A battery resembles a living organism that cannot be measured; only estimated by diagnostics similar to a doctor examining a patient. The accuracy of rapid-testing varies according to symptoms that change with state-of-charge (SoC), agitation after charge and discharge, temperature and storage. A well-functioning rapid-test must distinguish between a partially charged good battery and a fully charged weak pack. Both deliver similar momentary performance characteristics.
The leading health indicator of a battery is capacity. Capacity represents the energy storage, a quantity that gradually and permanently depletes with use. Other characteristics responsible for state-of-charge (SoH) are internal resistance (Ri) that governs load current and self-discharge that affects the mechanical integrity of a battery. On multi-cell packs, the balance of the cells connected in series and parallel should also be checked. (See BU-302: Series and Parallel Battery Configurations) These four characteristics should be met to give a battery a clean bill of health.
This article describes analog and digital battery test methods. Analog involves an electrical load to measure capacity as a function of time until the charge is depleted. The load also measures internal resistance (Ri) based on voltage drop.
The digital test method involves smart battery technology that assesses SoC and capacity by measuring in- and outflowing coulombs* (see BU-605 Testing and Calibrating Smart Batteries). With periodic calibration, smart batteries provide valuable SoH information on the fly. Here is a summary of analog and digital battery test methods.
Here is a summary of simple to complex test methods to examine batteries.
| Voltage | Reveals SoC. A load connected to the battery and agitation after loading or charging will affect the voltage and the battery needs several hours rest. (See BU-903: How to Measure State-of-charge) Capacity estimation is not possible. |
| Ohmic test | Measures internal battery resistance to check loading characteristics and to identify fault conditions. This is done by AC or DC method; both provide different readings. (See BU-802a: How does Rising Internal Resistance affect Performance? and BU-902: How to Measure Internals Resistance) Resistance readings do not correlate with capacity. |
| Full cycle | Reads the capacity of the chemical battery with a charge/discharge/charge cycle. The results are accurate but a battery must often be removed from service and the test takes several hours. (See BU-909: Battery Test Equipment) |
| Rapid-test | Most rapid-test methods are based on time domain or frequency domain analysis. Time domain excites the battery with pulses to observe ion-flow of Li-ion batteries. Frequency domain scans the battery with multiple frequencies to generate a Nyquist plot for analysis. Both methods require complex algorithms with parameters or matrices that serve as lookup tables. Quick-sort Model Specific (QSMS) observes the difference in resistive value when testing a battery with DC and AC methods. The algorithm is relatively simple and the test time is short, but each battery type requires a battery-specific parameter. Electrochemical Dynamic Response (EDR) measures the mobility of ion-flow between electrodes by applying load pulses and evaluating the response time on attack and recovery. The recovery times are compared with stored parameters relating to battery performance. (See BU-907: Testing Lithium-based Batteries) The diffusion coefficient of Li-ion differs according to active material and electrolyte additives used. Electrochemical Impedance Spectroscopy (EIS) injects AC signals at different frequencies to create a Nyquist plot. The Nyquist signature is superimposed onto electrochemical models that enable the estimation of capacity, CCA and SoC non-invasively. The typical test time is 15-60 seconds. (See BU-904: How to Measure Capacity) |
| BMS | Battery Management Systems estimate SoC by monitoring voltage, current and temperature. Some BMS for Li-ion also counts coulombs. A BMS can identify a battery defect but is unable to estimate capacity accurately(See BU-908: Battery Management Systems) |
| Battery parser | The parser measures the capacity of a Li-ion battery by taking a snapshot of the residual charge with the Extended Kalman Filter (EKF), followed by a coulomb* count to achieve full charge. The sum of residual charge plus added energy reveals the usable capacity. Each battery model requires a onetime calibration by cycling a known good pack. Parser parameters are stored in the test system. |
| SOLI | The state of life indicator (SOLI) predicts the Remaining Useful Life (RUL) of a battery by tracking delivered coulombs* as a percentage of total life expectancy revealed in Coulombic Energy Life (CEL). Definition: CEL represents the energy of a fully charged battery multiplied by the cycle count the manufacturer specifies. CEL of a new battery is 100%, a level that gradually depletes with usage until the specified life expectancy is delivered. The process is analogous to a vehicle being replaced on the odometer reading. SOLI can be added in wheelchairs, computers-on-wheels, golf cars, floor cleanser and scissor lifts. Cloud analytics assess RUL for scheduled battery replacements. * One coulomb is equal to the amount of charge delivered by 1A of current in one second |
No single test can capture all health characteristics of a battery. Many rapid-test devices look only at voltage and internal resistance. Stating the ability to estimate capacity with such methods makes industry believe that complex results are attainable with simplistic methods. Resistance-based instruments can indeed identify a dying; but so does the user.
Battery results are affected by SoC levels, agitation and temperature. Cadex laboratories further discovered differences in how batteries are aged. What is most puzzling is why natural aging gives dissimilar results to stress testing in an environmental chamber.
Summary
Battery testing always produces outliers that defy test protocols. Correct predictions for batteries in service should be 9 out of 10. Outliers may include batteries that are new and have not been fully formatted, or packs that have been in storage. Low SoC also causes errors.
Capacity is the gate keeper of battery health that relates to runtime and predicts end-of-life. The term capacity is poorly understood. A battery is typically replaced when the capacity fades to 80 percent. Some applications allow a lower capacity threshold, and a starter battery should be replaced when the capacity falls below 40%. (See BU-504: How to Verify Sufficient Battery Capacity)
When choosing the end-of-life threshold, an organization must ensure that the lowest performing battery delivers the expected runtime. Diagnostic Chargers and battery analyzers are becoming available that assess Minimal Operational Reserve (MOR), a verification standard that is governed by a user-adjustable Target Selector. Diagnostics enables the full use of each battery, reduces cost, improves system reliability and protects the environment.
Last Updated: 8-Apr-2022
Batteries In A Portable World