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The Truth About Windows Phone Persistent WiFi

Last week I stumbled upon a trick that will keep WiFi running on your WP device long after the screen has gone to sleep. The bug, or feature, was met with mixed reactions from; “great news” to “oh, we better tell Microsoft about this so we all don’t inadvertently kill our batteries.” That was enough ammunition for me to run a test. I did some preliminary testing on my non-SIM HTC Surround, which turned out to be very promising, but wanted to be absolutely certain I got my facts right, so here we go.

To recap, my phone (Lumia 900) is setup with the following:

  • – Data (mostly LTE) and Wi-Fi ON – Bluetooth OFF
  • – 8 Active Background Tasks (3 Internal)
  • – Location Services ON
  • – 5 Email accounts syncing (3 push, 1 every 15 minutes, 1 every 2 hours)
  • – Phone set down and untouched for a 8 hour period

My previous baseline tests have resulted in an 16%, or 2% per hour, battery drain in an overnight 8 hour test. For clarification, my Background Tasks include; USA Today, Fox News, Weather Channel, Mehdoh, Urban Dictionary, Network Dashboard, Clever-To-Do and Battery Meter (a homebrew app set to update every 10 minutes).

Test 1 – To setup for this test, I started by fully charging my L900. After taking the phone off the charger, I opened the iHeartRadio app (Skype, Spotify will work the same) and started to play some music. After about 5 seconds I paused the music and put my phone to sleep. I waited about 2 minutes and turned my phone on, verifying that WiFi was active at wakeup (it was) and then used the Back button to close out of iHeartRadio and any other running apps. Put the phone to sleep again and waited 2-3 minutes before waking a second time to verify that WiFi was remaining on. I then put the phone to sleep again and let it sit untouched for my 8 hour test. FYI, when I woke the phone after 8 hours I did verify that WiFi was still running (persistent WiFi).

In the immortal words of Joe Pesci (My Cousin Vinny) , “So what did you find out”. The results: 100% at 12:33AM / 85% at 8:32AM – Difference of 15% or 1.875% per hour. Wow, 1% less than the baseline. Let’s try this again.

Test 2 – Same setup as above. The results: 100% at 1:07AM / 86% at 9:06AM – Difference of 14% or 1.75% per hour. Dang, that’s even better. Think I’ll have another.


Test 3 – Same setup as above. The results: 100% at 12:25AM / 80% at 8:27AM – Difference of 20% or 2.5% per hour. Ok, I have seen enough anomalies running these test to not be all that surprised, but I should do one more test just to be certain.

Test 4 – Same setup as above. The results: 100% at 12:28AM / 84% at 8:28AM – Difference of 16% or 2% per hour. So what is going on here. if I average all four results I am at 16.25% in my 8 hour test, which is almost the same as my baseline. And if I take out the anomaly, I am at 15%. So does persistent WiFi actually “save” battery power. Before making that kind of claim I wanted to be sure that my baseline had not changed. So a couple more baseline tests are in order.

Test 5 – This was a baseline test, so my phone was setup as normal. In other words, no persistent WiFi. The results: 100% at 12:49AM / 82% at 8:50AM – Difference of 18% or 2.25% per hour. Whoops, a little baseline slip there. One more time to be sure.

Test 6 – Another baseline test. The results: 100% at 12:24AM / 84% at 8:31AM – Difference of 16% or 2% per hour. That’s more like it. As a side note, I generally use the lower of multiple numbers and here is why. There are a lot of things that might increase the overnight drain on my battery; a poorer than usual cellular signal, those gremlins working on the towers, some background tasks that Microsoft is performing (I do have all their update, feedback, marketplace services turned on), but the only way that the drain could be lower would be if something on my phone was turned off or disabled. And that just does not happen by itself.


In case you are beginning to doubt that WiFi is actually running persistently in the background, and that icon at the top of the screen is simply a placebo, my non-SIM Surround has had WiFi running persistently almost 24/7 for the past week while I run various tests. Because I have my Hotmail account set to "push” on the Surround and L900, every time a new message is received, day or night, I get stereo as both phones tingle at exactly the same time. So yes, WiFi is on and working constantly.

So what does this mean. How can LTE and  persistent WiFi both running on a phone use less power than LTE running alone. Well think about this. You cellular radio has multiple power levels. While your phone is sleeping, the radio is at its lowest power level, but when it needs to move some data, the power level jumps up to get you that background email as fast as possible. Now, we already know that when Data and WiFi are both on, WiFi is in control of your data exchange. So, when persistent WiFi is on, your cellular radio never needs to wake up. That means whatever power your persistent WiFi is consuming it is returning back to you with more efficient data transfer. Can I prove this. Well, maybe not, but here is a piece of anecdotal evidence. I have been running close to 300MB of data usage per month on my Unlimited AT&T Data Plan (I know, dumb me) . With several nights of WiFi testing behind me, I am currently at 133.5MB at 17 days into my monthly cycle. Extrapolated out to 30 days, that would be 235MB of data usage for the month. The lowest it has been in six or eight months. That’s good enough for me.

Eighth Truth – Persistent WiFi does not only use virtually zero power while running in the background. Combined with a data connection, especially power guzzling LTE, it can actually save you battery power. Let me state that again, more slowly this time. “Persistent……WiFi………..saves…….you……..battery……..power”.

I sense a serious rant coming on, so if you are sensitive to these kind of things, or are still only able to view PG programming, best to not visit the site this weekend. I gave you fair warning.