How does the Samsung Galaxy S5’s superb Ultra Power Saving Mode work?

As both Motorola and HTC have faltered, Samsung has taken over the Android device ecosystem with its Galaxy brand. That trend seems sure to continue with the Galaxy S5, which is just two days away from release and meeting with favourable reviews. Samsung managed to restrain itself to an astounding degree this time – whereas the Galaxy S4 was accompanied by innumerable mediocre software features, the Galaxy S5 focuses on a few aspects in particular, one of which is battery life.

In addition to the standard power saving mode, Samsung has created a new Ultra Power Saving Mode (UPSM). This feature leverages tightly integrated hardware and software to extend standby time many times over, apparently allowing the phone to squeeze 24 hours of standby time out of just 10 per cent battery life. Early impressions indicate it works even better than expected. Samsung may have under promised and over delivered on this one, but how?

Ultra Power Saving Mode is not simply a fancy way to restrict background data and dim the screen like most other OEMs do – this mode drastically reduces power drain by pulling back on almost everything the phone does. The most obvious change when activating UPSM is the switch to a greyscale display. This wouldn't make much difference at all to a phone with an LCD, but Samsung's Super AMOLED is playing to its strengths here.

An AMOLED panel differs from LCD in that it doesn't need a backlight. All the light is produced by the pixels, so a black pixel is simply off (it uses no power). By changing to a black and white UI, the Galaxy S5 needs to light fewer pixels, thus reducing power usage substantially. It also caps brightness at a maximum of 87 nits, a big drop from over 400 nits without Ultra Power Saver. Does that make it harder to read? Not really. AMOLEDs have very high contrast, so a black and white interface – even one that is very dim – is surprisingly readable.

Big changes happen on the inside of the Galaxy S5 when UPSM is engaged, too. The device comes with a Snapdragon 801, which has four Krait 400 CPU cores clocked to 2.5GHz. Ultra Power Saver Mode takes advantage of Qualcomm's custom ARM cores – two cores turn off completely, and the remaining two are downclocked and capped at 1.5GHz. To compensate for the reduction in processing power, Samsung also drops the display refresh from 60Hz to 30Hz.

Samsung has designed the new TouchWiz software to be ruthless when UPSM is turned on – all running processes and services are ended, and nothing is allowed to run in the background except for the bare essentials needed for calling and messaging. You can only access a subset of apps in this mode, including the stock Samsung browser, messaging, phone, clock, and ChatOn. Of course, this might just be a devious way to get people to actually use ChatOn.

With UPSM activated, the Galaxy S5 is able to drop its power usage to just 16.8mW, which could run the phone in standby for over 64 hours on a 10 per cent charge. Samsung only claimed 24 hours, which likely assumes you'll still wake it up on occasion. Our own informal testing showed 2 per cent battery drain for a 15-hour overnight stretch, during which the phone was in standby the entire time.

Ultra Power Saver Mode is an example of an OEM using its Android modification powers for good instead of evil. After all, Google hasn't even bothered with a power saving mode in stock Android. There are aspects of TouchWiz that still need hammering out, but it probably has the most impressive power saving feature of any smartphone today.