Disclosure: I was sent this battery by Seidio to review. I accepted with the stipulation that I could write an objective review (mostly numbers). Notice that I'm putting the disclaimer at the top; I don't want people accusing me of anything if I put a disclaimer at the bottom. Some (ok, a lot) of numbers for you guys: 1st Charge Cycle -- just played with battery; wore it down 2nd Charge Cycle -- Light to Moderate Usage (hrs:min) Up -- 25:55 Awake -- 17:18 Awake/Up Ratio: 0.67 Antennas: WiFi -- 6:24 * Turned off WiFi for a few hours, hence why 3G time is longer 3G -- 9:12 BT -- OFF GPS -- OFF Brightness -- 100% Activity: Sirius Radio streaming: 6:45 ( 6 hrs 45 min) Browsing -- 0:20 (20 min) Calls -- 1:00 (1 hr) SMS -- 6-8 texts Camera -- no shots taken 3rd Charge Cycle -- Very Heavy Usage Up -- 13:55 Awake -- 8:20 Ratio: 0.60 Antennas: (all antennas ON) WiFi -- 13:55 (approx; on the whole time) 3G -- 13:55 (approx; on the whole time) BT -- 13:55 (approx; on the whole time) GPS -- 13:55 (approx; on the whole time) Brightness -- 100% Activity: Sirius streaming -- 7:11 (approx) ** Revised Browsing -- 0:15 Calls -- 2:21 (2 hrs 21 min) SMS -- 8 Navigation (GPS turned ON) -- 0:45 (45 min) Camera -- 8 shots, still, no flash 4th Charge Cycle -- Moderate (what I consider my normal usage pattern) Up -- 31:13 Awake -- 14:39 Awake/Up Ratio -- 0.47 Antennas: WiFi -- ON -- (entire time) 3G -- ON -- 5:00 (approx 5 hrs) BT -- OFF GPS -- ON -- 0:15 Brightness -- 50% Activity: Sirius -- NO Browsing -- 0:40 Calls -- 0:38 SMS -- 22 Navigation -- 0:15 (see GPS time) Camera -- 7 shots, 1 flash ----------------------------------------------------- Thoughts: The 2150 would last me about 7-9 hrs of Awake time, 12-14 hrs Up time, with moderate use. The 3500 is roughly 63% more capacity (rated) than the 2150, so: 8 (avg awake time of 2150) * 1.63 = 13.04 hrs (approx awake time for 3500) That's close, meaning it is nearly a 1:1 correlation with the increase in rated capacity. However, consider that "moderate" is a highly variable range, so there is margin for error. I definitely had a tough time wearing down the 3500, though, as you can see from my Heavy Usage data. Compared to the 2600 mAh (13:47 awake time, light usage) I got roughly 3:30 (4 hrs, 30 min awake time) more juice, an increase of 25.4% 2600 mAh * 1.254 = 3260 mAh So, assuming the 2600 and 3500 are roughly the same rating vs. true capacity ratio, the 3500 mAh from Seidio is roughly 3260 mAh, if using the 2600 mAh rating vs. true capacity ratio as a reference. Juice, Weight, Back Cover The battery is hard to wear down. I've never run out of power before the end of a long day (wake up 6-7 am, go to school, then work, home around 8-9 pm). It feels heavy, but the heft is nice. It's noticeably heavier than stock. Back cover is much higher quality than the 2150 cover: it's thicker and the rubberized surface feels nice. More importantly, the 3500 cover does NOT make the power button feel mushy. For some reason, I was still burning through the 2150 at an unacceptable rate. I have the same widgets loaded that I did when I had the 2150. Yesterday, with light usage (not all Antennas on), I got: Up: 50:47 Awake: 15:32 Today, with WiFi OR 3G (not both), no BT/GPS, 100% brightness and moderate usage, I'm on pace for just over: Up: 34:23 Awake: 13:34 * I've been playing with the phone a lot (Facebook, texting), so less chance for phone to go to sleep. Summary You are look at a minimum of 13 hours with moderate use (possibly with all Antennas on, even) for Awake time, and at least double that in Up time. Not bad at all. Any questions? Please ask. I will try to take a picture of the hard silicone cover that I modified to use as a "frame." Comparison Photos Below Notes: In the pictures with two batteries, the red battery is the 2150 OEM, the other is the 3500. Notice the 3500 is barely over 1 mm thicker. In the pictures with two cases, the one in the foreground is the 2150 OEM back. The other is the 3500 back. Notice the difference in design. In the pictures with three cases, one is the 2600 back (flat), the others are the 3500 and 2150 backs. The picture with the flat 2600 back in the center has the 3500 cover to the left and the 2150 to the right. The picture with the 2600 back cover to the far left has the 3500 cover in the center, the 2150 to the right. In the picture with the phone with 3500 back cover on and on top of the red battery, I put a white background so you can judge the cover's design better (more contrast). Hope it helps. Notice the 3500 back has one extra "step" compared to the 2150 back. It actually feels better in the hand than the 2150 back cover. The 3500 actually fits inside the 2600 back. This is my preference, but the 2600 back I could not find a suitable case for. I bought a hard silicone case from Fommy and cut out the back to accommodate the 3500, so essentially the case has been turned into a protective "frame." I'll try to take pictures of that and post it as well.