BU-903: How to Measure State-of-charge

Explore SoC measurements and why they are not accurate.

Voltage Method

Measuring state-of-charge by voltage is the simplest method, but it can be inaccurate. Cell types have dissimilar chemical compositions that deliver varied voltage profiles. Temperature also plays a role. Higher temperature raises the open-circuit voltage, a lower temperature lowers it, and this phenomenon applies to all chemistries in varying degrees.

The most blatant error of voltage-based SoC occurs when disturbing the battery with a charge or discharge. This agitation distorts the voltage and no longer represents the true state-of-charge. To get accurate measurements, the battery needs to rest for at least four hours to attain equilibrium; battery manufacturers recommend 24 hours. Adding the element of time to neutralize voltage polarization does not sit well with batteries in active duty. One can see that this method is ill suited for fuel gauging.

Each battery chemistry delivers a unique discharge signature that requires a tailored model. While voltage-based SoC works reasonably well for a lead acid battery that has rested, the flat discharge curve of nickel- and lithium-based batteries renders the voltage method impracticable. And yet, voltage is commonly used on consumer products. A “rested” Li-cobalt of 3.82V/cell in open circuit indicates a SoC of roughly 40 percent.

Figure 1: Discharge OVC of typical cobalt-based Li-ion
After charge or discharge, the battery needs several hours rest for the voltage to neutralize.

The discharge voltage curves of Li-manganese, Li-phosphate and NMC are very flat, and 80 percent of the stored energy remains in this flat voltage profile. This characteristic assists applications requiring a steady voltage but presents a challenge in fuel gauging. The voltage method only indicates full charge and low charge and cannot estimate the large middle section accurately. 

 Lead acid has diverse plate compositions that must be considered when measuring SoC by voltage. Calcium, an additive that makes the battery maintenance-free, heat raises the voltage by 5–8 percent. Temperature also affects the open-circuit voltage; heat raises it while cold causes it to decrease. Surface charge further fools SoC estimations by showing an elevated voltage immediately after charge; a brief discharge before measurement counteracts the error. Finally, AGM batteries produce a slightly higher voltage than the flooded equivalent.

When measuring SoC by open circuit voltage, the battery voltage must be truly “floating” with no load present. Installed in a car, the parasitic load present makes this a closed circuit voltage (CCV) condition that will falsify the readings. Adjustments must be made when measuring SoC in the CCV state by including the load current in the calculation. In spite of the notorious inaccuracies, most SoC measurements rely on the voltage method because it’s simple. Voltage-based state-of-charge is popular for wheelchairs, scooters and golf cars.


The hydrometer offers an alternative to measuring SoC, but this only applies to flooded lead acid and flooded nickel-cadmium. Here is how it works: As the battery accepts charge, the sulfuric acid gets heavier, causing the specific gravity (SG) to increase. As the SoC decreases through discharge, the sulfuric acid removes itself from the electrolyte and binds to the plate, forming lead sulfate. The density of the electrolyte becomes lighter and more water-like, and the specific gravity gets lower. Table 1 provides the BCI readings of starter batteries.

Approximate state-of-charge

specific gravity

Open circuit voltage









8. 30


Table 1: BCI standard for SoC estimation of a maintenance-free starter battery with antimony. The readings are taken at room temperature of 26°C (78°F); the battery had rested for 24 hours after charge or discharge.

While BCI specifies the specific gravity of a fully charged starter battery at 1.265, battery manufacturers may go for 1.280 and higher. When increasing the specific gravity, the SoC readings on the look-up table will adjust upwards accordingly. Besides charge level and acid density, the SG can also vary due to low fluid levels, which raises the SG reading because of higher concentration. Alternatively, the battery can be overfilled, which lowers the number. When adding water, allow time for mixing before taking the SG measurement.

The specific gravity also varies according to battery type. Deep-cycle batteries use a dense electrolyte with an SG of up to 1.330 to get maximum runtime; aviation batteries have a SG of 1.285; traction batteries for forklifts are at 1.280; starter batteries come in at 1.265 and stationary batteries are at a low 1.225. Low specific gravity reduces corrosion. The resulting lower specific energy of stationary batteries is not as critical as longevity.

Nothing in the battery world is absolute. The specific gravity of fully charged deep-cycle batteries of the same model can range from 1.270 to 1.305; fully discharged, these batteries may vary between 1.097 and 1.201. Temperature is another variable that alters the specific gravity reading. The colder the temperature is, the higher (more dense) the SG value becomes. Table 2 illustrates the SG gravity of a deep-cycle battery at various temperatures.

Temperature of
the Electrolyte

Gravity at full charge

Table 2: Relation of specific gravity and temperature of deep-cycle battery
Colder temperatures provide higher specific gravity readings.




Errors can also occur if the acid has stratified, meaning the concentration is light on top and heavy on the bottom. [BU-804, Acid Stratification] High acid concentration artificially raises the open circuit voltage, which can fool SoC estimations through false SG and voltage indication. The electrolyte needs to stabilize after charge and discharge before taking the SG reading.


Coulomb Counting

Laptops, medical equipment and other professional portable devices use coulomb counting as a SoC indication. This method works on the principle of measuring the current that flows in and out of the battery. If, for example, a battery was charged for one hour at one ampere, the same energy should be available on discharge. This is not the case. Inefficiencies in charge acceptance, especially towards the end of charge, as well as losses during discharge and storage reduce the total energy delivered and skew the readings. The available energy is always less than what had been fed to the battery, and compensation corrects the shortage.

Disregarding these irregularities, coulomb counting works reasonably well, especially for Li-ion. However, the one percent accuracy some device manufacturers advertise is only possible in an ideal world and with a new battery. Independent tests show errors of up to 10 percent when in typical use. Aging causes a gradual deviation from the working model on which the coulomb counter is based. The result is a laptop promising 30 minutes of remaining runtime and all of a sudden the screen goes dark. Periodic calibration by applying a full discharge and charge to reset the flags reduces the error. [BU-603, Calibration]

There is a move towards electrochemical impedance spectroscopy and even magnetism to measure state-of-charge. These new technologies get more accurate estimation than with voltage and can be used when the battery is under load. Furthermore, temperature, surface charge and acid stratification do not affect the readings noticeably.

Last updated 12/29/2014

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On September 21, 2011 at 12:51pm
Joe Accetta wrote:

There is yet another and quite accurate means of measuring SOC in open port lead acid batterieis through the use of refractometry. The index of refraction of the electrolyte is directlly proportional to the SG. JSA Photonics has developed an in-cell immersion refractometer that yields accurate 24/7 SOC information along with cell temperature for monitoroing and temp correction and in some instances electrolyte level. The sensor replaces the existing battery cap. See www.jsaphotonics.com.

On October 13, 2011 at 4:02am
Josep wrote:

I’m investigating about, and I read that the most common, easy, and accurate way of measuring OSC is Ampere Hour Counting.


On November 29, 2011 at 7:15am
Qaisar Azeemi wrote:

Its really nice information about SOC is given here. But i request to please include measurement steps for all methods mentioned above.
Thank you

On January 20, 2012 at 5:05am
kishore kumar wrote:

Its good to know about soc. There is metal detector to detect metal likewise I need to know how to detect a battery? Can anyone help me.

On January 25, 2012 at 1:34pm
Diego wrote:

the problem of establishing the SOC is not trivial, expecially with LiFePO4 batteries. An interesting article “Fine Tuning TI-impedence Track (TM) battery fuel gauge with LiFePO4 cells in shallow discharge application” can be found at this link


On February 20, 2012 at 2:11am
Kumar wrote:

Please provide me the positive and negative plate behaviour of a lead acid battery during charge & discharge when it is measured with a cadmium rod as reference.

On June 22, 2012 at 1:09pm
TokenGimp wrote:

Assuming I need 2 gel batteries for a power wheelchair and they suggest 12V 75AH. Typically that’s good for 30 miles of rolling on a street..

How much less mileage would I get out of it using 55AH batteries under similar conditions?
I’m trying to decide if I should buy 55AH, 75, 90 or 115AH deep cycle gel if they will fit in the same area.

On June 23, 2012 at 2:17am
Josep wrote:

To TokenGimp,

If they told you 75AH is good for a 30 miles of rolling on a street, you can make rule of three: if 75->30, then 55-> 22miles, 90->36miles, 115->46miles.

That’s is not exact, but I think you can make an idea about how long.  You take the one you think will use.

I hope I was useful.


On August 30, 2012 at 3:29pm
P.F. wrote:


The metal detector use a General Electric MAGNISTOR , Ihave the chematic .

Error :A lead baterie contains sulfate no sulfite
SO4— no SO3 .

On September 6, 2012 at 6:15pm
Mark wrote:

I have a 24V lead acid battery (for a fork truck). I measure 9.0V from the negative terminal to the metal case. Can you suggest what might be causing this? I expected to find 0.0V.

Thank you,

On October 17, 2012 at 3:27am
kingk wrote:

what is the name of the composition of the battery positive and negative plates in a typical lead acid battery when in a charged condition ?

On January 13, 2013 at 11:45am
Sheldon Robidoux wrote:

The 1.98v number here for 0% charge doesn’t seem to line up with the 1.75 figure given for full discharge in another part of this site.  What am I missing?

On June 15, 2013 at 8:59pm
R.ashok kumar wrote:

Given every thing fine automative battery graveti explain detail

On July 10, 2013 at 10:13pm
Chetan Upadhyay wrote:

Nice detailing. Can anyone tell @ Impedance Spectroscopy requirements?

On October 1, 2013 at 3:11am
Jero wrote:

I have a question, is there not a relationship between the pH of the acid and the state of charge? I can’t seem to find anything on the net about this, but it is intuitive for me for them to have a relation, as the chemical reactions occur which also change the SG.

On November 2, 2013 at 12:03pm

i have 4 electric bike vrla based batteries showing 13.0v ocv, on 3 times load test shows 10.8v, but when put on road, does not give mileage.  even after discharge the ocv reduces momentarily but again the ocv goes upto 13.0v.  so while connecting a charger the battery gets charged much faster.  can anyone help and explain what is wrong inside the battery.
the batteries are hardly one year old.

On July 26, 2014 at 5:31am
peter carvalho wrote:

very good info.. thanks

On August 13, 2014 at 11:30pm
sarvani wrote:

Is there any relationship between the pH of the acid and the specific gravity?

On August 29, 2014 at 6:13am
satheesh wrote:

hi i want know the program for how to measure the battery level and indicator can any know??

On March 28, 2015 at 2:54pm
Lucy wrote:

What is the correlation of the acid level (in mm) of each cell of a SLI battery to its determination of defectiveness?