The section on Performance Characteristics describes how cells perform in practice.The section on Battery Pack Design describes some of the many possible battery packs functions and designs.A summary of the most commonly available cell technologies is given on the “Cell Chemistries” page with links to pages describing the advantages and disadvantages of each type.Most of the information needed to specify a battery for a particular application is listed on the “Request for Quotation” page and in the main this should be self explanatory.Some further explanation is however given below.
Trade offs
The starting point is the application and its power consumption requirements.For many consumer applications,the design trade offs are between cost and weight or volume and cycle life which can equally well be satisfied by a wide range of low power,low capacity batteries based on a range of cell chemistries.Customers for higher power applications and industrial users will more likely compare batteries on the cost of ownership taking into account the capital cost,the running costs and the battery cycle life.These industrial applications may require batteries working at the limits of the technology.In this case the trade off may be between power output and storage capacity.Knowledge of the application priorities is an important factor in determining the technology to be used.
Electrical Specification
The working voltages and currents required by the application are obviously needed for specifying the battery,not just the nominal,but also the maxima and minima.Beware of cell manufacturers’ published specifications.They are not deliberately misleading but they may need some interpretation.You may find the quoted Amphour capacity of a cell based on a prolonged discharge of 10 hours or more rather than the 1 hour at the C rate which is used for most low power batteries.This does not give a true indication of the capacity available if the cell were to be discharged at the C rate which could be as low as half the 20 hour capacity.
Voltage Requirements
The battery should be dimensioned to be able to support the full operating voltage range of all the devices in the application.There is no point however in specifying a battery operating voltage range which is greater than the operating voltage limits of the application for which the battery is intended.The battery will deliver excessive currents when it is fully charged or,if the application has a protection circuit,it will prevent the application from starting.At the other end of the discharge curve,when the application cuts off at its lower limit,the battery becomes unusable when there is still a substantial amount of energy left in the cells.The battery operating voltage range should therefore be less than the operating voltage range of the circuit it is designed to power.Note also that the battery terminal voltage decreases towards the end of the discharge cycle and this should be taken into account when specifying the battery voltage.
Multiple Voltages
Many designs require multiple supply voltages because of the range of active devices used in the application.In such cases it is not necessary to have multiple batteries.The various voltages can be derived from a single power supply rail using DC/DC converters,charge pumps and buck and boost and LDO and switch mode regulators singly or in combination.A wide range of power management integrated circuits is available for this purpose.
Capacity
The battery capacity required is determined by the usage profile of the application and the desired time between charges (or battery replacement in the case of primary batteries).In general it is the average current in amps multiplied by the time between charges.The battery pack should not be designed in isolation from the charger.Using inappropriate chargers can seriously shorten the cycle life of the battery and may even be dangerous.The pack designer needs to interface with the charger designer to ensure that the correct charger is specified for the chosen cell chemistry.
Environmental Conditions
Batteries have a limited temperature range over which they work.Attempting to use the battery outside these limits will usually result in a permanent degradation in performance or complete failure.The specification should therefore stipulate these limits.Note that the actual working temperature of the battery will not be the ambient temperature but some higher temperature depending on the heat generated by the battery application and the heat removed from the battery by conduction and radiation.If the product operating temperature requirements exceed the battery operating limits it will be necessary to incorporate heating or cooling into the pack as appropriate.
Dimensions,Weight and Construction
The pack designer also needs to know the dimensions of the battery or the cavity destined to contain it along with the location and specification of the connectors and the packaging requirements for interfacing with the intended product.The allowable weight can also be a deciding factor when choosing a cell.For a given capacity there is roughly a 4:1 ratio in the range weights of available cells with different technologies.
Safety Requirements
The battery pack should be intrinsically safe whether attached to,or separate from,the product in which it is used and so should include at least the necessary protection circuits.The battery will normally be designed to meet all international safety standards,but the application may have particular monitoring and control requirements that must be specified before work can commence on the pack design.
International Standards
The design,manufacturing,use and disposal of batteries,as with many electrical products,are subject to a wide variety of standards and regulations imposed by national and international regulatory organisations mainly to protect the user and the environment.The pack designer needs to be aware of the applicable regulations in the markets in which the product will be sold and must ensure that the pack design is fully compliant with these requirements.
Costs
The target cost of the battery is obviously an important part of the specification.The unit cost of the battery is dependent on the battery chemistry employed,however a high cost chemistry may offer a longer cycle life.One way of comparing different options is to use the cost per cycle as the basis of comparison,another is the cost per Watthour of delivered energy.For consumer products these may not be the ways the product is judged by the end user unless some attempt is made to explain the benefits of an alternative technology.More likely the battery will judged by its contribution to the initial product (capital) cost.
When comparing the cost of battery power with other sources of power it may be necessary to take into account net present values and cash flows as well as lifetime costing.For example,the capital cost of a lithium traction battery with its associated electric drive may be very high compared with the cost of a petrol or diesel engine but the running costs of the battery will be much lower since the electricity costs less than the fuel. Based on capital costs the battery solution is not attractive.But when purchasing a new vehicle the user does not purchase all of the fuel at the same time.It is purchased over the lifetime of the vehicle.If batteries were leased,a more reasonable comparison of initial costs and running costs could be made.
