Knowing when to Replace a Battery

Discovering the Weakest Link in a System

Batteries for critical missions must be taken seriously. A battery is a feeble vessel that begins to fade the moment it leaves the factory. A battery is also the most prone parts to fail . . .

Batteries for critical missions must be taken seriously. A battery is a feeble vessel that begins to fade the moment it leaves the factory. A battery is also the most prone parts to fail, requiring several replacements in the life of the host. When to replace is a blur for most battery users. While a faded pack in a personal device only causes a mild inconvenience, loss of mission-critical power can have serious consequences in the field.

The stubborn and unpredictable behaviour of a battery leaves many users in awkward situations. According to reports, up to 50% of system breakdowns are attributed to a battery. The Association for the Advancement of Medical Instrumentation (AAMI) has identified batteries as one of the top 10 challenges facing biomedical departments in a hospital setting. An US Army official was caught marking batteries that still function with tape so soldiers would not carry rocks to combat.

Batteries need good care because even with the best upkeep, some packs die early and scientists don’t know why. Batteries exhibit human-like characteristics and their state-of-health rests on environmental conditions and user patterns. Most batteries follow the path of a gradual capacity loss that is predictable. Keeping them cool and applying moderate charge and discharge currents prolongs life.

All batteries generally work well in the first year of service but confidence fades in the second and third year. New packs are added and in time the battery fleet becomes a jumble of good and failing batteries. That’s when the headache begins. Unless batteries in a fleet are examined regularly as part of quality assurance, the user has little knowledge on the trustworthiness of each pack. 

The energy in a battery can be divided into three segments: available energy, the empty zone that can be refilled, and the unusable part, or rock content, that has become dormant and is growing. Figure 1 illustrates these three sections graphically.

The three segments of a battery

The “ready” light on a charger does not mean “able;” it only indicates that the battery is fully charged. As charge acceptance decreases with time, the charge time also shortens because there is less to fill. Being ready first may hint to a dud battery, only to be picked by an unsuspecting user who placed faith in the ready light. To confuse the issue further, a partially charged battery also has a short charge time. Charge times cannot be used as a reliable indicator for battery state-of-health (SoH); discharging a fully charged battery is the most reliable method.

Product reliability falls under regulatory authorities and being labeled uncontrollable, the battery often evades the scrutiny of inspection. Meanwhile seemingly irrelevant regulatory issues are being tightened that add to the logistic overburden of a business. Authorities like to rule where laws can firmly be set.

To pass the regulatory approval, the device manufacturer picks the best battery from the lot. This satisfies the moment but it does not take fading into account as part of normal use. Once approved and released, the agency washes its hands and pass the responsibility on to the user. Few rules apply that assures continued reliability in the field. Most efforts in the approval process are geared to “birth-to-graduation” while the all-important “workforce-to-retirement” is being ignored.

Batteries should receive the same treatment as a critical part in an aircraft or machine where wear-and-tear falls under strict maintenance guidelines. Whistleblowers begin to speak up and a biomed technician said: “Batteries are the most abused components. Staff care little about them and only do the bare minimum. References to battery maintenance are vague and hidden inside service manuals.”

Device manufacturers have solved battery replacement in part with date stamping. The method is simple and moves inventory, but date stamping has flaws. Some batteries are in constant use delivering full discharge cycles, others are deployed for infrequent missions, and a third group sits on standby. To cover all cases, the battery stock is replaced after 2–3 years when most packs still have a capacity of over 90%. This adds to operational expense and causes environmental concerns.

The drop in battery capacity is gradual and the fade goes mostly unnoticed by the user. Although some batteries feature a state-of-charge indicator, the reading does not disclose state-of-health. The capacity may have dropped to 50% and delivers only half the runtime, but the fuel gauge always shows 100% after a full charge. Capacity is the leading health indicator of a battery that, in almost all cases, also determines when it should be replaced. Batteries and chargers that reveal SoC and SoH are in development.

Verifying Battery Capacity

To assure reliable service during the life of the battery, design engineers oversize the pack to provide spare capacity. This is similar to an airplane carrying extra fuel to enable a delayed landing when conditions dictate. No regulations exist as to the amount of spare capacity a battery should provide for each mission.

New batteries operate (should operate) at a capacity of 100%; replacement typically occurs when the packs fade to about 80%. Environmental conditions must also be considered as cold temperature lowers the capacity, especially with Li-ion. The capacity loss of a Li-ion (Energy Cell) is about 17%  at 0°C (32°F), 34% at –10°C (14°F) and 47% at –20°C (–4°F). Not taking cold temperatures into account can leave a rescue mission in limbo.

Systems commonly fail during emergencies when increased demand is place on the battery. During routine events, marginal batteries can hide comfortably among their peers. A system is only as good as the weakest link and battery maintenance program as part of quality control assures that all batteries in the fleet meet the minimal required performance criteria.

Figure 2 demonstrates the breakdown of a battery that includes capacity fade and spare capacity. Adding 20% for fade and 20% for spare as a safety net leaves only 60% for the actual capacity. Such a generous allowance may not be practical for all cases, and effective battery maintenance will allow for tighter tolerances.

Calculating spare battery capacity

To verify sufficient spare capacity in a battery fleet, identify older batteries that are close to retirement and spot-check their capacity after a busy day with a battery analyzer. Advanced battery analyzers (Cadex) provide a special program (Prime) that applies a discharge before charge. The first reading on the display reflects the spare capacity and the second represents the full capacity after a charge.

If older packs with fringe capacity levels come back from a full-day shift with less than 10% spare capacity, increase the pass/fail target capacity from 80 to 85% to gain five extra points. If, on the other hand, these old-timers come back with 30% before charging, keep the packs a bit longer by lowering the target capacity to, say, 70%. Modern battery analyzers (Cadex) offer adjustable capacity thresholds that can be set for each use.

Knowing the energy needs for each application during a typical shift makes power requirements transparent. This establishes the “sweet spot” between risk management and economics. Operations using such systems have reported payback periods of less than one year on battery savings alone, not to mention increased reliability and protecting our environment as fewer batteries are discarded.

Last updated 2016-07-28

About the Author

Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. For three decades, Buchmann has studied the behavior of rechargeable batteries in practical, everyday applications, has written award-winning articles including the best-selling book “Batteries in a Portable World,” now in its fourth edition. Cadex specializes in the design and manufacturing of battery chargers, analyzers and monitoring devices. For more information on batteries, visit; product information is on