Even with ongoing improvements in battery technology, many mobile phone users remain less than satisfied with the battery life of their devices. When a device cannot make it through a full day without needing to be recharged, this can result in a significant inconvenience for the mobile user.

It is common knowledge that a smartphone’s display consumes large amounts of energy and therefore has a significant impact on battery life. Less well-known is the “silent killer” of battery life: energy consumption by the device radio. We have found that for typical users, the radio can account for between one-third and two-thirds of battery use. What’s more, much of the radio activity draining the battery occurs when the user is not actively using the device. (This is in contrast to battery consumption by the screen, since the screen being on correlates strongly with active use). Many of the most popular applications continue to interact with the network to receive updates when the screen is off. These background interactions result in frequent device connections that cause the radio to remain in a higher energy state.

To illustrate the impact of connections and time connected on battery life, here is a plot of current draw in milliamps (mA) over time as a device connected to an actual LTE network: The brief, smaller spikes are the result of the device synchronizing with the network, which occurs at intervals of about a second and a quarter. These smaller spikes have a short duration, so they do not have a significant effect on overall battery use.

The device connection is the large period of increased energy draw in the middle of the chart. We can clearly see that immediately prior to initiation of the network connection at approximately 2836 seconds, the average current draw is negligible. As soon as the connection is initiated, the current draw jumps up to more than 300 mA, and subsequently drops down to roughly 180 mA until the connection is dropped at 2852 seconds.

From this example we can see that the current draw when the device is connected averages about 200 mA, compared to about 4 mA when the device is not connected. This indicates that a device connection causes the battery to drain fifty times faster than it does in the idle state. Another way of looking at this is that one second of connection time is equivalent to fifty seconds in the “not connected”, or “standby” state. Our tests also show that time connected has a much stronger effect on battery drain than the number of device connections.

These results parallel those presented by Google as part of their Android L feature announcement at the Google I/O developers’ conference. They examined the battery impact of several scenarios in which a device wakes up from an idle state, involving the screen, the radio, and/or the CPU. Their research showed that for every second that a device is active, standby time is reduced by a full two minutes. This 120-1 ratio is consistent with our results, given that their tests included scenarios where the screen was on.

The significance of all of this is that device connections happen frequently during the day, most of them the result of background activity created by chatty mobile apps. To the degree that we can reduce the number and duration of device connections, we can both relieve signaling congestion in the network and extend device battery life. Our traffic optimization technology is able to do this without adversely impacting end-user experience.