BU-801b: How to Define Battery Life

Become familiar with battery fade and how the ready light can deceive the user.

Folks have been using rechargeable batteries for over 100 years but this marvelous power source is still poorly understood. The battery is a silent worker that delivers energy until it quits of exhaustion and old age. It is more prone to failure than most other parts in a system. Much is expected but little is given in return. With a shorter life span than the host device, battery replacement becomes an issue, and the “when” and “what if” are not well defined by the device manufacturer. Some batteries are replaced too soon but most stay too long.

A portable system works well when the batteries are new but confidence drops after the first packs need replacing due to capacity fade. In time, the battery fleet becomes a jumble of good and bad batteries, and that’s when the headache begins. Battery management mandates that all batteries in a fleet are kept at an acceptable capacity level. Packs that fall below a given threshold must be replaced to keep system integrity. Battery failure occurs most often on a heavy traffic day or in an emergency when more than normal service is demanded.

Batteries exhibit human-like qualities and need good nutrition. Care begins by operating at room temperate and discharging them at a moderate current. There is some truth as to why batteries cared for by an individual user outperform those in a fleet; studies can back this up.

Charging is generally well understood, but the “ready” light is misconstrued. Ready does not mean “able.” There is no link to battery performance, nor does the green light promise full runtime. All batteries charge fully, even if weak; “ready” simply means that the battery is full.

The capacity a battery can hold diminishes with age and the charge time shortens with nickel-based batteries and in part also with lead acid, but not necessarily with Li-ion. Lower charge transfer capability that inhibits the flow of free electrons prolongs the charge time with aged Li-ion. (See BU-409a: Why do Old Li-ion Batteries Take Long to Charge?)

A short charging time propels faded batteries to the top, disguised as combat ready. System collapse is imminent when workers scramble for freshly charged batteries in an emergency; those that are lit-up may be deadwood. (Note that the charge time of a partially charged battery is also shorter.)  Figure 1 shows the “ready” light that is known to lie.

The “ready” light lies

Figure 1: The “ready” light lies. The READY light indicates that the battery is fully charged. This does not mean “able” as there is no link between “ready” and battery performance.

Courtesy of Cadex

The amount of energy a battery can hold is measured in capacity. Capacity is the leading health indicator that determines runtime and predicts end of battery life when low. A new battery is rated at 100 percent, but few packs in service deliver the full amount: a workable capacity bandwidth is 80–100 percent. As a simple guideline, a battery on a two-way radio having a capacity of 100 percent would typically provide a runtime of 10 hours, 80 percent is 8 hours and 70 percent, 7 hours.

The service life of a battery is specified in number of cycles. Lithium- and nickel-based batteries deliver between 300 and 500 full discharge/charge cycles before the capacity drops below 80 percent.

Cycling is not the only cause of capacity loss; keeping a battery at elevated temperatures also induces stress. A fully charged Li-ion kept at 40°C (104°F) loses about 35 percent of its capacity in a year without being used. ( See BU:808: How to Prolong Lithium-based Batteries ). Ultra-fast chargers and harsh discharging is also harmful. This cuts battery life to half, and hobbyists can attest to this.

Last updated 2017-08-16

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On May 28, 2014 at 11:18pm

i am doing some practical for new lead acid battery , i develop electronic circuit which is doing good performs to increase the life of battery, so i hope any lead acid battery can double the life by attaching my circuit.

On May 2, 2015 at 8:14am
Taufique wrote:

I understood the meaning of cycle life. But i am concerned of how much time a battery takes for a complete charge/discharge cycle? Can i assume that 1 cycle=1 day? If i can then how a lead acid battery having 200 cycle life lasts for more than 5 years? Is Li-ion battery’s one cycle greater than Lead acid battery’s?

On September 17, 2015 at 2:54pm
Dixon wrote:

If I have a solar charging system that keeps my AGM batteries in “float” or “absorption” stages of charge, how do you calculate a “cycle”? Is it only when they discharge to their 50% DOD and then recharge, that an official “cycle has occurred?