[Xperia Z1 Q&A, Help & Troubleshooting] luism9916:
Hello everyone

I recently bought a used Xperia Z1 (C6903), which came with Lollipop 5.0.2 installed (Build .270). It has a defective screen, which is going to be replaced shortly. I mostly use it with a mouse via USB. Other than that, it is perfectly fine.

On Thursday, I inserted my SIM card on the device and installed the latest build available for this device, which is 14.6.A.1.216 (Lollipop 5.1.1) via Flashtool, making sure I did a factory reset. On Friday, I rooted it using KingRoot (promptly replacing it with SuperSU) and, as I did my research first, installed Wakelock Detector to compare the battery between each Android version. On 5.0.2 battery was pretty good, maybe because the phone didn't have the SIM card inserted, but still used WiFi and had a lot of apps which were left by the previous owner (Such as Facebook and FB Messenger, which both are battery hoggers). Even then, the phone had tremendously good battery, managing to reach 3 days on a full charge (I observed that before buying it). The only problem was that the phone overheated a bit.

Now that I have installed 5.1.1, things got worse. The only good things are that the overheating problem is gone and the performance seems a bit better. However I saw that for many, the battery was way better on 5.1.1 than on 5.0.2. This isn't the case for me. The battery drops approximately 1% each 30 minutes, with the screen off. I don't have mobile data on, as I always use WiFi, so the radio active bug doesn't affect me (right?). I have disabled most, if not all useless apps or things that would keep using the phone's resources. I disabled them, not uninstalled them because the phone has a problem with root permissions, saying that system is not mounted as R/W but still giving apps root access and they can disabled apps just fine. Currently I have none of my apps installed with the exception of Wakelock Detector, thanks to the battery drain and the defective screen which prevent me from doing much. STAMINA mode is on, but I don't notice it working too well, as it doesn't turn WiFi off when the screen is off, even though that option is turned on. I cannot deliver an exact screen on time as don't use the phone that much, but it is average 3-4 hours.

Wakelock Detector tells me that the phone has been awake by 12%, even though there aren't any wakelocks nor apps that would take batery. I would have tried with BetterBatteryStats but it is a bit confusing to use for me. Most people here on this phone's XDA forum complain about the battery in Lollipop, but Kitkat has provided them very good battery life.

Is there any way to fix this? I will provide anything that is asked, such as screenshots of Wakelock Detector, etc.

Should I downgrade to 4.4.4? (Build .157 or whichever you recommend) or perhaps install a custom rom based on 4.4.4, like iHackers? I would test them myself, as it is the best way of telling the difference, but I have a very bad connection and downloading files over 1GB+ takes 5 or 6 hours, only if it doesn't disconnect automatically. The most important things for me are the performance of the phone and the battery life.

Thanks in advance, and sorry for my bad english.