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What is the default setting? I unchecked it and not sure how it was originally set. Also, what is the best setting?

I have been successful with 24 to 36 plus hours in between charges with it unchecked. Having it checked means that your data is always on vs. letting the phone periodically turn on the data.
Whenever I have had the always on checkmarked, my battery life is reduced by 10% to 20%.
This is one of those settings that really irk me.
The wording of the setting and its description makes no sense to me. To this day, I still don't really know what this setting is for. For example, what kind of condition would this setting restrict data? What qualifies as "mobile" data? Is there any other kind?
And there's no reliable way to test it to see how much impact it really makes. My daily fluctuation of usage is greater than 10-20%; therefore, if the setting makes a 10-20% difference, I won't know if it's because of the setting, or my usage, or variations in signal strength, where I went that day, etc. If I don't use the device at all, then the setting technically shouldn't have to do anything, so I still can't tell what its impact is.
I've tried to observe a change from a black box perspective and could not see any benefit or detriment. So I leave it on. For sure, there are lots of other things you can do to really improve battery life, so I don't consider this one a very important setting to modify.
I'm having an extremely hard time comprehending how you're getting 24 hours out of this phone. I've had two now. The first one barely lasted 6 hours and this one will last 8-9 hours. I use it mainly for texting, wifi disabled and I can't even touch 4G if I want the phone to last more than a few hours. I probably send 300 texts a day and maybe 1 phone call.
I turned off always on data and I stopped getting texts. It was a mess
Not sure why you weren't getting text messages but Always on Data has nothing to do with texting (with the exception of MMS but they force data on regardless). As for the rest I will tell you my set up which is pretty easy: I use WiFi all day at my office and then at night at my house (saves a significant amount of battery life compared to 3G), I have always on data disabled, I turned off Automatic brightness and have it set around 20% (guesstimation here), and I use 4g when it calls for it. I also have Facebook for HTC Sense syncing once per day, Google Syncing, LinkedIn for Sense Once per day, Twitter every 4 hours, and weather every 3 hours. Every thing else in Accounts and Sync has been disabled. I send and receive texts all day and average about 1 hour or so of phone calls (maybe more maybe less depending on the day). I also receive send and receive emails and Gmail throughout the day. Generally on a day like that I get an average of 18 hours or so, some days less, a lot of days I get closer to 23-24 hours of usage. The biggest saver I have found on this phone is turning down the display rather than using automatic brightness.
Unchecking always on mobile data can delay regular text messages as well, not just MMS. Using an app like Juicedefender may also cause text message delays. I recommend leaving your data on always and if you're having battery issues find other areas to fine tune (email fetching, display brightness, haptic feedback, etc.)
Not sure why you weren't getting text messages but Always on Data has nothing to do with texting (with the exception of MMS but they force data on regardless). As for the rest I will tell you my set up which is pretty easy: I use WiFi all day at my office and then at night at my house (saves a significant amount of battery life compared to 3G), I have always on data disabled, I turned off Automatic brightness and have it set around 20% (guesstimation here), and I use 4g when it calls for it. I also have Facebook for HTC Sense syncing once per day, Google Syncing, LinkedIn for Sense Once per day, Twitter every 4 hours, and weather every 3 hours. Every thing else in Accounts and Sync has been disabled. I send and receive texts all day and average about 1 hour or so of phone calls (maybe more maybe less depending on the day). I also receive send and receive emails and Gmail throughout the day. Generally on a day like that I get an average of 18 hours or so, some days less, a lot of days I get closer to 23-24 hours of usage. The biggest saver I have found on this phone is turning down the display rather than using automatic brightness.
I feel like doing that isn't even utilizing the phone as it's supposed to be utilized. Your phone is generally out of sync every time you use it weather it be weather, gmail, facebook, etc. That'd be frustrating.
I'll start using Wifi now to see if that helps but I figured having an additional antenna on would hurt battery life even more. I've turned brightness down and turned off heptic feedback. My weather is already set to 3 hours and Gmail is push notifications. GTalk is disabled, data is always on, and Facebook/Twitter will still send me push notifications. If this doesn't increase me to atleast 13 hours I'm going to be disappointed.
I'll start using Wifi now to see if that helps but I figured having an additional antenna on would hurt battery life even more.
This setting simply turns off 3G around when you turn your screen off and it isn't in use....
Unchecking always on mobile data can delay regular text messages as well, not just MMS. Using an app like Juicedefender may also cause text message delays. I recommend leaving your data on always and if you're having battery issues find other areas to fine tune (email fetching, display brightness, haptic feedback, etc.)
. I think it is a mileage may vary thing here. I think, I am no expert and don't know for sure, that even with always on data off certain things might kick it back on when they need it. But that doesn't seem to be the case for me. With always on disabled my phone loses maybe 3% tops over an 8 hour idle period (while sleeping) vs with it on significantly bigger drain (completely unscientific). Having said all that you will definitely save more by having 3G completely off.Wow, if that's true, then I should be seeing a MUCH bigger savings with this checkbox off. Because I normally keep 3G off unless I need it, and with the 3G radio off (and my phone screen off), my battery barely drains. If I uncheck this setting, however, my drain is still quite significant, so I think there's more going on.
But, let's assume you're completely correct. Why can't they call the setting "Disable 3G when screen off?" Wouldn't that make PERFECT sense to anyone who saw that setting?
Is leaving this unchecked better than using Juice Defender, and will this affect my HTC widgets updating regularly when the screen is off?
Is it recommended to use Juice Defender?