Tracking Battery Capacity and Resistance as part of Aging

Let’s examine the aging mechanism of batteries in terms of fading capacity and increasing internal resistance. Figure 1 shows a battery with high capacity and another that has aged. The capacity loss is illustrated with growing “rock content;” the rocks mark the unusable part of the battery. Figure 2 looks at resistance and illustrates a good battery with a free-flowing tap and a high-resistance one with restricted flow.

New battery has high capacity

Aged battery has low capacity

Figure 1: Battery capacity illustrated as liquid content. Both batteries are fully charged, but the “rock-content” limits the amount of energy being stored.

Battery with high CCA

Battery with low CCA

Figure 2: Free-flowing and restricted taps representing CCA performance.The cranking current is about 300A. (A golf cart typically draws 56A.)

Automotive technicians are most familiar with CCA, but this reading reflects engine cranking only. Capacity, the energy storage component, remains mostly unknown. Figure 3 illustrates the relationship between CCA and capacity on hand of a fluid-filled container. The liquid represents capacity and the taps symbolize CCA at different loading capabilities.

Figure 3: Relationship of CCA and capacity of a starter battery Capacity represents energy content and CCA is power delivery. A battery with 40% capacity can still have a healthy crank but the low capacity indicates end-of-life.

Most rechargeable batteries maintain low internal resistance during the service life, and this reflects in a high CCA (cold cranking amps) on starter batteries. Capacity, on the other hand, begins to drop gradually as the battery ages. To study these changes, Cadex measured the capacity and CCA of 20 aging starter batteries. The results are laid out in Figure 4, sorted according to capacity levels in percentage.

Figure 4: Capacity and CCA readings of 20 aging batteries. Batteries 1 to 9 have good CCA and high capacity; the CCA of batteries 10 to 20 remains reasonably strong but suffers from capacity loss. CCA tends to remain high while the capacity drops steadily as part of aging.

Test method: CCA was estimated with the Spectro CA-12 and the capacity was measured with an Agilent load bank by applying full discharges according to BCI standards.
Courtesy of Cadex
Batteries 1 to 9 perform well on capacity and CCA, but batteries 10 to 20 show notable capacity loss while the CCA remains strong. The motorist is unaware of the fading capacity until the car won’t start one morning. This is especially critical during a cold spell, which further reduces the capacity.
Capacity is the leading health indicator of a battery, and car manufacturers often use 65 percent as the pass/fail threshold for warranty replacement. This magic level forms a natural bend, a cliff between a high performing battery and one that is beginning to age. Service garages usually take 40 percent as an end-of-life indication.  Even though a starter battery with 40 percent capacity may still crank well and have 6 to 12 months of service left before it will finally quit, the battery should be replaced. Thrifty drivers, (including this author) prefer to wait, but invariably get caught with a dead battery at the worst possible moment.
 Evaluating the capacity of a starter battery gives the most accurate end-of-life prediction. Capacity sets the floor upon which CCA and other readings are compared. Without knowing capacity, other measurements mean little.

Testing Lead Acid Batteries

Many manufacturers of battery testers claim to measure battery health on the fly. These instruments work well in finding battery defects that involve voltage anomalies and elevated internal resistance, but other performance criteria remain unknown. Stating that a battery tester based on internal resistance can also measure capacity is misleading. Advertising features that are outside the equipment’s capabilities confuses the industry into believing that multifaceted results are attainable with basic methods. Manufacturers of these instruments are aware of the complexity involved, but some like to add a flair of mystery in their marketing scheme, similar to a maker of a shampoo product promising to grow lush hair on a man’s bald head. Here is a brief history of battery testers for lead acid and what they can do.
The carbon pile, introduced in the 1980s, applies a DC load of short duration to a starter battery, simulating cranking. The voltage drop and recovery time provide a rough indication of battery health. The test works reasonably well and offers evidence that power is present. A major advantage is the ability to detect batteries that have failed due to a shorted cell (low specific gravity in a cell due to high self-discharge). Capacity estimation, however, is not possible, and a battery that has a low state-of-charge appears as weak. A skilled mechanic can, however, detect a faulty battery based on the voltage signature and loading behavior. To do a CCA pass/fail test, load a fully charged starter battery with half the rated CCA value for 15 seconds. To pass, the voltage must stay above 9.6V at 10º C (50º F) and higher. Colder temperatures cause a large voltage drop.
The AC conductance meters appeared in 1992 and were hailed as a breakthrough. The non-invasive method injects an AC signal into the battery to measure the internal resistance. Today, these testers are commonly used to check the CCA of starter batteries and verify resistance change in stationary batteries. While small and easier to use, AC conductance cannot read capacity, and the resistive value gives only an approximation of the real CCA of a starter battery. A shorted cell could pass as good because in such a battery the overall conductivity and terminal voltage are close to normal, even though the battery cannot crank the motor. AC conductance testers are common in North America; Europe prefers the DC load method.
Critical progress has been made towards electrochemical impedance spectroscopy (EIS). Cadex took the EIS technology a step further and developed battery specific models that are able to estimate the health of a lead acid battery. Multi-model electrochemical impedance spectroscopy, or Spectroäfor short, reads battery capacity, CCA and state-of-charge in a single, non-invasive test.Figure 1 illustrates the Spectro CA-12 handheld battery tester.

Figure 1: Spectro CA-12 battery tester Compact battery rapid tester displays capacity, CCA and state-of-charge in 15 seconds. Courtesy Cadex

The Spectro CA-12 handheld device, in which the Spectro™ technology is embedded, excites the battery with signals from 20–2000Hz. A DSP deciphers the 40 million transactions churned out during the 15-second test into readable results. To check a battery, the user simply selects the battery voltage, Ah and designated matrix. Tests can be done under a steady load of up to 30A and a partial charge, however, if the state-of-charge is less than 40 percent, the instrument advises the user to charge and retest.
The Spectro method is a further development of EIS, a technology that had been around for several decades. What’s new is the use of multi models and faster process times. Cost and size have also shrunk. Earlier models cost tens of thousands of dollars and traveled on wheels. The heart of Spectro is not so much the mechanics but the algorithm. No longer do modern EIS devices accompany a team of scientist to decipher tons of data. Experts predict that the battery industry is moving towards the multi-model EIS technology to estimate batter performance
Nowhere is the ability to read capacity more meaningful than with deep-cycle batteries in golf cars, aerial work platforms and wheelchairs, as well as military and naval carriers. Getting a readout in seconds without putting the vehicles out of commission allows for a quick performance check on a suspect battery before deployment in the field. Figures 2, 3 and 4 show typical battery problems and how modern test technologies can detect them.

	Figure 2: Low charge Drive is sluggish; Spectroäreads low SoC. Capacity estimation is correct in spite of low charge.
	Figure 3: Low capacity Battery has good drive but short runtimes. Spectroäreads good impedance but low capacity.
	Figure 4: Faulty set Spectroäfinds low performing and shorted blocks in a string. Good batteries can be regrouped and reused. All figures Courtesy of Cadex

Matrices

Measurement devices, such as the Spectro CA-12, are not universal instruments capable of estimating the capacity of any battery that may come along; they require battery specific matrices, also known as pattern recognition algorithm. A matrix is a multi dimensional lookup table against which the measured readings are compared. Text recognition, fingerprint identification and visual imaging operate on a similar principle in that a model exists, with which to equate the derived readings.
This book identifies three commonly used measuring methods. The principle in all is to take one or several sets of readings and compare them against known reference settings or images to disclose the characteristics of a battery. The three methods are as follows.

Scalar:     The single value scalar test takes a reading and compares the result with a stored reference value. In battery testing this could be measuring a voltage, interrogating the battery by applying discharge pulses or injecting a frequency and then comparing the derived result against a single reference point. This is the simplest test, and most DC load and single-frequency AC conductance testers use this method.

Vector:    The vector method applies pulses of different currents, or excites the battery with several frequencies, and evaluates the results against preset vector points to study the battery under various stress conditions. Typical applications for this one-dimensional scalar model are battery testers that apply multi-tier DC loads or inject several test frequencies. Because of added complexity in evaluating the different data points and limited benefits, the vector method is seldom used.

Matrix:    The matrix method scans a battery with a frequency spectrum as if to capture the image of a landscape and compare the imprint with a stored model of known characteristics. This multi-dimensional set of scalars, which form the foundation of Spectroä, provides the most in-depth information but is complex in terms of evaluating the data generated. With a proprietary algorithm, the Spectroätechnology is able to estimate battery capacity, CCA and SoC.

Matrices are primarily used to estimate battery capacity, however, CCA and state-of-charge also require matrices. These are easier to assemble and serve a broad range of starter batteries. While the Spectroämethod offers an accuracy of 80 to 90 percent on capacity, CCA is 95 percent exact. This compares to 60 to 70 percent with battery testers using the scalar method. Service personnel are often unaware of the low accuracy; verifications are seldom done, as this would involve several days of laboratory testing.
A further drawback of scalar battery testers is obtaining a reading that is neither resistance nor CCA. While there are similarities between the two, no standard exists and each instrument gives different values. In terms of assessing a dying battery, however, this method is adequate as it reflects conductivity. The larger disadvantage is not being able to read capacity. Table 5 illustrates test accuracies using scalar, vector and matrix methods.

Measuring units	Scalar Single value	Vector One-dimensional set of scalars	Matrix Multi-dimensional set of scalars
CCA	60–70% accurate		90–95% accurate
Capacity	N/A		80–90% accurate
SoC	Voltage-based; requires rest after charge and discharge		90–95% accurate (with new battery)

Table 5: Accuracy in battery readings with different measuring methodsScalar and vector provide resistance with references to CCA on starter batteries. The matrix method is more accurate and provides capacity estimations but needs reference matrices.
To generate a matrix, batteries with different state-of-health are scanned. The more batteries of the same model but diverse capacity mix are included in the mix, the stronger the matrix will become. If, for example, the matrix consists only of two batteries, one showing a capacity of 60 percent and the other 100 percent, then the accuracy would be low for the batteries in between. Adding a third battery with an 80 percent capacity will solidify the matrix, similar to placing a pillar at the center of a bridge. To cover the full spectrum, a well-developed matrix should include battery samples capturing capacities of 50, 60, 70, 80, 90 and 100 percent. Batteries much below 50 percent are less important because they constitute a fail.
It is difficult to obtain aged batteries, especially with newer models. Forced aging by cycling in an environmental chamber is of some help; however, age-related stresses from the field are not represented accurately. It also helps to include batteries from different regions to represent unique environmental user patterns. A starter battery in a Las Vegas taxi has different strains than that of a car driven by a grandmother in northern Germany.
Different state-of-charge levels increase the complexity to estimate battery health. The SoC on a new battery can be determined relatively easily with impedance spectroscopy, however, the formula changes as the battery ages. A battery tester should therefore be capable of examining new and old batteries with a charge level of 40 to 100 percent. With ample data, this is possible because natural aging of a battery is predictable and the scanned information can be massaged to calculate age. This is similar to face recognition that correctly identifies a person even if he or she has developed a few wrinkles and has grown gray hair.
Simplifications in matrix development are possible by grouping batteries that share the same chemistry, voltage and a similar capacity range into a generic matrix. This simplifies logistics; however, the readout is classified into categories rather than numbers. Figure 6 illustrates the classification scheme of Good, Low and Poor. Good passes as a good battery; Low is suspect and predicts the end of life; and Poor is a fail that mandates replacement.

Heat, Loading and Battery Life

Heat is a killer of all batteries and high temperatures cannot always be avoided. This is the case with a battery inside a laptop, a starter battery under the hood of a car and stationary batteries in a tin shelter under the hot sun. As a guideline, each 8°C (15°F) rise in temperature cuts the life of a sealed lead acid battery in half. A VRLA batteryfor stationary applications that would last 10 years at 25°C (77°F) would only live for five years if operated at 33°C (95°F). Once the battery is damaged by heat, the capacity cannot be restored. The life of a battery also depends on the activity and is shortened if the battery is stressed with frequent discharge.
According to the 2010 BCI Failure Mode Study, starter batteries have become more heat-resistant over the past 10 years. In the 2000 study, a change of 7°C (12°F) affected battery life by roughly one year; in 2010 the heat tolerance has widened to 12°C (22°F). In 1962, a starter battery lasted 34 months, and in 2000 the life expectancy had increased to 41 months. In 2010, BCI reports an average age of 55 months of use. The cooler North attains 59 months and the warmer South 47 months.
Cranking the engine poses minimal stress on a starter battery. This changes in a start-stop function of a micro hybrid. The micro hybrid turns the IC engine off at a red traffic light and restarts it when the traffic flows. This results in about 2,000 micro cycles per year. Data obtained from car manufacturers show a capacity drop to about 60 percent after two years of use in this configuration. To solve the problem, automakers are using specialty AGM and other variations that are more robust than the regular lead acid.  Figure 5 shows the drop in capacity after 700 micro cycles. The simulated start-stop test was performed in Cadex laboratories. CCA remains high.

Test method:   The test battery was fully charged and then discharged to 70 percent to resemble the SoC of a micro hybrid in real life. The battery was then discharged at 25A for 40 seconds to simulate engine off condition at stoplight with the headlight on, before cranking the engine at 400A and recharging. The CCA readings were taken with the Spectro CA-12.

The cell voltages on a battery string must be similar, and this is especially important for higher-voltage VRLA batteries. With time, individual cells fall out of line, and applying an equalizing charge every six months or so should theoretically bring the cells back to similar voltage levels. While equalizing will boost the needy cells, the healthy cell get stressed if the equalizing charge is applied carelessly. What makes this service so difficult is the inability to accurately measure the condition of each cell and provide the right dose of remedy. Gel and AGM batteries have lower overcharge acceptance than the flooded version and different equalizing conditions apply. Always refer to the manufacturer’s specifications.

Water permeation, or loss of electrolyte, is a concern with sealed lead acid batteries, and overcharging contributes to this condition. While flooded systems accept water, a fill-up is not possible with VRLA. Adding water has been tried, but this does not offer a reliable fix. Experimenting with watering turns the VRLA into unreliable battery that needs high maintenance.

Flooded lead acid batteries are one of the most reliable systems. With good maintenance these batteries last up to 20 years. The disadvantages are the need for watering and providing good ventilation. When VRLA was introduced in the 1980s, manufacturers claimed similar life expectancy to flooded systems, and the telecom industry switched to these maintenance-free batteries. By mid 1990 it became apparent that the life for VRLA did not replicate that of a flooded type; the useful service life was limited to only 5–10 years. It was furthermore noticed that exposing the batteries to temperatures above 40°C (104°F) could cause a thermal runaway condition due to dry-out.

A new lead acid battery should have an open circuit voltage of 2.125V/cell. At this time, the battery is fully charged. During buyer acceptance, the lead acid may drop to between 2.120V and 2.125V/cell. Shipping, dealer storage and installation will decrease the voltage further but the battery should never go much below 2.10V/cell. This would cause sulfation. Battery type, applying a charge or discharge within 24 hours before taking a voltage measurement, as well as temperature will affect the voltage reading. A lower temperature raises the OCV; warm ambient lowers it.  

How to Store Batteries

The recommended storage temperature for most batteries is 15°C (59°F); the extreme allowable temperature is –40°C to 50°C (–40°C to 122°F) for most chemistries. While lead acid must always be kept at full charge during storage, nickel- and lithium-based chemistries should be stored at around a 40 percent state-of-charge (SoC). This level minimizes age-related capacity loss while keeping the battery in operating condition and allowing self-discharge.
Finding the 40 percent SoC level is difficult because the open circuit voltage (OCV) of batteries does not lend itself well to state-of-charge estimations. For lack of better methods, voltage is nevertheless used as a rough fuel gauge indicator. The SoC of Li-ion is roughly 50 percent at 3.80V/cell and 40 percent at 3.75V/cell. Allow Li-ion to rest 90 minutes after charge or discharge before taking the voltage reading to get equilibrium.
SoC on nickel-based batteries is especially difficult to measure. A flat discharge curve, agitation after charge and discharge, and voltage change on temperature contribute to the fluctuations. Since no other estimation tool exists that is practical, and the charge level for storage is not all too critical for this chemistry, simply apply some charge if the battery is empty, and then make sure that the battery is kept in a cool and dry storage.
Storage will always cause batteries to age. Low temperature and partial SoC only slow the effect. Table 1 illustrates the recoverable capacity of lithium- and nickel-based batteries at various temperatures and charge levels over one year. The recovered capacity is defined as the available battery capacity after storage with a full charge.

Temperature	Lead acid at full charge	Nickel-based at any charge	Lithium-ion (Li-cobalt)
			40% charge	100% charge
0°C 25°C 40°C 60°C	97% 90% 62% 38% (after 6 months)	99% 97% 95% 70%	98% 96% 85% 75%	94% 80% 65% 60% (after 3 months)

Table 1: Estimated recoverable capacity when storing a battery for one yearElevated temperature hastens permanent capacity loss. Depending on battery type, lithium-ion is also sensitive to charge levels.
Lithium-ion batteries are often exposed to unfavorable temperatures, and these include leaving a cell phone in the hot sun or operating a laptop on the power grid. Elevated temperature and allowing the battery to sit at the maximum charge voltage for expended periods of time explains the shorter than expected battery life. Elevated temperature and excessive overcharge also stresses lead and nickel-based batteries. All batteries must have the ability to relax after charged, even when kept on float or trickle charge.
Nickel-metal-hydride can be stored for about three years. The capacity drop that occurs during storage can partially be reversed with priming. Nickel-cadmium stores well, even if the terminal voltage falls to zero volts. Field tests done by the US Air Force revealed that NiCd stored for five years still performed well after priming cycles. It is believed that priming becomes necessary if the voltage drops below 1V/cell. Primary alkaline and lithium batteries can be stored for up to 10 years with minimal capacity loss.
You can store a sealed lead acid battery for up to two years. Since all batteries gradually self-discharge over time, it is important to check the voltage and/or specific gravity, and then apply a charge when the battery falls to 70 percent state-of-charge. This is typically the case at 2.07V/cell or 12.42V for a 12V pack. (The specific gravity at 70 percent charge is roughly 1.218.) Some lead acid batteries may have different readings and it is best to check the manufacturer’s instruction manual. Low charge induces sulfation, an oxidation layer on the negative plate that inhibits current flow. Topping charge and/or cycling may restore some of the capacity losses in the early stages of sulfation.
Sulfation may prevent charging small sealed lead acid cells, such as the Cyclone by Hawker, after prolonged storage. If seemingly inactive, these batteries can often be reactivated by applying a higher than normal voltage. At first, the cell voltage under charge may go up to 5V and absorb only a small amount of current. Within two hours or so, the charging current converts the large sulfate crystals into active material, the cell resistance drops and the charge voltage gradually normalizes, and at a voltage of 2.10–2.40V the cell is able to accept a normal charge. To prevent damage, set the current limit to a very low level. Do not attempt to perform this service if the power supply does not allow setting current limiting.

Simple Guidelines for Storing Batteries

• Primary batteries store well. Alkaline and primary lithium batteries can be stored for 10 years with moderate loss capacity.
   
• Remove battery from the equipment and store in a dry and cool place.
   
• Avoid freezing. Batteries freeze more easily if in discharged state.
   
• Charge lead acid before storing and monitor the voltage or specific gravity frequently; apply a boost if below 2.10V/cell or an SG below 1.225.
   
• Nickel-based batteries can be stored for five years and longer, even at zero voltage; prime before use.
   
• Lithium-ion must be stored in a charged state, ideally 40 percent. This assures that the battery will not drop below 2.50V/cell with self-discharge and fall asleep.
   
• Discard Li-ion if the voltage has stayed below 2.00/V/cell for more than a week.

Caution:

When charging an SLA with over-voltage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting and observe the battery voltage and temperature during charge. In case of rupture, leaking electrolyte or any other cause of exposure to the electrolyte, flush with water immediately. If eye exposure occurs, flush with water for 15 minutes and consult a physician immediately. Wear approved gloves when touching electrolyte, lead and cadmium. On exposure to skin, flush with water immediately.

How to Repair a Battery Pack

Batteries for power tools and other industrial devices can often be repaired by replacing one or all cells. Finding a NiCd and NiMH cell is relatively easy; locating the correct Li-ion cell can be more difficult. Naked Li-ion cells are not readily available off the shelf and a reputable battery manufacturer may only sell to certified pack assemblers. Incorrect use or lack of an protection circuit could cause stress and disintegration of the replaced cell. When repairing a Li-ion pack make certain that each cell is properly connected to a protection circuit.

If a relatively new pack has only one defective cell, you may replace only the affected cell. On an aged battery, it’s best to replace all cells. Adding a new cell with full capacity in between neighboring cells that have faded would cause a cell mismatch. Matching the replacement cell with one of a lower rating may work but this fix is often of short duration. Always replace with the same chemistry cell.

A well-matched battery pack means that all cells have similar capacities. An anomaly can be drawn with a chain in which the weakest link determines the performance of the battery. Read more about When replacing all cells, the rating is less important as long as the differences are not too large for the charger to handle. Cells with higher Ah will simply take a bit longer to charge.  The state-of-charge of all cells being charged for the first time should have a similar charge level, and the open-circuit voltages should be within 10 percent of each other
Many visitors of BatteryUniversity.com ask if NiCd can be replaced with NiMH? Theoretically, this should be possible but charging may be an issue. NiMH uses a more defined charge algorithm than NiCd. A modern NiMH charger can charge both NiMH and NiCd; the old NiCd charger could overcharge NiMH by not properly detecting full charge state and applying a trickle charge that is too high.
Welding the cells is the only reliable way to get dependable connection. Limit the heat transfer to the cells during welding to prevent excess heat buildup.
Simple Guidelines when Repairing Battery Packs

• Only connect cells that are matched and have the identical state-of-charge. Do not connect cells of different chemistry, age or capacity.
   
• Never charge or discharge Li-ion batteries without a working protection circuit unattended. Each cell must be monitored individually.
   
• Include a temperature sensor that disrupts the current on high heat.
   
• Apply a slow charge only if the cells have different state-of-charge.
   
• Pay special attention when using an unknown brand of cells. Some may not contain a high level of intrinsic safety.
   
• Li-ion is sensitive to reverse polarization. Observe correct polarity.
   
• Do not charge a Li-ion battery that exhibits physical damage or has dwelled at a voltage of less than 1.5V/cell.
   
• When repairing Li-ion, assure that each cell is connected to a protection circuit

How to Know End-of-Battery-Life

critical concern among battery users is knowing “readiness” or how much energy a battery has at its disposal at any given moment. While installing a fuel gauge on a diesel engine is simple, estimating the energy reserve of a battery is more complex — we still struggle to read state-of-charge (SoC) with reasonable accuracy. Even if SoC were precise, this information alone has limited benefits without knowing the capacity, the storage capability of a battery. Battery readiness, or state-of-function (SoF), must also include internal resistance, or the “size of pipe” for energy delivery. Figure 1 illustrates the bond between capacity and internal resistance on hand of a fluid-filled container that is being eroded as part of aging; the tap symbolizing the energy delivery.

Figure 1: Relationship of CCA and capacity of a starter battery The liquid represents capacity, the leading health indicator; the tap symbolizes energy delivery or CCA. While the energy delivery remains strong, the capacity diminishes with age. Courtesy Cadex

Most batteries for critical missions feature a monitoring system, and stationary batteries were one of the first to receive supervision in the form of voltage check of individual cells. Some systems also include cell temperature and current measurement. Knowing the voltage drop of each cell at a given load provides cell resistance. Elevated resistance hints to cell failure caused by plate separation, corrosion and other malfunctions. Battery management systems (BMS) are also used in medical equipment, military devices, as well as the electric vehicle.
Although BMS serves an important role in supervising of batteries, such systems often falls short of expectations and here is why. The BMS device is matched to a new battery and does not adjust well to aging. As the battery gets older, the accuracy goes down and in extreme cases the data becomes meaningless. Most BMS also lack bandwidth in that they only reveal anomalies once the battery performance has dropped to 70 percent. The all-important 70–100 percent operating range is difficult to gauge and the BMS gives the battery a good bill-of-health. This prevents end-of-life prediction in that the operator must wait for the battery to show signs of wear before making a judgment. These shortcomings are not an oversight by the manufacturers, and engineers are trying to overcome them. The problem boils down to technology, or the lack thereof. Over-expectation is common and the user is stunned when stranded with a dead battery. Let’s look how current systems work and examine new technologies.
The most simplistic method to determine end-of-battery-life is by applying a date stamp or observing cycle count. While this may work for military and medical instruments, such a routine is ill suited for commercial applications. A battery with less use has lower wear-and-tear than one in daily operation and to assure reliability of all batteries, the authorities may mandate that all batteries be replaced sooner. A system made to fit all sizes causes good batteries to be discarded too soon, leading to increased operational costs and environment concerns.
Laptops and other portable devices use coulomb counting for SoC readout. The theory goes back 250 years when Charles-Augustin de Coulomb first established the “Coulomb Rule.” Coulomb counting works on the principle of measuring in- and out-flowing current of a battery. If, for example, a battery is charged for one hour at one ampere, the same energy should be available on discharge, but this is not the case. Internal losses and inaccuracies in capturing current flow add to an unwanted tracking error that must be corrected with periodic calibrations.
Calibration occurs naturally when running the equipment down. A full discharge sets the discharge flag, and the subsequent recharge establishes the charge flag (Figure 2). These two markers allow the calculation of state-of-charge by estimating the distance between the flags.

Figure 2: Discharge and charge flags Calibration occurs by applying a full charge, discharge and charge. This can be done in the equipment or externally with a battery analyzer as part of battery maintenance. Courtesy Cadex

Coulomb counting should be self-calibrating, but in real life a battery does not always get a full discharge at a steady current. The discharge may be in form of a sharp pulse that is difficult to capture. The battery may then be partially recharged and be stored at high temperature, causing elevated self-discharge that cannot be tracked. To correct the tracking error, a “smart battery” in use should be calibrated once every three months or after 40 partial discharge cycles. This can be done by a deliberate discharge of the equipment or externally with a battery analyzer. Avoid too many intentional deep discharges as this stresses the battery.
Fifty years ago, the Volkswagen Beetle had few battery problems. The only battery management was ensuring that the battery was being charged while driving. Onboard electronics for safety, convenience, comfort and pleasure have added to the demands of the battery in modern cars. For the accessories to function reliably, the battery state-of-charge must be known at all times. This is especially critical with start-stop technologies, a future requirement in European cars to improve fuel economy.
When the engine of a start-stop vehicle turns off at a stoplight, the battery continues to draw 25–50 amperes to feed the lights, ventilators, windshield wipers and other accessories. The battery must have enough charge to crank the engine when the traffic light changes; cranking requires a brief 350A. To reduce engine loading during acceleration, the BMS delays charging for about 10 seconds.
Modern cars are equipped with a battery sensor that measures voltage, current and temperature. Packaged in a small housing and embedded into the positive battery clamp, the electronic battery monitor (EBM) provides a SoC accuracy of about +/–15 percent on a new battery. As the battery ages, the EBM begins to drift and the accuracy drops to 20–30 percent. This can result in a false warning message and some garage mechanics disconnect the EBM on an aging battery to stop annoyances. Disabling the control system responsible for the start-stop function immobilizes engine stop and reduces the legal clean air requirement of the vehicle.
Voltage, current and temperature readings are insufficient to assess battery SoF; the all-important capacity is missing. Until capacity can be measured with confidence on-board of a vehicle, the EBM will not offer reliable battery information. Capacity is the leading health indicator that in most cases determines the end-of-battery-life. Imagine measuring the liquid in a container that is continuously shrinking in size. State-of-charge alone has limited benefit if the storage has shrunk from 100 to 20 percent and this change cannot be measured. Capacity fade may not affect engine cranking and the CCA can remain at a vigorous 70 percent to the end of battery life. Because of reduced energy storage, a low capacity battery charges quickly and has normal vital signs, but failure is imminent. A bi-annual capacity check as part of service can identify low capacity batteries. Battery testers that read capacity are becoming available at garages.
A typical start-stop vehicle goes through about 2,000 micro cycles per year. Test data obtained from automakers and the Cadex laboratories indicate that the battery capacity drops to approximately 60 percent in two years when in a start-stop configuration. The standard flooded lead acid is not robust enough for start-stop, and carmakers use a modified AGM (Absorbent Glass Mat) to attain longer life.
Automakers want to make sure that no driver gets stuck in traffic with a dead battery. To conserve energy when SoC is low, the BMS automatically turns unnecessary accessories off and the motor stays running at a stoplight. Even with this preventive measure, SoC can remain low when commuting in gridlock. Motor idling does not provide much charge and with essential accessories engaged, such as lights and windshield wipers, the net effect could be a small discharge.
Battery monitoring is also important in hybrid vehicles to optimize charge levels. The BMS prevents stressful overcharge above 80 percent and avoids deep discharges below 30 percent SoC. At low charge level, the internal combustion engine engages earlier and is left running for additional charge.
The driver of an electric vehicle (EV) expects similar accuracies on the energy reserve as is possible with a gasoline-powered car. Current technologies do not allow this and some EV drivers might get stuck with an empty battery when the fuel gauge still indicates reserve. Furthermore, the EV driver anticipates that a fully charged battery will travel the same distance, year after year. This is not possible and the range will decrease as the battery fades with age. Distances between charges will also be shorter than normal when driving in cold temperatures because of reduced battery performance.
Some lithium-ion batteries have a very flat discharge curve and the voltage method does not work well to provide SoC in the mid-range. An innovative new technology is being developed that measures battery SoC by magnetic susceptibility. Quantum magnetism (Q-Mag™) detects magnetic changes in the electrolyte and plates that correspond to state-of-charge. This provides accurate SoC detection in the critical 40-70 percent mid-section. More impotently, Q-Mag™ allows measuring SoC while the battery is being charged and is under load.
The lithium iron phosphate battery in Figure 3 shows a clear decrease in relative magnetic field units while discharging and an increase while charging, which relates to SoC. We see no rubber band effect that is typical with the voltage method in which the weight of discharge lowers the terminal voltage and the charge lifts it up. Q-Mag™ also permits improved full-charge detection; however, the system only works with cells in plastic, foil or aluminum enclosures. Ferrous metals inhibit the magnetic field.

Figure 3: Magnetic field measurements of a lithium iron phosphate during charge and discharge Relative magnetic field units provide accurate state-of-charge of lithium- and lead-based batteries.   Courtesy of Cadex (2011)

Q-Mag™ also works with lead acid. This opens the door to monitor starter batteries in vehicles. Figure 4 illustrates the Q-Mag™ sensor installed in close proximity to the negative plate. Knowing the precise state-of-charge at any given moment optimizes charge methods and identifies battery deficiencies, including the end-of-battery-life with on-board capacity estimations.

Figure 4: Q-Mag™ sensor installed on the side of a starter battery   The sensor measures the SoC of a battery by magnetic susceptibility. When discharging a lead acid battery, the negative plate changes from lead to lead sulfate. Lead sulfate has a different magnetic susceptibility than lead, which a magnetic sensor can measure. Courtesy of Cadex (2009)

Q-Mag™ is also a candidate to monitor stationary batteries. The sensing mechanism does not need to touch the electrical poles for voltage measurements and this poses an advantage for high-voltage batteries. Furthermore, Q-Mag™ can assist EVs by providing SoF accuracies not possible with conventional BMS. Q-Mag™ may one day assist in the consumer market to test batteries by magnetism. It is conceivable that one day an iPhone or iPad can be placed on a test mat, similar to a charging mat, and read battery SoC and performance.

How to Define Battery Life

Most new batteries go through a formatting process during which the capacity gradually increases and reaches optimal performance at 100–200 cycles. After this mid-life point, the capacity gradually begins decreasing and the depth of discharge, operating temperatures and charging method govern the speed of capacity loss. The deeper the batteries are discharged and the warmer the ambient temperature is, the shorter the service life. The effect of temperature on the battery can be compared with a jug of milk, which stays fresh longer when refrigerated.

                           Most portable batteries deliver between 300 and 500 full discharge/charge cycles. Fleet batteries in portable devices normally work well during the first year; however, the confidence in the portable equipment begins to fade after the second and third year, when some batteries begin to lose capacity. New packs are added and in time the battery fleet becomes a jumble of good and failing batteries. That’s when the headaches begin. Unless date stamps or other quality controls are in place, the user has no way of knowing the history of the battery, much less the performance.

                         The green light on the charger does not reveal the performance of a battery. The charger simply fills the available space to store energy, and “ready” indicates that the battery is full. With age, the available space gradually decreases and the charge time becomes shorter. This can be compared to filling a jug with water. An empty jug takes longer because it can accept more water than one with rocks. Figure 1 shows the “ready” light that often lies.

Figure 1: The “ready” light lies The “ready” light on a charger only reveals that the battery is fully charged; there is no relationship to performance. A faded battery charges faster than a good one. Bad batteries gravitate to the top. Courtesy of Cadex

Many battery users are unaware that weak batteries charge faster than good ones. Low performers gravitate to the top and become available by going to “ready” first. They form a disguised trap when unsuspecting users require a fully charged battery in a hurry. This plays havoc in emergency situations when freshly charged batteries are needed. The operators naturally grab batteries that show ready, presuming they carry the full capacity. Poor battery management is the common cause of system failure, especially during emergencies.

Failures are not foreign in our lives and to reduce breakdowns, regulatory authorities have introduced strict maintenance and calibration guidelines for important machinery and instruments. Although the battery can be an integral part of such equipment, it often escapes scrutiny. The battery as power source is seen as a black box, and for some inspectors correct size, weight and color satisfies the requirements. For the users, however, state-of-function stands above regulatory discipline and arguments arise over what’s more important, performance or satisfying a dogmatic mandate.

Ignoring the performance criteria of a battery nullifies the very reason why quality control is put in place. In defense of the quality auditor, batteries are difficult to check, and to this day there are only a few reliable devices that can check batteries with certainty.