No practical method exists to quantify all conditions of a battery in a short, comprehensive test. State-of-health (SoH) cannot be measured per se, it can only be estimated to various degrees of accuracy based on available symptoms. If the symptoms are vague or not present, a reliable measurement is not possible. When testing a battery, three SoH indicators must be evaluated:
- Capacity, the ability to store energy
- Internal resistance, the capability to deliver current, and
- Self-discharge, reflecting mechanical integrity and stress-related conditions
Batteries come in many conditions and a charge can easily mask a symptom allowing a weak battery to perform well. Likewise, a strong battery with low charge shares similarities with a pack that exhibits capacity loss. Battery characteristics are also swayed by a recent charge, discharge or long storage. These mood swings must be clearly identified when testing batteries.
The leading health indicator of a battery is capacity, a measurement that represents energy storage. A new battery should deliver 100 percent of the rated capacity. This means a 5Ah pack should deliver five amperes for 1 hour. If the battery quits after 30 minutes, then the capacity is only 50 percent. Capacity also supports warranty obligations with a replacement due when falling below 80 percent. Most importantly, capacity defines end of battery life.
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. Nickel-based batteries need priming to reach full capacity when new or after a long storage.
Manufacturers base device specifications on a new battery. This state is temporary and does not represent a battery in real-life situations because fading begins from the day it is made. The decrease in performance only becomes visible once the shine of a new device has worn off and daily routines are being taken for granted. An analogy is an aging man whose endurance begins to wear off after the most productive years.
Knowing when to replace a battery is a blur for many battery users. When asked, “At what capacity do you replace the battery?” most reply in confusion, “I beg your pardon?” Few are familiar with the term capacity as a measurement of runtime, and fewer know that capacity is used as a threshold for retiring batteries. In many organizations, battery problems only become apparent with increased breakdowns, which may be caused by a lack of battery maintenance.
Battery retirement depends on the application. Organizations using battery analyzers typically set the replacement threshold at 80 percent. Some industries can keep the battery longer than others and a toss arises between “what if” and economics. Scanning devices in warehouses may 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, but that is cutting it thin.