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How To Specify Batteries

 

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The battery industry has provided us with an enormous range of battery sizes , shapes, voltages, capacities and chemistries. With so many to choose from, how does the applications engineer select the optimum battery for the intended application? This section outlines the information needed to specify a battery.

Don't leave it too late. To avoid problems of last minute changes to the system power budget, or in finding a battery to fit the available space left in the product casing, the requirements for the power source should be considered as early as possible in the design cycle.

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The section on Performance Characteristics describes how cells perform in practice. The section on Battery Pack Design describes some of the many possible battery pack 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. Convenience is also an issue.

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.

 

Performance Requirements

Details and explanations of cell behaviours are given in the section on Performance Characteristics. The applications engineer must match the cell performance to the requirements of the application.

 

Requirement for Recharging?

The pros and cons of using rechargeable cells for the application are discussed in separate sections on Primary Cells and Secondary Cells .

 

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. (The SAE uses 20 hours as standard). 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. On the other hand the cell capacity may be quoted at the C rate but the cell may be designed for low discharge rates only and may not be able to sustain a prolonged continuous discharge at the C rate.

 

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. Similarly, at lower temperatures the battery internal impedance may increase resulting in a lower terminal voltage available to the application.

 

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.

 

Current Requirements

The average and the pulsed current drains required by the application are key factors in determining the battery capacity and for specifying the associated protection circuits.

 

One factor often overlooked is the start-up or surge current. When equipment is first switched on, extremely high currents may flow for a very short time until the circuit reaches its steady state condition. This could be due to capacitors charging up or other effects. This is a particular problem for circuits powering electric motors. The current through the motor is controlled by the difference between the battery voltage and the motor's generated voltage (otherwise called the back EMF). When the motor is first connected up to the battery (with no motor speed controller) there is no back EMF. So the current is controlled only by the battery voltage, motor resistance (and inductance) and the battery leads. Without any back emf before the motor starts to turn, it therefore draws a very large large surge current. It may be necessary to program a delay into fast acting protection circuits to avoid false triggering during start-up. Alternatively it may be possible to minimise the problem by applying the load progressively rather than instantaneously.

 

Similarly batteries used in UPS applications often experience a dramatic initial voltage drop when called upon This is known as a "coup de fouet" or "whiplash." The voltage recovers after a short time once the electro-chemical discharge process is stabilised and protection circuits should specified accordingly.

 

Limiting the quiescent current when the system is powered down is also an important consideration particularly if primary cells are used.

 

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. See also the section on Chargers and the Charger Specification Checklist.

 

Duty Cycle

It is important to understand the pattern of current usage in order to calculate the overall current drain on the battery. The table below shows typical usage of portable telecommunications equipment.

 

Corner  
Standby Mode
  Receive Mode
Transmit Mode
Corner
Percent
80%
10%
10%
Minutes per hour
48
6
6
Current drain during mode
10 mA
62 mA
325 mA

Average current for mode

8 mA

6.2 mA

32.5 mA

 

More complex applications such as electric vehicles have a much more variable load pattern depending on driving conditions. An example is given in the section on Battery Testing. These simulated load predictions are an essential tool for dimensioning a new automotive battery systems.

 

The battery must be able to handle the peaks as well as the average load.

 

Timing Issues

Specifying the usage profile of the battery is particularly important for high power applications.

Standby and emergency power applications impose particular timing requirements on the battery. Depending on the application, the battery may be required to react to a power outage within milliseconds or seconds to avoid loss of data, process interruption or other system failures. Such events don't occur very often but when they do the battery may be required to provide power for very short periods, just long enough for the system to achieve a graceful shut down, or long enough for a standby generator to fire up. In these cases where the battery is expected to take over from an AC mains source, it must be supported by a fast acting electronic inverter to convert the DC battery output to the required clean AC voltage and frequency. The battery should therefore be specified in conjunction with the associated inverter. See also AC Batteries.

 

Depth of Discharge

In load levelling applications, batteries are required to provide power when the prime power source is not available or uncontrolled and intermittent. Examples are solar and wind powered applications. In these applications the battery will be required to provide power for prolonged periods on a daily basis. The battery is thus subject to deep discharges and many charge - discharge cycles. Special battery constructions are needed to meet these very onerous loading conditions. For these applications it is important to specify the expected Depth of Discharge which the battery must support as well as the Duty Cycle.

Similar requirements apply to electric and hybrid vehicle applications.

 

Self Discharge

All batteries progressively lose their charge over time, some much more quickly than others, even if they are not used. It is important to be aware of the time finished batteries may be in the consumer supply pipeline, often called the shelf life, after they leave the factory. This is particularly true with primary cells, to ensure they still have a reasonable remaining lifetime when they are eventually sold. Problems can be avoided by minimising the time the batteries will be in the supply pipeline (including shipping times and times in the manufacturers', assemblers', wholesalers' and retailers' inventories) or by choosing cells with a low self discharge rate.

Secondary batteries usually have a higher self discharge rate but at least they can be recharged. For rechargeable batteries the self discharge rate affects the time between charges and this could influence the choice of cell chemistry. Some examples of typical self discharge rates are given on the section on Performance Characteristics.

 

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. See Thermal Management.

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. See more in the section on Battery Safety.

 

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. A list of major standards setting organisations and typical standards is given in the section on International Standards.

 

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.

 

 

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