BU-904: How to Measure Capacity

The traditional charge/discharge/charge cycle still offers a dependable way to measure battery capacity. Alternative methods have been tried but none deliver reliable readings. Inaccuracies have led users to adhere to the proven discharge methods even if the process is time-consuming and removes the battery from service for the duration of the test.

While portable batteries can be discharged and recharged relatively quickly, a full discharge and recharge on large lead acid batteries gets quite involved, and service personnel continue to seek faster methods even if the readings are less accurate. This section explains what’s available in new technologies, but first we look at the discharge method more closely.

Discharge Method

One would assume that capacity measurement with discharge is accurate but this is not always the case, especially with lead acid batteries. In fact, there are large variations between identical tests, even when using highly accurate equipment and following established charge and discharge standards, with temperature control and mandated rest periods. This behavior is not fully understood except to consider that batteries exhibit human-like qualities. Our IQ levels also vary depending on the time of day and other conditions. Nickel- and lithium-based chemistries provide more consistent results than lead acid on discharge/charge tests. 

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 capacity. The batteries were prepared in the Cadex laboratories according to SAE J537 standards by giving them a full charge and a 24-hour rest. The capacity was then measured by applying a regulated 25A discharge to 10.50V (1.75V/cell) and the results plotted in diamonds (Test 1). The test was repeated under identical conditions and the resulting capacities added in squares (Test 2). The second reading exhibits differences in capacity of +/–15 percent across the battery population. Other laboratories that test lead acid batteries experience similar 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.

Courtesy of Cadex (2005)

Capacity vs. CCA

Starter batteries have two distinct values, CCAand capacity.These two readings are close to each other like lips and teeth, but the characteristics are uniquely different; one cannot predict the other. [BU-806, Changes in Capacity and Resistance]

Measuring the internal battery resistance, which relates to CCA on a starter battery, is relatively simple but the reading provides only a snapshot of the battery at time of measurement. Resistance alone cannot predict the end of life of a battery. For example, at a CCA of 560A and a capacity of 25 percent, a starter battery will still crank well but it can surprise the motorist with a sudden failure of not turning the engine (as I have experienced).

The leading health indicator of a battery is capacity,but this estimation is difficult to read. A capacity test by discharge is not practical with starter batteries; this would cause undue stress and take a day to complete. Most battery testers do not measure capacity but look at the internal resistance, which is an approximation of CCA. The term approximationis correct — laboratory tests at Cadex and at a German luxury car manufacturer reveal that the readings are only about 70 percent accurate. A full CCA test is seldom done; one battery can take a week to measure.

The SAE J537 CCA test mandates to cool a fully charged battery to -18°C (0°F) for 24 hours, and while at subfreezing temperature apply a high-current discharge that simulates the cranking of an engine. A 500 CCA battery would need to supply 500A for 30 seconds and stay above 7.2V (1.2V/cell) to pass. If it fails the test, the battery has a CCA rating of less than 500A. To find the CCA rating, the test must be repeated several times with different current settings to find the triggering point when the battery passes through 7.2V line. Between each test, the battery must be brought to ambient temperature for recharging and cooled again for testing. (For CCA DIN and IEC norms, please refer to “Test Method” on this essay.)

To examine the relationship between CCA and capacity, Cadex measured CCA and capacity of 175 starter batteries at various performance levels. Figure 2 shows the CCA on the vertical y-axis and reserve capacity* readings on the horizontal x-axis. The batteries are arranged from low to high, and the values are given as a percentage of the original ratings.

Figure 2: CCA and reserve capacity (RC) of 175 aging starter batteries

Figure 2: CCA and reserve capacity (RC) of 175 aging starter batteries

The CCA of aging starter batteries gravitates above the diagonal reference line. (Few batteries have low CCA and
high capacity.)

Courtesy of Cadex

Test method: The CCA and RC readings were obtained according to SAE J537 standards (BCI). CCA (BCI) loads a fully charged battery at –18°C (0°F) for 30s at the CCA-rated current of the battery. The voltage must stay above 7.2V to pass. CCA DIN and IEC are similar with these differences: DIN discharges for 30s to 9V, and 150s to 6V; IEC discharges for 60s to 8.4V. RC applies a 25A discharge to 1.75V/cell and measures the elapsed time in minutes.

The table shows noticeable discrepancies between CCA and capacity, and there is little correlation between these readings. Rather than converging along the diagonal reference line, CCA and RC wander off in both directions and resemble the stars in a clear sky. A closer look reveals that CCA gravitates above the reference line, leaving the lower right vacant. High CCA with low capacity is common, however, low CCA with high capacity is rare. In our table, one battery has 90 percent CCA and produces a low 38 percent capacity; another delivers 71 percent CCA and delivers a whopping 112 percent capacity (these are indicated by the dotted lines).

As discussed earlier, a battery check must include several test points. An analogy can be made with a medical doctor who examines a patient with several instruments to find the diagnosis. A serious illness could escape the doctor’s watchful eyes if only blood pressure or temperature was taken. While medical staff are well trained to evaluate multiple data points, most battery personnel do not have the knowledge to read a Nyquist plot and other data on a battery scan. Nor are test devices available that give reliable diagnosis of all battery ills.


*   North America marks the reserve capacity (RC) of starter batteries in minutes; RC applies a 25A discharge to 1.75V/cell and measures the elapsed time in minutes. Europe and other parts of the world use ampere/hours (Ah). The RC to Ah conversion formula is as follows: RC divided by 2 plus 16.

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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