by Charles Miller

There is one element that is found on the screen of every portable device. I refer here to cell phones, laptop computers, tablets, etc. That element is a battery meter. Some battery meters read out a numeric percentage while others might show bars or a small graph. Something I wish was included together with all of these is an asterisk. Putting the * next to the battery meter would serve to remind everyone that what you see there is a SWAG.

Across many technical disciplines, technicians know that SWAG is short for Systematic Wild-Ass Guess. The information provided by a battery meter is systematic, but in the end it is nothing more than an extrapolation, a guess. The reason for this is that modern science has yet to discover any 100% accurate method to predict how much electrical power is stored in a battery. Since there exists no way to predict with 100% accuracy, engineers have come up with logical ways to estimate, ways with rather less than 100% accuracy.

Every battery meter is based on a battery management system that uses indirect methods to calculate an estimated state of charge. Basically this consists of tracking how long the battery lasted the last time it was charged then extrapolating figures for how long the current charge might last. Over time, that calculation tends to drift and become less accurate due to many factors including age, temperature, etc.

All batteries naturally degrade over time, and their capacity slowly decreases. After a few months your battery might have degraded to the point it only has 90% the capacity it had when it was new, yet the battery meter still shows it to be 100% charged. Sometimes, your phone’s battery management system tries to account for this, but it is still only guessing.

Some web pages truthfully, and irresponsibly, instruct how to improve the accuracy of the battery meter by recalibrating your phone’s battery. This process involves draining your phone battery completely until it is dead, then charging it uninterrupted back up to 100%. This does work to improve the accuracy of the SWAG, but it also damages the battery reducing its capacity and shortening its lifespan. Moreover, due to the inherent complexities of battery technology the recalibration often does not help for long.

And now let us switch gears and leave the subject of how the battery meter estimates how much battery power remains, and mention how the battery meter behaves during charging cycles. I recently received an email from a very intelligent iPhone user saying "as recently as last Christmas, I got a 1% charge for every minute the iPad was plugged in. Now it takes 40 minutes to recharge up to 20%." What this sharp user correctly observed is that since the battery meter is based on a SWAG that applies to both discharging and charging. Those percentage numbers that allegedly showed the progress of charging the battery were just a guess based on how long it took to charge the battery last time. This means your device might not really be fully charged when the battery meter shows 100%. For this reason it is recommended to always use only the charger that came with your device because that charger will be the one that is best matched to your device.

In writing this column I found a lengthily blog posting describing the procedure for disassembling an iPhone then using a multimeter to test the voltage, current, and resistance of components in order to estimate a battery’s health. After detailing the complicated steps for how to disassemble then hopefully reassemble the phone, the post also said "You could test the battery by charging it overnight and then unplugging it in the morning and seeing how long it lasts." That is what I have been saying for years.

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Charles Miller is a freelance computer consultant with decades of IT experience and a Texan with a lifetime love for Mexico. The opinions expressed are his own. He may be contacted at 415-101-8528 or email FAQ8 (at) SMAguru.com.

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