Battery characteristics

The following battery characteristics must be taken into consideration when selecting a battery:

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1) Type

See primary and secondary batteries page.

 

2) Voltage

The theoretical standard cell voltage can be determined from the electrochemical series using Eo values:

Eo (cathodic) &#; Eo (anodic) = Eo (cell)

This is the standard theoretical voltage. The theoretical cell voltage is modified by the Nernst equation, which takes into account the non-standard state of the reacting component. The Nernstian potential will change with time either because of use or self-discharge by which the activity (or concentration) of the electro-active component in the cell is modified. Thus the nominal voltage is determined by the cell chemistry at any given point of time.

The actual voltage produce will always be lower than the theoretical voltage due to polarisation and the resistance losses  (IR drop) of the battery and is dependent upon the load current and the internal impedance of the cell. These factors are dependent upon electrode kinetics and thus vary with temperature, state of charge, and with the age of the cell. The actual voltage appearing at the terminal needs to be sufficient for the intended application.

Typical values of voltage range from 1.2 V for a Ni/Cd battery to 3.7 V for a Li/ion battery.

The following graph shows the difference between the theoretical and actual voltages for various battery systems:

 

3) Discharge Curve

The discharge curve is a plot of voltage against percentage of capacity discharged. A flat discharge curve is desirable as this means that the voltage remains constant as the battery is used up.

 

4) Capacity

The theoretical capacity of a battery is the quantity of electricity involved in the electro-chemical reaction. It is denoted Q and is given by:

$$Q = xnF$$

where x = number of moles of reaction, n = number of electrons transferred per mole of reaction and F = Faraday's constant

The capacity is usually given in terms of mass, not the number of moles:

\[Q = {{nF} \over {{M_r}}}\]

where Mr = Molecular Mass. This gives the capacity in units of Ampere-hours per gram (Ah/g).

In practice, the full battery capacity could never be realised, as there is a significant weight contribution from non-reactive components such as binders & conducting particles, separators & electrolytes and current collectors & substrates as well as packaging. Typical values range from 0.26 Ah/g for Pb to 26.59 Ah/g for H2.

 

5) Energy density

The energy density is the energy that can be derived peer unit volume of the weight of the cell.

 

6) Specific energy density

The specific energy density is the energy that can be derived per unit weight of the cell (or sometimes per unit weight of the active electrode material). It is the product of the specific capacity and the operating voltage in one full discharge cycle. Both the current and the voltage may vary within a discharge cycle and thus the specific energy derived is calculated by integrating the product of current and voltage over time. The discharge time is related to the maximum and minimum voltage threshold and is dependent upon the state of availability of the active materials and/or the avoidance of an irreversible state for a rechargeable battery.

 

7) Power density

The power density is the power that can be derived per unit weight of the cell (W/kg).

 

8) Temperature dependence

The rate of the reaction in the cell will be temperature dependant according to theories of kinetics. The internal resistance also varies with temperature; low temperatures give higher internal resistance. At very low temperatures the electrolyte may freeze giving a lower voltage as ion movement is impeded. At very high temperatures the chemicals may decompose, or there may be enough energy available to activate unwanted, reversible reactions, reducing the capacity.
The rate of decrease of voltage with increasing discharge will also be higher at lower temperatures, as will the capacity- this is illustrated by the following graph:

Contact us to discuss your requirements of ni-cd battery pack. Our experienced sales team can help you identify the options that best suit your needs.

 

9) Service life

The battery cycle life for a rechargeable battery is defined as the number of charge/recharge cycles a secondary battery can perform before its capacity falls to 80% of what it originally was. This is typically between 500 and cycles.

The battery shelf life is the time a battery can be stored inactive before its capacity falls to 80%. The reduction in capacity with time is caused by the depletion of the active materials by undesired reactions within the cell.

Batteries can also be subjected to premature death by:

• Over-charging
  
• Over-discharging
  
• Short circuiting
  
• Drawing more current than it was designed to produce
  
• Subjecting to extreme temperatures
  
• Subjecting to physical shock or vibrations
  

Voltage Delay

Battery Death due to Aging

 

10) Physical requirements

This includes the geometry of the cell, its size, weight and shape and the location of the terminals.

 

11) Charge/Discharge cycle

There are many aspects of the cycle that need consideration, such as:

• Voltage necessary to charge
  
• Time necessary to charge
  
• Availability of charging source
  
• Potential safety hazards during charge/discharge

 

12) Cycle life

The cycle life of a rechargeable battery is the number of discharge/charge cycles it can undergo before its capacity falls to 80%.

 

13) Cost

This includes the initial cost of the battery itself as well as the cost of charging and maintaining the battery.

 

14) Ability to deep discharge

There is a logarithmic relationship between the depth of discharge and the life of a battery, thus the life of a battery can be significantly increased if it is not fully discharged; for example, a mobile battery will last 5-6 times longer if it is only discharged 80% before recharging.

Special deep discharge batteries are available for applications where this might be necessary.

Nickel-Cadmium Batteries

 

15) Application requirements

The battery must be sufficient for the intended application. This means that it must be able to produce the right current with the right voltage. It must have sufficient capacity, energy and power. It should also not exceed the requirements of the application by too much, since this is likely to result in unnecessary cost; it must give sufficient performance for the lowest possible price.

 

Whats the difference between Nickel Cadmium (Nicad), Nickel-metal hydride (NiMH), and Lithium Ion (Li-Ion)?

 
The three most popular battery chemistries have very special qualities each. I'll start with the oldest first. 
 
 
Nickel Cadmium
 
Nicad batteries are very robust. They are good for working in extreme environments, such as cold or hot weather. They also have a longer life cycle than NiMH or Li-ion, with about 700- life cycles. They are very robust for high output deep discharge applications. On the downside, they have a charging problem called the "memory effect". That is, if they don't get completely charged after each use, they will potentially only charge up to the last highest charge. This can shorten the lifespan of the battery. They can be reconditioned but at the cost of at least 3 life cycles.
 
The benefits of using a Nicad battery are extreme temperature tolerance, deep discharge capabilities, 
 
Nicad cell availability. Since Panasonic has bought Sanyo, the availability of quality cells has gone down. Panasonic has discontinued manufacturing several sizes of Nicad batteries. As a result, the sizes that have been discontinued are now only available by Chinese manufacturers. This is an inferior battery.
 
 
Nickel Metal Hydride
 
NiMH batteries offer a higher capacity than Nicad batteries, and less capacity than Li-Ion. They are nearly twice as heavy as Nicad batteries. They also don't have a memory effect. They are a good medium temperature battery. You can operate with them usually in -5 to 95 degrees farenheit with no adverse problems. They have good deep-discharge qualities and can store nearly twice the capacity of nicad cells. Their life-cycle is generally lower than nicad, at 500-800 life cycles. They are very similar to nicad when it comes to charge and discharge characteristics, and are safer than Lithium ion with thermal runaway.
 
NiMH batteries have more of a tendency to have weak-cell syndrome. That is, when you charge a battery pack all the way, then you go to use, it it dies right away. When you test it, it will say that its fully charged. This is because some or all of the cells can no longer hold power. NiMH batteries have a tendency to do this more than any other battery type.
 
NiMH cells are better protected from thermal runaway than Lithium Ion, however not as good as nicad. They have similar safety characteristics as nicad and are better for the environment than nicad.
 
The availability of NiMH cells is very good. Several manufacturers produce them in many countries.
 

Lithium Ion (Li-Ion)
 
Lithium Ion battery cells are known for their enormous energy density. They are able to store more energy per pound than any of the traditional battery packs. That makes them very popular for portable electronics, vehicles, etc. They don't have the memory effect that nicad does, and they perform the best at deep discharge applications compared to nicad or nimh. Environmentally they are safer to dispose than nicad as they don't taint water supplies, and from a mining point of view, there is no benefit or negative aspect either way.
 
Li-Ion is not a good battery chemistry for extreme temperatures. According to Nasa, the maximum capacity of lithium ion cells at -40 degrees C is 12% of its room temperature capacity. We've had customers who have had li-ion radio batteries stop working at -5 degrees farenheit.
 
Safety is another issue with lithium Ion. All lithium ion batteries have to be controlled with an integrated circuit to control input and output voltage. If the circuit is not present, the cell could have thermal runaway. I'm sure you've all heard of laptop batteries catching fire. That is an instance of thermal runaway. Another safety issue is water. In the presence of H2o, li-ion will oxidize extremely rapidly (hint explode).
 
The life cycle of li-ion is approximately 700 to 950. This is ever changing with the testing of different lithium salts but as of November , when you purchase batteries for your consumer electronics, this will be the approximate range.
 
The charge and discharge curve of li-ion is extraordinary. It can handle heavy input and output voltage, making it ideal for use in power tools, electric vehicles, mobility devices, and the like. 
 
The availability of lithium Ion is very good. There are several areas of the world that mine it, and pricing should be stable for the foreseeable future. Efforts from Wall Street to commoditize it would have negative effects on its price and availability.

Future Battery Products

Lithium Sulphur

Aluminum Air

Solid State

 

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