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The Truth About Battery Saver

Battery Saver (Settings>Battery Saver) was introduced to Windows Phone with the Mango update. What it essentially does is limit automatic updates on your phone when it is active. So, while the feature is enabled; email will not sync, background tasks will not refresh (except locally) and push applications will not update your Live Tiles. Battery Saver has two options. The first, “Always turn on Battery Saver when battery is low”, does what it says. When your battery dips below 20%, Battery Saver will turn on automatically, attempting to preserve the little juice you have left. You know it’s active as a little heart is displayed over the battery meter in the top tray.

The second option, “Turn on Battery Saver now, until next charge”, can be toggled on when you need to retain as much power as you can, or if you are going to be away from your phone for an extended period and updates are not very important. As soon as you plug your device into any USB port, this second option will disable automatically. This feature is working well on my SIM-Less Surround, which still has most of my apps installed, although everything has been set to manual update. Even with Wi-Fi turned on, my Surround is only down to 78% after a few days. Prior to activating Battery Saver, I needed to recharge every 2 to 3 days, even without using the phone.

The first Battery Saver option is really useful and everyone should probably enable it by default. The second option; well it does sort of limit what a WP is know for – updatable Live Tiles, but it will preserve more of that valuable battery power. Let’s find out how much.

As a refresher, all of the below tests were preformed on my Lumia 900 with the following settings;

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

As I have already done several tests to develop a “baseline” for my device, we will use my typical 18% drain (or 2.25% per hour) over an 8 hour test period for comparison.

Test 1 – I left my phone set as noted above and turned on Battery Saver-Option2. Note that I always have Battery Save-Option 1 enabled, but as that does not kick in till the battery drops below 20% it has zero effect on testing. RESULTS: 99% at 12:07AM – 73% at 8:08AM – Difference – 26% or 3.25% per hour. WTH! How can that be? I thought Battery Saver was supposed to SAVE battery, not suck it up. If you look at the graph below you will see a significant drop early in the testing period. I noticed when I woke my phone up in the morning that I had 2 app updates waiting so I decided to take a closer look.


Breaking the 8 hour test down into 4 – 2 hour segments, these were the results: 12:07 to 2:08 – 12% drop, 2:08 to 4:08 – 5% drop, 4:08 to 6:08 – 4% drop, 6:08 to 8:08 – 5% drop. So, my best guess is one of two things happened; Marketplace was already checking my phone for updates when I turned Battery Saver-Option 2 ON, or App Update initiated after I activated Battery Saver. With nearly 200 apps/games installed, I suppose there is a lot to check, but wow that is a lot of power drain. FYI, over the past 18 months I have noticed that I usually get App Update notifications between 12:00am and 1:00am, so this is not unusual. 

Either way, this is a serious flaw in the way Battery Saver works. App Update checking should be disabled as soon as Battery Saver is enabled, especially with the potential draining effect it can have on your battery. For those following my “Truths” series, I had an anomaly when testing Push Email, and also noticed 1 App Update waiting for me in the morning. Coincidence, probably not. But as I can’t predict when App Update will actually run, really hard to prove.


Test 2 – After using my phone normally all day, I turned Battery Saver-Option 2 back on just before putting my phone down for its overnight test. RESULTS – 99% at 12:18AM – 89% at 8:08AM – Difference – 10% or 1.25% per hour. Now that’s more like it. An overnight savings of 8% vs. my baseline testing. I also took a closer look at this data to compare to Test 1: 12:18 to 2:19 – 3% drop, 2:19 to 4:19 – 3% drop, 4:19 to 6:20 – 2% drop – 6:20 to 8:20 – 2% drop. That’s pretty consistent, but not sure why, aside from the first 2 hour block, Test1 averaged 4-5% per 2 hours and Test 2 only 2-3%.

I did notice that my Background Tasks that require Data access, did not update overnight as expected. But most did almost immediately as soon as I turned Battery Saver-Option 2 OFF, as did delivery of email messages. One News app took several minutes to update. This indicates to me that Tasks are held in a queue and will initiate as soon as Battery Saver says it’s OK. Note, that my Internal Background Tasks, defined as tasks that “do not” require Data access are NOT disabled by Battery Saver. One example is the homebrew Battery Meter app that continued to check battery status every 10 minutes throughout the night. Another is Network Dashboard, a Marketplace app, that I tested later in the day with Battery Saver turned on, noting that it was able to update its Live Tile.


Test 3 – Repeated everything I did in Test 2 to validate my findings. RESULTS: 99% at 12:14AM – 88% at 8:14AM – Difference of 11% or 1.375% per hour. So effectively, with rounding, the same result as Test 2. Verified. But wait, the story does not end here. Being Memorial Day, after checking my phone and taking the screen shot (below left) I jumped back in bed for a bit and then got up to do some busy stuff before getting back to my device.

When I checked it at around 11:00am with Battery Saver-Option 2 still active, I noticed that it had dropped from 88% to 72%, a difference of 16%, in less than 3 hours. That was a surprise. Before everyone starts guessing at possibilities, note that I have a habit from my WM days and use the Back button on my WP a lot more than I should. So, except for rare occasion, I almost always back out of all the apps tombstoned on my phone, and I did the same that morning. You can see the excessive drop in the screen shot (right) below.  For those of you keeping score, the three hour breakdown went like this: 8:15 to 9:15 – 2% drop, 9:15 to 10:15 – 7% drop, 10:15 to 11:07 – 7% drop. No idea what caused that kind of drain as my phone sat on my desk in the same test position for those three hours. Oh, and I had no App Updates waiting for me.


So there you have it. Battery Saver-Option 1 is a good thing to always have enabled on your phone. Battery Saver-Option 2; it will usually save battery power, at the cost of not getting scheduled updates, disabling one of the most exciting feature of Windows Phone. But it looks like Microsoft has some work to do regarding what can actually run when Battery Saver is enabled, as a user can get a big surprise when they find their battery drained despite enabling this feature.

Third Truth: Enabling Battery Saver (both option) can save up to half the battery power your phone normally consumes when sleeping, and a smaller percentage when awake. But, some tasks like checking for app updates, will still run in background draining your battery unexpectedly. Next up, those pesky Background Tasks. From the tests above, we already know a bunch of them collectively consume less than 1% per hour, but let’s see how much they really burn.  


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