04May 2017

A long time ago in a galaxy far, far away

smartphones used primitive energy sources called batteries 

that users could easily replace.

Then came the Apple iPhone and made it difficult to swap out the battery.

Batteries failed too often and even caught fire. Users got upset.

But the labels on the batteries stayed the same.


Whether you browse the web searching for a teardown of your favorite smartphone, or are sufficiently skilled to take a smartphone apart, you will always find a battery, a lithium-ion battery, with a whole bunch of markings on it.  Some of them are obvious to decipher, such as the name of the manufacturer. Other label marks may be puzzling such as a dog safety mark — yes, dogs seem to occasionally savor batteries. Then there are cryptic numbers that can mean very little to the average reader. The purpose of today’s post is to shed some light on what one can glean from the label of a lithium-ion battery.

The left photograph above is for the battery used in the iPhone 7 while the right photograph is for the Samsung S8 battery. The iPhone 7 battery has fewer markings than its Samsung S8 counterpart. That is typical of Apple’s batteries. It clearly shows the Apple logo but it does not say who manufactured the cell. Rumors abound on who manufactures Apple’s batteries in Asia, but Apple does not disclose this information on their battery labels. By contrast, the Samsung label clearly states that Samsung SDI manufactured this particular cell in Korea, and assembled it with its electronics in its factory in Vietnam.

Battery labels also state some required product certification marks depending on where the smartphone is sold. Both of these cells carry the PSE mark required by the Japanese Electrical Appliance and Material Safety Law. The Samsung S8 cell also carries the European CE mark as well as the Korean KC certification mark indicating compliance with the European and Korean product safety requirements. The iPhone battery carries the UL recognized component mark for the US market (which looks like a cRUus logo). These marks usually indicate that the product conforms with certain guidelines established by a regulatory body or government, but they do not guarantee the safety of the battery. Safety remains the responsibility of the smartphone manufacturer.

Both iPhone 7 and Samsung S8 battery labels also state some important electrical characteristics, in particular the battery’s capacity and its voltage. Battery capacity is stated in two units: maximum charge capacity measured in milli-amp-hours (mAh), and maximum energy stored in the battery measured in Watt-hours (Wh). The first is a measure of electrical charge (how many ions the battery can hold). The latter measures the total amount of energy. If you recall your high-school physics, energy is electrical charge multiplied by voltage. That is the third figure that one can read on the battery label.

For the iPhone 7, the maximum charge capacity is 1,960 mAh. For the Samsung S8, it is a nominal 3,000 mAh. In terms of maximum energy stored, the iPhone 7’s figure is 7.45 Wh which pales in front of the S8’s value of 11.55 Wh. So when we say that the Samsung S8 has a bigger battery than the iPhone 7, we mean that its  capacity is larger, not that it is physically bigger.

Now we get to the tricky conversation regarding voltage. First, we notice that the iPhone 7 battery reads only one value, 3.8 V. The Samsung S8 batteries reads two values: (i) a nominal voltage of 3.85 V and (ii) a charge voltage of 4.4 V. What do they mean?

Let’s start with the easy one. The charge voltage is the maximum voltage that the battery can be used in charging the cell. The Samsung S8 cell is rated to a maximum of 4.4 V. It does not mean that the charging is at 4.4 V. It only means that it can go as high as 4.4 V. We know that Samsung derates the cell to 4.35 V instead of 4.4 V to mitigate concerns about safety.

The nominal voltage needs a lot more explaining.  For that, we will need to examine the next graph showing the battery’s voltage and its dependence on state of charge (the measure of how full it is).

When a typical lithium-ion battery is empty (at zero percent), the voltage across its two terminals is low, about 2.9 V. As the battery is charged, its voltage will rise to its maximum charge voltage.  The “average” voltage throughout this charging process is called “nominal voltage.” It turns out that if the maximum voltage is 4.4 V, the corresponding nominal voltage is 3.85 V. But if the maximum voltage is only 4.35 V, then the nominal voltage is 3.80 V. So it becomes easy to figure out that the iPhone 7 has a maximum voltage of 4.35 V even though it is not stated on its battery label.

You have now become an expert in reading battery labels. But whatever you do, always remember to stay safe and keep your battery away from metal objects.