Knowing the health of a battery is important, but no practical method exists that can quantify all conditions in a short, comprehensive test. State-of-health (SoH) cannot be measured per se, only estimated to various degrees of accuracies based on available symptoms.
A battery behaves like a living organism that is swayed by conditions such as state-of-charge (SoC), charge and discharge events, rest periods and age. In addition, a battery with low SoC behaves similarly to a pack exhibiting capacity loss and these two symptoms become a blur. Test methods must isolate mood swings and only capture characteristics that relate to SoH. Figure 1 illustrates the usable capacity in form of a liquid that can be dispensed, the “rock content” that presents a permanent loss of capacity and the tap symbolizing power delivery as part of internal resistance.
Figure 1: Conceptual battery
Symbolizing the usable capacity, empty portion that can be refilled, permanent capacity loss as “rock content” and the tap symbolizing power delivery as part of internal resistance.
The leading health indicator of a battery is capacity; a measurement that represents the actual energy storage. A new battery delivers (should deliver) 100 percent of the rated capacity. Lead acid starts at about 85 percent and increases in capacity through use before the long and gradual decrease begins. Lithium-ion starts at peak and begins its decline immediately, albeit very slowly, while nickel-based batteries need priming to reach full capacity when new or after a long storage.
Manufacturers base device specifications on a new battery, but this is only a temporary states and does not represent a battery in real life situations. Performance will decrease with use and time, and the loss will only become visible after the shine of a new device has worn off and daily routines are taken for granted. An analogy is an aging man whose decreased endurance begins to show after the most productive years draw to an end. Figure 1 demonstrates such an aging process.
Few people know when to replace a battery; some are replaced too early but most are kept too long.
When asking battery users: “At what capacity do you replace the battery?” most would reply in confusion: “I beg your pardon?” Few are familiar with the term capacity as a measurement of runtime, and even less as a threshold when to retire them. Performance loss only becomes apparent when breakdowns begin to occur and the battery becomes a nuisance.
Battery retirement depends on the application. Organizations using battery analyzers typically set the replacement threshold at 80 percent. [See Battery Test Equipment: BU-909] There are applications where the battery can be kept longer and a toss arises between “what if” and economics. Some scanning devices in warehouses can go as low as 60 percent and still provide a full day’s work. A starter battery in a car still cranks well at 40 percent. Engine-starting only requires a short discharge that is replenished while driving, but letting the capacity go much lower may get the driver stranded without warning. No one gets hurt if a battery cuts off a phone call, but a failing medical device can put a patient at risk. Running out of power in an industrial application can also incur high logistic costs.
The best indicator for battery retirement is checking the spare capacity after a full shift. The Cadex battery analyzers (www.cadex.com) do this by applying a discharge before charge. A battery should have 10 to 20 percent spare at the end of a day to cover unknowns and emergencies. If the lowest performing battery in the fleet comes back with 30 percent, then the target capacity can safely be lowered from 80 percent to 70 percent. Knowing the energy requirement creates a sweet spot between risk management and economics.
Let’s take a drone that is specified to fly for 60 minutes with a good battery. Unknown to mission control, the capacity may have dropped to 75 percent, reducing the flying time to 45 minutes. This could crash the $50,000 vehicle when negotiating a second landing approach. With the reserve capacity marked on each pack, batteries delivering close to 100 percent can be assigned for long hauls while older packs may be sent for shorter errands. This allows the full use of each battery and establishes a sound retirement policy based on application. The analyzer’s label print option enables this feature. [See How to Maintain Fleet Batteries: BU-810c]
Many batteries and portable devices include a fuel gauge. [See Battery fuel Gauge: BU-602] While this shows the amount of energy left during use, the readout only measures the remaining charge; capacity estimation is sketchy. SoC always shows 100 percent after a full charge whether the battery is new or faded. This creates a false sense of security by assuming that a fully charged battery will always deliver the anticipated runtime. Runtime data get inaccurate with use and time and the battery needs calibration. [See Battery Calibration: BU-603]
In the absence of maintenance, some device manufacturers mandate to replace a battery on a date-stamp or cycle count. A pack may fail before the appointed time but most last far longer, prompting perfectly good batteries to be discarded prematurely. Dr. Imre Gyuk, manager of the Energy Storage Research Program at DOE, says that “every year roughly one million usable lithium-ion batteries are sent in for recycling with most having a capacity of up to 80 percent.” Lack of suitable battery diagnostics also affects heathcare. An FDA survey says that “up to 50% of service calls in hospitals surveyed relate to battery issues.” Healthcare professionals at AAMI say that “battery management emerged as a top 10 medical device challenge.” (AAMI stand for Association for the Advancement of Medical Instruments.)
Batteries do not exhibit visible changes as part of usage; they look the same when fully charged or empty, new or old and in need of replacement. A car tire, in comparison, distorts when low on air, shows signs of wear, and indicates end-of-life when the treads are worn. Batteries should receive the same treatment as a critical aircraft part, a medical device and an industrial machine where wear and tear falls under strict maintenance guidelines. Authorities struggle to implement such procedures for batteries, but lack of suitable test technology makes this almost impossible. Bad batteries thus enjoy immunity as they can hide comfortably among the peer. It is no wonder then that batteries escape the scrutiny of vigorous inspection and are declared “uncontrollable.”
Battery analyzers are effective in managing small to mid-sized batteries with a discharge/charge function; rapid-test methods are available for single Li-ion cells. Testing and monitoring technologies are being developed for larger batteries used in vehicles and stationary applications but the advancements seem slow. It appears not much has changed since the invention of the lead acid battery by Gaston Planté in 1859. We don’t even have a reliable method to measure state-of-charge; not to mention attaining accurate capacity assessments as part of rapid-testing. Simply measuring voltage and internal resistance, as was done in the past, is no longer sufficient to estimate SoC and battery capacity today.