Comparison of Commonly Used Rechargeable Batteries

There is no single perfect battery. Every battery has its own advantages and disadvantages and their uses range from airplanes and forklifts to smartphones. Consequentially, manufacturers have to choose the most suitable battery for an electrical appliance. The following table shows the characteristics of the four most widely use rechargeable batteries today.

Type of Battery

Advantages

Disadvantages

Uses in Daily life

Nickel Cadmium

High capacity (700mAh)

Long service life (1000+ cycles)

Able to work well in low temperatures

Economical

Deep discharge

Fast charge

Low energy density

Environmentally unfriendly

When unused for long time, crystals called 'memory' form on cell plates - must be regularly maintained.

Two way radios, emergency medical equipment, power tools

Nickel Metal Hydride

High capacity (987mAh)

High energy density

Environmentally friendly

Expensive

Limited service life (200 - 300 cycles)

Shallow discharge

Slow charge

Damaged by high temperatures

Satellites, wireless communications, mobile computing

Lead Acid

Very high capacity

Long shelf life

Low maintenance and manufacture costs - durable

Low energy density

Short service life (200 - 300 cycles)

Highly toxic and environmentally unfriendly (high lead content)

Shallow discharge

Slow charging

Poor performance at low temperatures

Automobiles, forklifts (heavy duty machines)

Lithium Ion

Relatively high capacity

Highest energy density (twice of NiCd)

Low maintenance costs

Fast charge

Limited service life - prone to ageing

Need to be stored in cool temperatures

Expensive to manufacture

Unsafe - explode if overheated

Mobile devices, laptops, tablets, garden tools

Batteries for benefits

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Which is the Best Battery?

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Parameter	What is it?	Unit	In Batteries
Voltage & Resistance	Electromotive force generated by the potential difference between the anode and the cathode. Theoretical voltage of a cell: Eo cathode - Eo anode = Eo cell The higher the voltage of a cell, the more powerful the battery.	Volts (V)	Let us model the battery using Tevenin's equivalent. Source: http://sub.allaboutcircuits.com/images/00236.png Voltage is a force. When current flows through a resistance, some of this force is used to overcome the resistance and hence voltage drops. Every cell contains some internal resistance Rthevenin. Voltage drop can be calculated using the formula V = IR, where: V is voltage in V I is current in A R is resistance in Ω Knowing the amount of current flowing and the internal resistance of the cell would enable you to calculate the voltage drop across the cell. The lower the internal resistance, the lower the voltage drop (or the wasted voltage) and the better the battery.
Discharge Curve	A battery is in the process of being discharged when it is connected to a circuit and current flows through a load. This causes the voltage of the battery to drop with time as the capacity of the battery (stored chemical energy) is used up.	-	An ideal battery would have an almost flat discharge curve, meaning that the voltage remains constant until the capacity of the battery is almost entirely used up, where it will taper off. Source:http://www.ae5x.com/blog/wp-content/uploads/2009/07/compare-smaller.gif Battery with red discharge curve is most ideal - curve is flattest and longest. Battery with blue discharge curve is least ideal - curve is steep and short.
Capacity	The capacity of a battery is a measurement of how much stored chemical energy there is within the battery. It is defined as how long a battery sustain a certain current before it is completely discharged: Capacity = Power x time Power = VI Therefore, Energy = VIT	Ampere-hr (Ah) or Watt-hr (Wh)	The higher the capacity of a battery, the better the battery. However, in real-life, every battery has a minimum discharge rate. When this maximum discharge rate is exceeded, the capacity of the battery will be reduced. For example, a battery has: Capacity: 1000mAh (milliampere-hour) Maximum discharge: 250mA Time sustained at 250mA = 1000mAh/250mA = 4hr BUT. Time sustained at 500mA is less than 1000mAh/500mA = 2hr, because the cell is already overworked. So you get less work done out of the battery.
Energy Density	Energy density is the amount of energy contained within a certain unit volume or mass of the battery. For the same amount of energy stored, a battery with higher energy density is more compact and requires less raw materials.	Watt-h per g (Wh/g) Or Watt-h per cm3 (Wh/cm3)	The higher the energy density of the battery, the better the battery. Assuming these two batteries provide the same power: Low energy density: Source: http://img.diytrade.com/cdimg/748276/7846985/0/1231745788/sealed_lead_acid_battery_12V65Ah.jpg High energy density: Source: http://www.clker.com/cliparts/0/b/9/b/1194989707579933886battery_01.svg.hi.png
Temperature Dependence	The temperature of the battery affects its performance. At low temperatures, the electrolyte might freeze, impeding the movement of ions, whereas at high temperatures, chemicals might decompose or have enough energy to undergo other unwanted chemical reactions.		A good battery should be resistant to temperature changes, particularly at low temperatures. Why low temperatures? Generally, moderately high temperatures are preferable to moderately low temperatures. From the graph, the voltage of batteries at higher temperature are higher than lower temperatures. The voltage drop is also slower. Source: http://www.doitpoms.ac.uk/tlplib/batteries/battery_characteristics.php
Service Life/Shelf life	Service life only applies to rechargeable batteries. It is defined as the number of recharging cycles before the its capacity drops to less than 80%. Shelf life refers to how long a battery can be inactive before self-discharge (leakage) causes battery capacity to drop to less than 80%.	Unit of time	The longer the shelf/service life of a battery, the better the battery. However, sometimes poor handling and other factors could cause the battery to die early, such as extreme temperatures, shock, short circuiting etc.
Charge/discharge cycle	Charge/discharge cycles is a general term that refers to the ease at which a battery can be charged. (Applies only to rechargeable batteries.)		In general, a battery with good charge/discharge cycle should: · Require low voltage to charge · Require short time to charge · Be safe to charge/discharge
Cost	How expensive the battery is to buy and to maintain.	$	THE CHEAPER THE BETTER.
Ability to deep discharge	Ability to deep discharge refers to how much of the battery's capacity can be discharged without damaging the battery. Most batteries should not be discharged fully, which means they cannot be used to their full capacity. Constantly doing so will shorten the service life of the battery.		The greater the ability of a battery to deep discharge, the better the battery.

Type of Battery	Advantages	Disadvantages	Uses in Daily life
Nickel Cadmium	High capacity (700mAh) Long service life (1000+ cycles) Able to work well in low temperatures Economical Deep discharge Fast charge	Low energy density Environmentally unfriendly When unused for long time, crystals called 'memory' form on cell plates - must be regularly maintained.	Two way radios, emergency medical equipment, power tools
Nickel Metal Hydride	High capacity (987mAh) High energy density Environmentally friendly	Expensive Limited service life (200 - 300 cycles) Shallow discharge Slow charge Damaged by high temperatures	Satellites, wireless communications, mobile computing
Lead Acid	Very high capacity Long shelf life Low maintenance and manufacture costs - durable	Low energy density Short service life (200 - 300 cycles) Highly toxic and environmentally unfriendly (high lead content) Shallow discharge Slow charging Poor performance at low temperatures	Automobiles, forklifts (heavy duty machines)
Lithium Ion	Relatively high capacity Highest energy density (twice of NiCd) Low maintenance costs Fast charge	Limited service life - prone to ageing Need to be stored in cool temperatures Expensive to manufacture Unsafe - explode if overheated	Mobile devices, laptops, tablets, garden tools