26% installed on my Lumia 640XL. I didn't have any battery problems. I wonder why some did and some didn't have a problem?

Shawn's link is broken in the OP. Here it is without the desktop info,

Windows 10 Mobile preview build 14291 released to more Fast ring Insider devices | Windows Central

and list of known issues,

• If you reset your phone on this build (Build 14291) and restore your phone from a backup, restore will fail to install apps from your backup's apps list leaving blank grey titles on Start. If you delete the grey tiles and attempt to reinstall the same apps from the Store, you will not get app data restored. The next backup will over-write the app backup data as well. To avoid hitting this bug, avoid resetting your phone on this build. If for some reason you do need to reset your device on this build, don't restore your phone from a backup and you should turn off backup for apps and OS data to avoid creating a corrupt backup via Settings > Update & Security > Backup.
• If you have a Microsoft Band 1 or 2 paired to your phone, it will no longer sync after updating to this build due to a system API failure that occurs after the update. If you want to get your Band syncing with your phone again – you can temporarily change the language of your phone as a short-term workaround until we release a fix. Additionally, you can also choose to reset your phone to get out of this state – however you may experience this update issue again with the next build until we fix this issue. This issue may also impact Skype video and audio calls.
• The Gadgets app will not detect the Microsoft Display Dock on phones running Windows 10 Mobile Insider Preview builds, and thus cannot update the firmware version. If you have a dock which has already been updated to version 4 then this will not affect you. If you have a dock that has not been updated, then you may experience some minor issues with USB-C stability. You will still be able to use your dock and Continuum.
• There is a new option under Settings > Update & security for Windows Insider Program. This is a work-in-progress option to improve managing your Windows Insider Program settings on your device. Right now, if you go to this option it crashes the Settings app. Please continue to use the Windows Insider app to manage your settings for now.

Installed successfully on my L640,



Don't have a Band or use Gadgets, so don't have those problems. The Insider bug is minor seeing as you can still use the standalone app instead of the new integrated one. Haven't done a hard reset yet, and at the moment it doesn't feel like it needs one.

As for the battery issue, from looking around it appears that it may be related to Skype running in the background.

Installed on my Lumia 640XL without any problems. I only tried out a few apps but everything seems OK. :)