BU-603: How to Calibrate a “Smart” Battery

Learn why calibration is needed and how often it is required.

A common error in fuel gauge design is assuming that the battery will always stay young and that the setting will remain unchanged as the battery ages. Such an oversight creates a false sense of security by clinging to displayed data that may increasingly become inaccurate. For the casual user of a mobile phone or a laptop, fuel gauge errors will only be mild irritants but the problem is more acute with medical and military devices, as well as the electric drivetrain that depends on precise predictions to reach the destination.|

A “smart” battery can be viewed as consisting of two parts: the chemical battery and the digital battery. The chemical battery represents the energy storage vessel while the digital battery serves as peripheral support system gathering data, establishing communications and relaying the information to the user.

A “smart” battery should be self-calibrating by taking advantage of occasional full discharges and recharges, but in real life this does not always happen. The discharge is often in form of sharp random pulses that are difficult to capture. The partially discharge pack may then be partially recharged and stored at a high temperature, causing elevated self-discharge that cannot be tracked.

These anomalies contribute to errors that are inherent to all smart batteries and in time manifest themselves in false state-of-charge estimations and other discrepancies. Figure 1 shows a digital battery that is drifting away from the chemical battery; calibration periodically corrects the tracking discrepancy. The values are assumed and accentuated.

battery as a function of time

 

Figure 1: Tracking of Electrochemical and digital battery as a function of time

With use and time the electro-chemical and digital battery drift apart; calibration corrects the error.

Note: The accumulating error is application related; the values on the chart are accentuated.

To correct the tracking error that occurs, a “smart battery” should periodically be calibrated. Calibration occurs by running the battery down in the equipment until “Low Battery” appears. This sets the discharge flag and the subsequent recharge establishes the charge flag. The distances between the flags enable state-of-charge (SoC) estimations for a time. After a while, the reference lines become blurred again and the battery requires recalibration. Figure 2 illustrates the full-discharge and full-charge flags.

Full-discharge and full-charge flags

 

Figure 2: Full-discharge and full-charge flags

Calibration occurs by applying a full charge, discharge and charge. This is done in the equipment or with a battery analyzer as part of battery maintenance.

Courtesy Cadex

A better calibration method is using a battery analyzer. In addition to calibration a controlled discharge also provide the all-important capacity readings. Capacity is the leading health indicator that governs battery runtime and predicts replacement when low.

How often does a battery need calibrating? The answer depends on the application. For a battery in continued use, a calibration should be done once every three months or after 40 partial cycles. If the portable device applies a periodic full deep discharge cycles on its own, no additional calibration is needed.

What happens if the battery is not calibrated regularly? Can such a battery be used with confidence? Most smart battery chargers obey the dictates of the chemical battery rather than the digital battery and there are no safety concerns. The battery should function normally, but the digital readout may become increasingly more skewed.

The SMBus battery relies exclusively on the information obtained from charge and discharge cycles. The chemical battery is the master while the digital battery is the slave providing peripheral support. Although the digital battery enables stunning and believable readouts, the data are estimations only.

Some smart batteries come with impedance tracking that are self-learning and reduce or eliminate the need to calibrate. Calibration is still recommended and smart batteries with impedance tracking may require several discharge/charge cycles to correct the tracking error. The iPad Instruction reads: “For proper reporting of SoC, be sure to go through at least one full charge/discharge cycle per month.”


Max Error

The traction between the chemical and digital battery is measured by the Max Error. A low reading indicates good traction, and as the battery is being cycled the number tends to move up. The Max Error serves as policeman and if the number rises too high, the SMBus battery prompts for calibration. Many host devices and SMBus chargers call for calibration when the Max Error reaches 10.

Some manufacturers recommend calibration at a Max Error of 8 percent; readings above 12 percent may trigger an alarm, and higher levels could render the battery unserviceable. Although the SMBus follows an established protocol, no unified standard exist as to what Max Error level requires service or constitutes an error. Every battery manufacturer applies its own formula. Figure 3 illustrates a screenshot of the data stored in an SMBus battery.

SMBus battery

Figure 3: Screenshot of a typical SMBus battery   
Courtesy: Texas Instrument

Last updated 2015-06-08


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Comments

On March 9, 2011 at 4:40am
Bakhtawar Gul wrote:

Thanks for these lessons, it will be highly appreciated if you kindly covert these lesson in pdf format for downloading the whole document….....
Thanks

On July 22, 2011 at 12:58am
saad wrote:

dear sir ,

how i can calculate the 50 % of battery life and how i now the battery voltage when i get the 50 %

On September 5, 2011 at 10:15pm
Lydia wrote:

When applying “Full Discharge”, should we discharge battery to 0% ?
Figure 1. shows 10%. Is it means that we should start to charge battery after battery is discharged to 10% ?

On January 2, 2012 at 6:48pm
Yuttana wrote:

Can this calibration apply to lead-acid battery?

On March 11, 2012 at 11:23am
Lindsay wrote:

I have two batteries for my Canon 50d camera.  The Canon battery seems to work just fine.  The other is an aftermarket battery.  It has worked fine for about 2 years, but two days ago I charged it in anticipation of taking a number of photos yesterday.  At the start of the day I put the aftermarket battery into my camera and the camera showed it as discharged.  The original Canon battery performed for the day.

The aftermarket battery showed as completely charged when I put it on the charger at home, and checking with a simple volt meter showed a voltage of 7.4 - 8 volts.

Is there some sort of circuitry that is forcing the camera to no longer recognize this aftermarket battery?  Am I doing something wrong?  Does the camera or battery need to be ‘reset’?

Thanks for any assistance you can provide.

Lindsay

On April 8, 2012 at 5:48pm
khalil wrote:

if the battery is not calibrated, it may slow down my laptop ?

On September 22, 2012 at 10:57pm
mahabubur rashid wrote:

the article will help me to sustain making quality batteries. I will ever great full to you & battery university.

On December 9, 2012 at 10:01pm
steve h wrote:

If “calibration should be done once every three months or after 40 partial cycles”, then would this not lessen the charge cycles of a Li-Ion cell?  Full depletion of a Li-Ion will do more harm than good to the lifespan of the battery, will it not?

On December 17, 2012 at 3:58pm
Tim wrote:

@Steve H. Battery’s have a limited cycling. As you said full depletion of the battery will hurt the battery, but the Li-Ion cell reserves enough charge from sustaining permanent damage. Whether you keep it in storage or not calibrate it all, it will still degrade over time. So to save yourself from increasing your risk in losing data or your device not showing proper percentage of charge… Calibration is fine if you don’t do it often. Just think of it as going through 1 complete cycle when you do the calibration.

On December 30, 2012 at 5:44am
John Manning wrote:

Thanks 4 all the tips on how 2 Maximize Laptop Batteries

On December 30, 2012 at 1:00pm
Bart wrote:

@Tim: But this article says:
“(...) until the battery is fully depleted and “Low Battery” appears”
which means, that the full depletion, when device won’t start up, is not necessary - the battery is fully-discharged since low battery notification appears (at 10% charge?). Please correct me if I’m wrong.

On December 30, 2012 at 1:38pm
Tim wrote:

@Bart Full depletion as in the device won’t start anymore. For example, charging your phone to 100% then run it down until the device shuts itself off. That’s a full depletion cycle. Look over at other manufacturers recommendation, such as… http://support.apple.com/kb/HT1490 They also recommend doing this procedure. So, yes you would need to run it down until it totally shuts off. That’s the only way it will be able to set the appropriate flag.

If you haven’t calibrated from 100% to full depletion then the gauge wouldn’t know what charge it would be at. That’s the point of having this calibration, so that it sets the flag at the lowest and highest flag point of being full.

On December 30, 2012 at 1:43pm
Tim wrote:

@Bart In addition to my response to you. Always follow per manufactures recommendation if present. For example, the new Apple Macbook with the non-removable battery has already been calibrated from factory and does not need this procedure. I do not know the reason so I cannot answer that question, but only to postulate that they are using a different method to gauge the remaining charge of the battery. If anyone has information and link to the source it would be greatly appreciated.

On December 30, 2012 at 4:52pm
Bart wrote:

@Tim: Thank you so much for your valuable information. Everything is clear now (except Apple’s non-removable battery, so I would also appreciate if someone could post any details).

On January 9, 2013 at 11:00pm
Kevin wrote:

@Tim Once I go from 100% to the full-discharge flag (drained iPhone until it shuts off), do I need to fully charge to 100% without unplugging from power source to complete the process? Apple’s website does not specify that you need to wait until it’s 100% after you discharge it to calibrate.

Kevin

On January 10, 2013 at 5:48am
Tim wrote:

@Kevin Yes you would need to charge it right back up to 100%. Logic is still the same in terms of setting the low flag(most depleted point) and charging it to the 100% mark thereby setting the full flag.

As quoted by their webpage:
“Be sure to go through at least one charge cycle per month (charging the battery to 100% and then completely running it down).”

One charge cycle still means to charge the device back up. If you run it down and not complete the charge, your efforts would be for nothing.

On January 10, 2013 at 8:58am
Kevin wrote:

@Tim That’s what I figured, but wanted verification. Thanks for all the valuable information!

On January 10, 2013 at 11:18am
@Kevin wrote:

no problem smile

On February 2, 2013 at 2:49pm
Potato wrote:

But why does the red flag on the diagram appear at 10%? Does the battery really need to reach 0% or anywhere below 11%?

On February 9, 2013 at 2:52pm
Clintz wrote:

@KEVIN I did my first charge cycle with open all apps,turn the brightness into full,notifications on etc..

Am i right or i did it wrong?i mean it’s ok to do charge cycle while all my apps was open etc?

On February 23, 2013 at 5:57pm
Tim wrote:

@Kevin If you read here: http://batteryuniversity.com/learn/article/low_voltage_cut_off

You’ll see that lithium ion batteries in nature have a protection circuit to protect the battery from overly discharged. The graph that you see from what I gather is the cut-off of the batteries capacity. So in sense when your device states it has nearly reached 0% that means it hit the cut-off or at least close to it.

You can however hit the “True” 0% capacity of your lithium ion battery, but if you read the article on here that I provided for you… You’ll see that you are going to prematurely damage the battery.

On April 29, 2013 at 8:11am
viva wrote:

i have a xperia ion and i caliberated by a different method . but still im having issues with my battery ...As soon as the battery reaches 45% it shows the message that 1% battery is left .i tried calibration again but it didnt help ...any suggestion

On June 7, 2013 at 8:41am
Manu wrote:

@viva May be there’s a problem with the software, try using 3rd party application or try flashing the phone.

On June 26, 2013 at 4:58am
garvit dahiya wrote:

this is a really good website. it tells almost everything one needs to know about a battery. i think that you should also include article and category about li-po battery used in RC systems.

On August 28, 2013 at 11:19am
Monaem Hossen Totul wrote:

From long time I’ve worked with battery & charger’s, in these period I used same battery’s in many ways. Result is different from them, and I learned battery life depends on it’s charge & discharge conditions. From that event I decided, I’ve to fix battery maintenance policy.

On May 6, 2014 at 1:27pm
hisanr wrote:

The most adequate method to do a full discharge (100% to a minimum of 3%) consists of the following procedure:

  Fully charge the battery to its maximum capacity (100%);

  Let the battery “rest” fully charged for 2 hours or more in order to cool down from the charging process. You may use the computer normally within this period;

  Unplug the power cord and set the computer to hibernate automatically at 5% as described by the image sequence below (click images to enlarge). If you cannot select 5%, then you should use the minimum value allowed, but never below 5%;

  Leave the computer discharging, non-stop, until it hibernates itself. You may use the computer normally within this period;

  When the computer shuts down completely, let it stay in the hibernation state for 5 hours or even more;

  Plug the computer to the A/C power to perform a full charge non-stop until its maximum capacity (100%). You may use the computer normally within this period.

After the calibration process, the reported wear level is usually higher than before. This is natural, since it now reports the true current capacity that the battery has to hold charge. Lithium Ion batteries have a limit amount of discharge cycles (generally 200 to 300 cycles) and they will retain less capacity over time.

Many people tend to think “If calibrating gives higher wear level, then it’s a bad thing”. This is wrong, because like said, the calibration is meant to have your battery report the true capacity it can hold, and it’s meant to avoid surprises like, for example, being in the middle of a presentation and suddenly the computer shuts down at 30% of charge.

On August 25, 2014 at 8:04am
lule ramzan wrote:

would like to know if deep cycle batteries when stored on concrete floors or metalic casing happen to have a problem of high discharge rate. Also inform me of any other storage problems.
thank you for the lessons.

On June 2, 2015 at 11:10pm
Ashton wrote:

So today on Jun 2nd (around 11pm-12am), my LG optimus L90 stopped charging at 97% and the charger plug was still plugged in, i was wondering if i needed a new battery or is there a way to fix it?