BU-904: How to Measure Capacity

Learn about various test methods and why none is fully satisfactory.

Capacity is the leading health indicator of a battery but estimating it on the fly eludes scientists. The traditional charge/discharge/charge cycle still offers a dependable way to measure battery capacity. While portable batteries can be cycled relatively quickly, a full discharge and recharge on large lead acid batteries is not practical. Scientists continue to seek fast and non-intrusive test methods even if the readings are less accurate and applying a full discharge. This section explains what’s available in new technologies.

Discharge Method

One would assume that capacity measurement by discharge is the most accurate method but this may not always be the case, especially with lead acid batteries. Even when using highly accurate equipment in a temperature controlled environment and following established charge and discharge standards, variations between identical tests occur. This is not fully understood except considering that batteries exhibit human-like qualities. Our IQ level also varies depending on the time of day and other conditions. Lithium- and nickel-based chemistries provide more consistent discharge results than lead acid. 

To verify the capacity on repeat tests, Cadex checked 91 starter batteries with diverse performance levels and plotted the results in Figure 1. The horizontal x-axis shows the batteries from weak to strong, and the vertical y-axis reflects the capacity. The tests followed SAE J537 standards by applying a full charge and a 24-hour rest, followed by a regulated 25A discharge to 10.50V (1.75V/cell). The results in diamonds represent Test 1. The test was repeated under identical conditions and the capacities shown in squares embody Test 2. Although done within days of each other, Test 1 and 2 differ much as +/–15 percent in capacity. Other laboratories are also observing these discrepancies.

Capacity fluctuations on two identical charge/discharge tests of 91 starter batteries

Figure 1: Capacity fluctuations on two identical charge/ discharge tests of 91 starter batteries
The capacities differ +/–15% between Test 1 and Test 2. Tests were done according to SAE J537
Courtesy of Cadex (2005)

When evaluating battery test results, the question is asked: “Against what standard are the readings compared?” If the classic charge/discharge provides inaccuracies, assessing a modern test technology is put in question by asking: “Which method is more correct?” Could electrochemical impedance spectroscopy be better than a discharge/charge cycle, we ask? This might be true in some cases but not in all.

Capacity vs. CCA

Starter batteries have two distinct values, CCA and capacity. These two readings are close to each other like lips and teeth, but the characteristics are uniquely different; one cannot predict the other and correlation between the two is almost non-existent. (See BU-806, Tracking Battery Capacity and Resistance as part of Aging)

Most rapid-testers look at the internal resistance, which is an approximation of CCA. Reading battery resistance is relatively simple but this alone cannot predict capacity, nor can it tell when to replace a battery as end-of-life is primarily capacity related. A battery will crank the engine as long as there is enough capacity to do so and a sudden failure might occur when the capacity drops below 30 percent. A low capacity with good CCA does not necessarily show down cranking until all energy is spent.

Some battery testers, including Spectro™, display a “Resistance High” error when the internal resistance is elevated. A working starter battery provides single-digit mOhm values that are represented by R1 in the Randles model on the right. (See BU-902: How to Measure Internal Resistance) Batteries developing high resistance move into double-digit readings caused by any or all of the following conditions:
Randles Model
  1. Low electrolyte level (See BU-804C: Water Loss, Acid Stratification and Surface Charge)
  2. Stratification of electrolyte (See BU-804C: Water Loss, Acid Stratification and Surface Charge)
  3. Sulfation of electrodes (See BU-804b: Sulfation and How to Prevent it)
  4. Bad or deteriorated weld connections of the collector plates and posts
  5. Collector plate cracking corroded (See BU-804a: Corrosion, Shedding and internal Short)
  6. Poor battery connection at the clamps or internal to the battery

The dominant quantity for internal resistance should be R1. R1 represents the electrolyte resistance, which is affected by points 1 and 2 above. Points 3 to 6 affect the contact resistance from the battery posts to the electrodes, or the electrodes to the electrolyte that adds to the R1 resistance.

R2 represents the charge transfer resistance, or the speed at which electrons are transferred from the electrodes to ions in the electrolyte. Being part of the inner battery, R2 is immune to the outer mechanical conditions and should have a lower resistive value than R1.

Being able to separate the individual components in the Randles model allows for a more comprehensive battery test than is possible with a single reading. An analogy is a doctor who must rely on several tests to diagnose the illness of a patient. Combining capacity, CCA and high resistance in an all-encompassing measurement, as Spectro™ does, enables improved battery assessment that will reduce unnecessary battery replacement, especially during the warranty period. (See BU-901: Difficulties with Testing Batteries)

SAE specifies the capacity of a starter battery by reserve capacity (RC). RC reflects the runtime in minutes at a steady discharge of 25A. DIN and IEC assess the battery in Ah and measure the runtime at a typical discharge rate of C/20 (5h rate) for starter batteries. A 60Ah would discharge at 12A.

No accurate RC to Ah conversion exists but the most common formula is RC divided by 2 plus 16. A short method is dividing RC by 1.9. Differences in discharge current produce some inaccuracies.

A full CCA test is tedious and is seldom done. CCA cannot be “measured” but only “guessed” and the process can take a week per battery. To test CCA, apply different discharge currents to see which amperage keeps the battery above a set voltage while in frozen state. Table 2 illustrates the procedures of SAE J537, IEC and DIN. The methods are similar and only differ in the length of discharge and the cut-off voltages.

Fully charge battery according
to SAE J537 and cool to -18°C (0°F) for 24 hours. While at subfreezing temperature, apply a discharge current equal to the specified CCA. (500 CCA battery discharges at 500A.) To pass, the voltage must stay above 7.2V (1.2V/cell) for 30 seconds.
Fully charge battery according
to SAE J537 and cool to -18°C (0°F) for 24 hours. While at subfreezing temperature, apply
a discharge current equal to the specified CCA. (500 CCA battery discharges at 500A.) To pass, the voltage must stay above 8.4V for 60 seconds.
Fully charge battery according
to SAE J537 and cool to -18°C (0°F) for 24 hours. While at subfreezing temperature, apply a discharge current equal to the specified CCA. (500 CCA battery discharges at 500A.) To pass, the voltage must stay above 9V for 30s and 6V for 150s.

Table 2: CCA test methods of SAE, IEC and DIN. The methods differ in the length of discharge and the cut-off voltages.

Advancement in electrochemical impedance spectroscopy (EIS) makes estimation of battery capacity possible. The non-invasive technology known by its trademark Spectro™ combines EIS with complex modeling to attain capacity, CCA and SoC with the help of a matrix. Here is how it works:

A sinusoidal signal of multiple frequencies is injected into the battery at a few millivolts. After digital filtering, the extracted signal produces a Nyquist plot onto which Nyquist plots for various electrochemical models are superimposed. The best matching electrochemical models within allotted margins are selected; non-fitting models are rejected. Data fusion correlates the values of the key parameters to derive at capacity and CCA estimations. Figure 3 illustrates the patented process in a simplified way.

Data Fusion Figure 3 Spectro™ combines EIS with complex modeling to estimate battery capacity and improve CCA measurements

A sinusoidal signal produces a Nyquist plot; data fusion correlates the values of the key parameters to estimate capacity and CCA.

“How accurate are the readings,” car mechanics and battery users ask? This depends on the tester and the test methods used, as well as the condition of the batteries. A battery fault can only be diagnosed if measurable symptoms are present and these indicators can get muddled on brand new batteries and those that have been in storage.

A test algorithm can be turned to optimize batteries in regular service, but this could give false readings on a new battery. The art is to correctly identify all living batteries, new and old, in service and rested, fully charged and partially empty. Although a test device can be optimized to give high accuracy on a given battery type, service technicians prefer a generic solution even if the hit rate is slightly lower. 

Batteries are marked with capacity in Ah (or RC in minutes) and starter batteries also include CCA. These published values are assumed correct, but this may not always so. The CCA of some starter batteries can be higher or lower than marked and few consumers verify the readings. In addition, deep-cycle batteries show low capacity when new but the readings will strengthen with use as the battery is being formatted.

Last updated 2015-06-02


*** Please Read Regarding Comments ***

Comments are intended for "commenting," an open discussion amongst site visitors. Battery University monitors the comments and understands the importance of expressing perspectives and opinions in a shared forum. However, all communication must be done with the use of appropriate language and the avoidance of spam and discrimination.

If you have a question, require further information, have a suggestion or would like to report an error, use the "contact us" form or email us at: answers@cadex.com. While we make all efforts to answer your questions accurately, we cannot guarantee results. Neither can we take responsibility for any damages or injuries that may result as a consequence of the information provided. Please accept our advice as a free public support rather than an engineering or professional service.

Or Jump To A Different Article

Basics You Should Know
The Battery and You
Batteries as Power Source


On March 30, 2011 at 5:26am
arup kumar hbussain wrote:

how can we calculate a 12volt/ 80ah battery efficiency is 90%(lead acid battery)

On November 22, 2011 at 8:07pm
nick bell wrote:

can i get tables or graphs or info on depth of discharge vs voltage .. i have 12 x 2v   1550 amp hour batteries and want to be able to monitor them if possible with a volt meter

On December 6, 2011 at 11:24am
Marcus wrote:

figure 1 doesn’t make sense. If capacity readings are +-15% inaccurate, how could the first test results draw such a perfect line?

On February 13, 2012 at 8:06am
hisannah wrote:

Plan is to install an 89 amphour agm for a bilge pump which draws 7 amps. Recharge will be with a solar panel. Runtime is estimated/desired to be 2 hours total, (intermittent 15 minute cycles) service. Will the battery be sufficient? What size solar output is needed to recharge in three days and still remain connected to float the battery?

The pump flowrate varies in direct proportion the speed.  Speed varies to the square of the voltage. Its pressure varies to the cube of the speed.  What is the expected fall-off of the voltage versus time? With this I can calculate the real-time flow and pressure and obtain a useful life of the pump without destrying the battery.

Anybody care to submit their ideas on this

On July 17, 2012 at 3:32pm
Wayne wrote:

Marcus,  the first line is straight because the graph was constructed by placing the 91 batteries in order of weak to strong based on the first test.  On the second test the batteries remain in the same order but the inacuracies cause the data points to spread out.

On April 17, 2013 at 3:58am
s. mateen wrote:

We have stored Lead acid batteries for about 12 months without charging and now we want to use them. We have measured the voltage of the batteries are more than 12.25vdc per battery (100 ah).

Are there any risk of using them? What should we do before using them? Should we just throw them away?

Please help me urgently. I need help to make the decision. Thanks a lot.

On April 27, 2013 at 6:03am
Robert H. wrote:

s. mateen:  I am a novice, but…1.25 is almost fully discharged.  The longer they were stored at this low open voltage, the greater likelyhood of a weakened battery.  The best option from my perspective would be to remove the batteries and gently rock them side to side for 3 - 4 minutes, check the acid levels (use only distilled water - assuming no battery leaks) and verify acid levels are at covering the plates.  If the levels are low but covering the plates, completely charge with a good quality smart charger (battery minder, schumacher), then check the specific gravity; I would want no more total variation of 0.02 from the stongest cell to the weakest.  Assuming an “o.k.” S.G. reading, finish filling the cells as needed (do not overfill), and charge again with a 2 amp smart charge until completely charged.  Be sure your smart charger is set for either “deep cycle” or “marine”, or “STD”, but for sure NOT AGM or GELL! Recheck S.G. - it should be 1.25 minimum on any one cell (in the green of the hydometer) and still withing the 0.02 S.G. variance maximum bron the best to the worst cell.  If this is good, follow battery universities’ instructions for measuring each cells individual voltage which I would want to vary by no more than 0.02.  If you can, then load test the battery with no more than a 25 amp draw. and it should test good for at least 3 1/2 hours.  If still good, repeat the 2 amp charge to full charge, and put into service.  There are a number of other tests, and if this is totally foriegn to you, have a qualified battery service check the batteries for fitness to use.  Always abide by safe battery handling precautions; eye, vapor, and skin protection, etc.  I believe this is all good advice and safe, but only if done correctly and by someone comfortable and familiar with batteries.  No guarntee though, and there are risks with handling batteries, so I must disclaim any responsibility for this information, “at your own risk”.  Please verify this information with someone qualified if your unsure of what your doing.

The short alternative version: Take them to a service shop with a good reputation, and have them tested and serviced before using or depending on them.

On April 27, 2013 at 6:16am
Robert H. wrote:

s. mateen:  I wrote the above for deep cycle or rv/marine start battery use because you mentioned “100 ah”, but if your battery is an auto battery or some other starting battery, simply do everything as described up to the load test point, then change the load test to accomodate a starting battery - which is a load test based on the CCA, or take it to your local auto parts store for them to check the batteries after you have charged, filled, topped off the charge, and let the battery rest for a full 24 hours.  If it doesn’t test good after this, my guess is it may need professional charging/servicing to be saved (if that will do it).

On April 27, 2013 at 6:33am
Robert H. wrote:

s. mateen: sorry for being the mistype and vagueness on my first post.  I meant to say 12.25 vdc is an almost dead battery, and the variances of 0.02 apply to both specific gravity differences on first mention, and 0.02 vdc variances on the individual cell voltage checks.  Btw, indidvidual cell voltages should be 2.1 (typically considered full charge on a servicable lead acid battery) on individual cells, but in no case would I be comfortable with less than 2.09 vdc (about 75% charge) if you want any measure of dependability. All measurements should be taken only when any charging has been completed for at least 24 hours.

On June 4, 2013 at 6:40am
Randolf de Leeuw wrote:

If the capacity fluctuations of a starter batterie is +/- 15%, wath wil be the fluctuation if you mesure this by li-ion betteries?

On March 6, 2014 at 10:00pm
Doug Devries wrote:

Article states Ah = RC / 2 + 16.  This seems incorrect. 
RC is defined as the time in minutes that a 12V battery can deliver 25A while maintaining 10.5V our greater.  Therefore:
Ah = 25 A * RC (minutes) / 60 min / h OR
Ah = RC * 0.416667

A battery with a RC of 120 minutes has a measured capacity of 50 Ah when discharged at 25A

The RC / 2 + 16 method of capacity calculation yields 76 Ah - clearly incorrect.

Can anyone explain the origins of the RC / 2 + 16 = Ah calculation?

On March 9, 2014 at 9:52am
mo wrote:

Is the internal construction of an IEC battery different than a CCA?
If I had both batteries infront of me but didn’t know wich one was wich, what would be a good bench mark test rating to use. I use a midtronics exp800 and have nine ratings to pick from.
If I test an IEC or EN battery with CCA or SAE rating the amp measurement is drasticly higher
Does one battery discharge differently than the other given the same conditions? thamks m

On March 11, 2014 at 6:34pm
AW wrote:

In relation to Doug Devries comment, I am unsure of the origin of the Ah = RC/2 + 16, but I imagine it is empirically determined. I have also seen Ah = 0.6*RC.

Your equations are correct, but this will always be an underestimation of the actual Ah rating. The reason for this is that the 25A current draw on the RC test will be higher than that used to determine the Ah rating. Because you will get more energy out of a battery if the total current draw is lower for a longer period of time (Peukert’s Law), your conversion will underestimate RC.

Hope that makes sense.

On April 24, 2014 at 5:16am
Nick Bordujenko wrote:

G’day all, I’d like to share my limited knowledge with some of you, if I may.

Most people I worked with often asked me how long does a car battery last? Well, how long is a piece of string? - Ambient temp effects how a battery performs, & how long it will last. The rule of thumb is the hotter the climate, the better cranking you will get out of the battery albeit with a shorter lifespan, but the opposite effect for colder climates - the battery will have a longer lifespan, but with less performance.

The quality of materials used to construct a battery internally has a lot to be said for. The Panasonic batteries that come out of Japan in Toyota’s are a great battery.  I have had 6 years out of one in my Toyota Hilux. I have also seen an Optima Red Top 34R starting SLAB get 8 years plus in the tropics. So choosing the right battery is just as important as maintaining it correctly. I believe it is fair to say that - You only get what you pay for.

I believe that it would be fair to say that most people’s knowledge or understanding of how to keep a battery in peak condition is pretty poor, either by lack of education or plain ignorance. Lack of regular maintenance charging, (short drives to and from work), and lack of maintenance in topping up water (if applicable) leads to a shorter battery life by the operator. A really good indication that your battery is sulphated, and is on the way out is a simple visual check. If the battery’s end cases are visually swollen, (bowed out), than the battery is sulphated and it’s life span has been severely shortened. You would expect to up for a new battery in the not too distant future.

The sure fire way of keeping your battery in ship shape is to regularly charge it with a smart charger, I personally use a CTEK 7 amp charger, but any other brand will do the job. Also if the battery is a vented lead acid battery (the type where distilled water is required to top up the cells), then regular inspections and top ups should be done iaw your vehicle’s or battery’s manufacturer’s recommendations. If you allow the cells to become exposed, you will kill a battery.

In the workshop, we kept our batteries fully charged, and never let the batteries open circuit (O/C) voltage to drop below 12.5 volts. This is where most people especially folks with fishing or ski boats get into trouble, especially neglecting their batteries and they go flat.. A fully charged battery, left sit for 24 hours should have a O/C voltage = to or greater than 12.5 to 12.6 volts. This is 100% charged. If you have a battery that loses voltage relatively quickly over a few days to a week, than you have some internal resistance discharging your battery - There could also be some minor current draw, so isolating your battery in these situations is recommended. This could mean that the battery is suffering from sulphation, and it’s time to get yourself a replacement battery pronto. When a battery’s voltage drops below 12v, then sulphation starts to build up on the cells. when this happens, your batteries internal capacity decreases. It is possible to do a load test of the battery via a carbon pile load tester, this is the best way I have found. Load a fully charged starting battery up to half the battery’s CCA rating for 10 - 15 sec. As long as the battery stays above 9.6v, then it’s serviceable, if not, charge and re-test. For a vented lead acid battery, using a hydrometer, check the specific gravity. If any of the cells are in the red, or are 50 points or more differnce, it’s time to get a new battery.
If you don’t have access to a carbon pile load tester, an easy way of doing a load test on your vehicles battery yourself on your car is to get a volt meter and measure the battery’s voltage prior to starting, and then during starting. As long as the battery’s voltage is healthy (above 12.5v) prior to starting and it doesn’t drop below 9.6 volts during the cranking of the starter motor, then you have yourself a serviceable battery.

Unfortunately there is no way knowing how much life you have left in your battery at any given point of time. It is important to remember to keep on top of the maintenance of your battery and keep it charged regularly, at least monthly, for a longer battery life.

A rough, but easy way to work out your batteries state of charge - O/C voltage is:
12.6v = 100% (Fully Charged)
12.5v = 90%
12.4v = 80%
12.3v = 70%
12.2v = 60%
12.1v = 50%
12.0v = 40%
11.9v = 30%
11.8v = 20%
11.7v = 10%
11.6v = 0% (Flat Battery)
Cut off voltage (DEAD BATTERY) = 10.5v

As you can see, 1 volt is all there is between a fully charged battery and a flat one, and shows the importance of keeping your battery charged, not only for a longer battery life, but to prevent an embarrassing situation of having a flat battery, and it always happens at the worst possible time, doesn’t it?. Cheers, Nick. 

On July 18, 2014 at 12:52pm
ziad wrote:

exclent site

On September 22, 2014 at 10:32pm
Zack Philip wrote:

Your site is fantastic…

How can i convert 350CCA to AMPHours?

On October 16, 2014 at 11:35pm
prakash wrote:

i need to find cca value of 12v,42ah battery
i need full calculaton

On December 29, 2014 at 2:17am
Ruparathna Amuwala wrote:

I need to know how to find CCA value of 12 v 200ah lead acid battery

On July 4, 2015 at 6:44pm
B.V.S.Prakash wrote:

26400mah li-ion

How do I convert in v / ah

On August 6, 2015 at 11:44am
Ashvin wrote:

How I can undestabilising 100 ah battery take how much load..?

On September 23, 2015 at 9:47am
timm wrote:

SAE specifies the capacity of a starter battery by reserve capacity (RC). RC reflects the runtime in minutes at a steady discharge of 25A. DIN and IEC assess the battery in Ah and measure the runtime at a typical discharge rate of C/20 (5h rate) for starter batteries. A 60Ah would discharge at 12A.

C/20 means 20 hour rate… 60ah discharge 3a in 20 hours….
Isnt it?

On October 6, 2015 at 1:50am
HW_Freak wrote:

My Load Power is 10W.
Battery : 4.2V max, 1800mA max
          3.7V nominal

What will the load current on the Li-ion Battery..?
Can anyone pl let know..

On October 25, 2015 at 11:29am
abdelmoniem hamed wrote:

Very informative. Can you send me quatation for atester to carry tests on lead acid storage batteries for capacity and cold cranklng for batteries up to 200 A/H.

On October 25, 2015 at 11:43am
abdelmoniem hamed wrote:

Very informative .Iwould grateful of could send me quotation for atester to carry capacity and cc/a test on autombile battaries cap. Up to 200A/H.

On November 17, 2015 at 3:36am
kiran wrote:

pls guide me ias i want to design battery analyser for my handheld radios battery 7.2vols .