BU-801c: How to Know End-of-Battery-Life

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.

Relationship of CCA and capacity of a starter battery

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.

Courtesy Cadex

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.


Figure 2:
Battery aging as an analogy of a man growing old.

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.

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On July 5, 2011 at 9:50am
Rob Van wrote:

The soon to be available LEAD/CARBON battery by AXION purportedly addresses the needs of EV applications including quick charging in only 20% the time , 70% the weight and much greater use of capacity with deep discharge and 2 or 3 times the life of traditional lead/acid batteries. Won’t this revolutionize the battery industry and therefore dramatically affect the performance, desireability, and practicality of EVs and dozens of other applications???

On July 5, 2011 at 4:08pm
shivakorn krieng ati-butr wrote:

I   feel the items useful both   for end-users &  for   service   technicians.
I need   more& deeper informations for vocational & academic   training   in school
since   actually   very   little people   know   really what   the battery is ,how   the
battery work   and   could not   verify   exactly   the   useful   life   of   the battery.
And since battery   becomes   energy storage   source for   many   electric   power
sources   i.e,  wind power, solar and water stream &  so on, most   vehicles   must
turn to be electric   motivated   as fossil energy   crisis also pollutions awareness,
not even hybrid , there ‘ll becomes   purely   electric[battery] driven   ones.

On July 6, 2011 at 6:41pm
Roger D. Rognas wrote:

As a former (retired) developer of remote telcom battery monitoring devices I found it most informative. Thank You.

On July 28, 2011 at 7:07am
Hans Welschen wrote:

Most lead acid batteries come in packs of e.g. 6 cells. Charging - discharging control is done with only the total pack information. In practise one cell, the worst one, determines overall performance but cannot be distinguished from outside. Do we need build-in Q-mag sensors per cell and how can we influence individual cell charge like with Lithium?

On August 15, 2011 at 9:27am
Philip Canard wrote:

As a recently retired biomedical engineering technician, I had much experience in the past with battery packs in portable medical devices. The most reliable method for guaranteed battery performance was to buy premium battery packs of recent manufacture and replace them on a regular basis based on history of past performance. Each device and its battery type and method of use determined frequency of replacement. I saw major differences between battery packs manufactured in USA/Europe/Japan and those from China. Low cost battery packs from distributors who sold on price instead of quality ended up costing more in the end. I can never remember a Hewlett-Packard supplied battery pack ever failing in a heart defibrillator as long as I observed their testing procedure or replaced it at 2 years, whichever came first. Their requirement was 90% of new rated capacity. My time requirement was to take the average time I saw major problems occur, and cut that time in half. All defibrillators had a new battery installed every 2 years even if well above 90% capacity, since 4 years was the expected time to failure of that battery chemistry.

On February 1, 2012 at 2:53am
John Fetter wrote:

Rob Van’s comments, (July 5, 2011), appear to be based on “spec. picking”. The batteries he describes are based on lead-acid technology but are functionally capacitors, with very large capacities.

And like capacitors, they can be charged quickly. And like capacitors they can be cycled many, many times. And like capacitors they can work effectively at a partial state of charge. But try using them as lead-acid BATTERIES and the self-same 130 year-old lead-acid shortcomings will once again prevail.

They are good at what they do but they are not good for everything.

On May 15, 2012 at 2:31pm
ron davison wrote:

Yes it will make a big impact.
Remember batteries suck so any significant improvement will have a big impact.
Not just a capacitor but also increases conductivity and reduce graidant build up all produce extra unwanted losses.
not sure if it will supplant lithum types or more anvanced types. But for cost sensitive applications (Can you say 10,000 dollar battery pack price tag?) and for making smaller lighter lead acid batteries for all standard application…slam dunk.

On June 13, 2013 at 9:35am
Kulchaya Tanong wrote:

I want to discharge all battery before i use for my experiment leaching. I have no idea how to do it. Do i need the machine for discharging or I can do it by something else?

On June 13, 2013 at 11:59pm
Hans Welschen wrote:


You can use any load, like a lamp or resistor to discharge the battery. Keep the initial current between C5 and C10. Most important: stop discharging at 1.75V per cel and recharge immediately after reaching that point. For this detection and switching to charge mode you might need some automation. Special equipment does excist but the investment is only worth if you use it frequently. Discharging the complete battery is not a guarantee all cells are discharged equally.

On June 14, 2013 at 11:55am
Kulchaya Tanong wrote:

Hans Welschen,

For me, i will get all financial support for the experiment. I think I will buy muiltimter..or something else to discharge battery autometically. Thank for your response smile.
In fact we did some battery cutting without discharging, it was quiet dangerous because all lithium when it reacts with an oxygen or humidity, hygroden gas will be produce…we need really speacial equipment and good planification.

On June 16, 2013 at 4:01am
Hans Welschen wrote:

Dear Kulchaya

Sorry for misunderstanding you original question. My answer was based on lead acid battaries and discharging for capacity measurements.

As you said, opening Li battaries is quite dangereous and should not be done without knowledge of the material and all protective measures required.

On September 25, 2013 at 9:41am
Jose Urena wrote:

If I’m a Distributor of batteries in my country, how the manufacturer should calculate my warranty claim percentage? They are only calculating the warranty percentage of sales and not considering the units in operations or units on field of previous years. Please help.

Thanks + Best Regards

On October 7, 2013 at 2:14am
Kabindra Kaphle wrote:

I want to repair inverter lead acid battery for using computer and lights. how to measure add sulfuric acid and distilled water?

On October 7, 2013 at 3:50am
Hans Welschen wrote:

Dear Kabindra,

If there was no spillage of acid from the battery you only have to top it up with destilled water. This, asuming you have a battery with removable caps. When it is a sealed or maintenance free version, topping up is not possible. Using a battery with low fluid level could have distroyed the battery. If the battery was discharged for a longer period, it can suffer from sulfation which is hard to remove if any. I wish you luck the battery is still in good health. If so, after topping up and fully recharging the acid density shouls read between 1.24 to 1.28 depending on the type.