Testing is designed to tell us things we want to know aboutindividual cells and batteries.
Some typical questions are:
Is it fully charged ?
How much charge is left in the battery ?
Does it meet the manufacturer's specification ?
Has there been any deterioration in performance since it was new?
How long will it last ?
Do the safety devices all work ?
Does it generate interference or electrical noise ?
Is it affected by interference or electrical noise ?
The answers are not always straightforward.
Although all of the cell parameters the design engineer may wish tomeasure can be quantified by direct measurement, this is not alwaysconvenient or possible . For example the amount of charge left inthe battery, the State of Charge (SOC) can be determined by fullydischarging the battery and measuring the energy output. This takestime, it wastes energy, each test cycle shortens the battery lifeand it may not be practical if the battery is in use. It would alsobe pointless for a primary cell. For more detailed information ofhow this is done see the State of Charge page.
Similarly, the remaining life of a secondary cell can be determinedby continuously cycling it until it fails, but there's no point inknowing the cell life expectation if you have to destroy it to findout. This is known as the State of Health (SOH) of thebattery.
What is needed are simple tests or measurements which can be usedas an approximation to, or indirect measure of, the desiredparameter. For more information see the State of Health page
In all of the following tests, and testing in general, the testconditions must be specified so that repeatable results can beobtained, and meaningful comparisons can be made. This includesfactors such as method, temperature, DOD, load and duty cycle. Forinstance the cell capacity and cycle life, two key performanceindicators could vary by 50% or more depending on the temperatureand the discharge rate at which the tests were carried out. Seealso cell Performance Characteristics.
Battery specifications should always include the test conditions to avoidambiguity.
Qualification testing is designed to determine whether a cell orbattery is fit for the purpose for which it was intended before itis approved for use in the product. This is particularly importantif the cell is to be used in a "mission critical" application.These are comprehensive tests carried out initially on a smallnumber of cells including testing some of them to destruction ifnecessary. As a second stage, qualification also includes testingfinished battery packs before the product is approved for releaseto the customer. The tests are usually carried out to verify thatthe cells meet the manufacturer's specification but they could alsobe used to test the cells to arbitrary limits set by theapplications engineer to determine how long the cells survive underadverse conditions or unusual loads, to determine failure modes orsafety factors.
The battery packs should also be tested with the chargerrecommended for the application to ensure compatibility. Inparticular the potential user patterns must be evaluated to ensurethat the batteries do not become inadvertently overcharged. Seealso the section on Chargers.
Shake and Bake
Typical tests are included in the safety standards below. Theyinclude simple tests for dimensional accuracy to dynamic testing toverify that the product can survive any static and dynamicmechanical stresses to which it may be subject.
Typical tests are included in the safety standards below. They aredesigned to exercise the product through all the environmentalconditions likely to be encountered by the product during itslifetime.
The purpose of abuse testing is to verify that the battery is not adanger to the user or to itself either by accidental or deliberateabuse under any conceivable conditions of use. Designing foolproofbatteries is ever more difficult because as we know, fools are soingenious.
Abuse testing (always interesting to witness) is usually specifiedas part of the Safety Testing (below). Recent accidents withLithium cells have highlighted the potential dangers and stricterbattery design rules and a wider range of tests are being appliedas well as stricter Transport Regulations for shipping the products.
Consumer products normally have to comply with national orinternational Safety Standards required by the safety organisations of the countries in which theproducts are sold. Examples are UL, ANSI, CSA and IECstandards.
Strength, rigidity and flammability
Mould stress (Temperature)
Electrolyte not under pressure
No explosion or fire risk
Protection from or tolerance to
Power output - Load test
Instructions for use
Nail penetration tests
Exposure to fire
The published safety standards specify the method of testing andthe limits with which the product must comply.
Cells used in military applications usually have to meet morestringent requirements than those used in consumer products.
This is perhaps the most important of the qualification tests.Cells are subjected to repeated charge - discharge cycles to verifythat the cells meet or exceed the manufacturer's claimed cyclelife. Cycle life is usually defined as the number of charge -discharge cycles a battery can perform before its nominal capacityfalls below 80% of its initial rated capacity. These tests areneeded to verify that the battery performance is in line with theend product reliability and lifetime expectations and will notresult in excessive guarantee or warranty claims.
Temperature, charge/discharge rates and the Depth of Discharge eachhave a major influence on the cycle life of the cells (See the pageon Cycle Life) Depending on the purpose of the tests, the temperature and theDOD should be controlled at an agreed reference level in order tohave repeatable results which can be compared with a standard.Alternatively the tests can be used to simulate operatingconditions in which the temperature is allowed to rise, or the DODrestricted, to determine how the cycle life will beaffected.
Similarly cycle life is affected by over charging and overdischarging and it is vital to set the correct voltage and currentlimits if the manufacturer's specification is to beverified.
Cycle testing is usually carried out banks of cells using multichannel testers which can create different charge and dischargeprofiles including pulsed inputs and loads. At the same timevarious cell performance parameters such as temperature, capacity,impedance, power output and discharge time can be monitored andrecorded. Typically it takes about 5 hours for a controlled fullcharge discharge cycle. This means testing to 1000 cycles will take208 days assuming working 7 days per week 24 hours per day. Thus ittakes a long time to verify the effect of any ongoing improvementsmade to the cells. Because the ageing process is continuous andfairly linear, it is possible to predict the lifetime of a cellfrom a smaller number of cycles. However to prove it conclusivelyin order to guarantee a product lifetime would require a largenumber of cells and a long time. For high power batteries thiscould be very expensive.
Load testing is used to verify that the battery can deliver itsspecified power when needed.
The load is usually designed to be representative of the expectedconditions in which the battery may be used. It may be a constantload at the C rate or pulsed loads at higher current rates or inthe case of automotive batteries, the load may be designed tosimulate a typical driving pattern. Low power testing is usuallycarried out with resistive loads. For very high power testing withvariable loads other techniques may be required. A Ward-Leonardcontroller may be used to provide the variable load profile withthe battery power being returned to the mains supply rather thanbeing dissipated in a load.