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December 15, 2014


Electronics & Systems
Fast Charging
Mobile Devices
Nadim Maluf

You are now asking, what is “step charging?” The standard charging methodology for lithium ion batteries has been something called CCCV, which stands for constant-current-constant-voltage. As the name implies, the charging starts with a phase of constant charging current into the battery, until the battery terminal voltage reaches 4.35 V, at which point the charging circuitry switch to maintaining a constant voltage at the battery terminals. This ensures that no excessive and unsafe voltages are applied. As I indicated in an earlier blog, CCCV has proven to be a major source of headaches for the battery. Over the past 20 years, a variation on CCCV called step charging, continues to come in and out of consideration.  The figure below illustrates how step charging introduces an additional charging step where the constant current value is reduced so that the charging is gentle on the battery.

But step charging is very misleading and does not solve the problem in fast charging. Instead, it forces the manufacturers to create more compromises that are covered up with marketing gimmicks. Here are a couple of reasons.


Step charging includes an initial phase of increased charging current, then drops into a second phase of much reduced charging current to be gentle on the battery. Therefore the fast charging is only limited to the first phase of charging. But this phase tends to be relatively short, typically taking the battery to only about 30 or 40% of charge. The overall charge time for the battery remains very long.  In other words, once the battery reaches 30 or 40%, the charge time slows down and your anxiety takes off. In fact, the overall charge time for step charging ends up being worse than that of CCCV. And users notice this very quickly. Browsing the comment sections on the Moto X (one of very few phones that use step charging) illustrate how users notice that the fast charging is really limited. Considering that very few consumers will wait to charge when the battery is below 20%, the benefit of step charging becomes very small.


One of the hidden secrets of step charging is that its charge time grows with battery age. If we look in greater detail at the current profile of step charging in the figure above, the duration of “fast charging” is directly related to the duration of the first constant current phase, i.e., when the charging current is high. The onset of the “step” is when the current drops in value, at which point the charging really slows down. Well, the hidden secret is that as the battery ages, the onset of this step moves to the left. In other words, the duration of the fast charging phase shrinks as the battery ages. So instead of reaching 40%, now this step happens near 20%. So it is a double whammy: as the battery ages and its amount of charge dwindles, and the user loses battery life, it now takes far longer to charge the battery. How much longer? The figure below shows after 500 cycles, the charge time from 0 to 50% of the battery, CCCV and Qnovo charging is about 32 minutes, whereas step charging has grown to 50 minutes! This is not acceptable!

So how can you tell whether your mobile device is using step charging. For now, the only devices on the North American market that are using step charging are made by Motorola. In particular, the Moto X makes an additional and very serious compromise: it reduces the capacity of the battery in order to use step charging. The Moto X has a battery capacity of 2,300 mAh, greatly insufficient for a full day of use, especially for this 5.2-in giant.  In other words, step charging creates more reasons for more compromising. This is not innovation!

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