MacBook can't complete installation of El Capitan without battery?

I doubt any one has done any testing doing an OS upgrade to El Capitan without a battery installed. With that said I don't see why the installer would care.

So lets see if we can isolate the possible issues here:

• First you should check to see what the systems firmware level is. Follow this Apple T/N: About EFI and SMC firmware updates for Intel-based Mac computers your firmware should be at this listed level or newer.

• The next thing I would do is check the HD, do you have enough space on it? You may want to run this program to clear out the old cache & log files: Disk Doctor.

• I would then boot up under an external drive and run from it Disk Utility to check & repair the drives permissions & disk structure if needed (make sure the drive is setup correctly GUID & Journaled file system). Next, I would delete as much as I could so the systems drive had 1/4 to 1/3 of free space. The last thing here is defrag the drive using an app like this one: Drive Genius. Before restarting the system I would also make sure the internal drive is set as the boot drive.

Now I would do the upgrade!

I should point out El Capitan does do a firmware update so it will appear to shutdown and take awhile to restart. You should hear a long beep tone different than you normally hear. Don't touch the system! Let it do its thing. In some cases it has to go though the process twice.