pixel batterysaver turns off a part of the pixels from your cube display to save more batterypower, swamming. vertical level 1 is the only profile where the screen doesn't start to flicker, otherways your screen may get broken. 

doesn't work, it costs more batterypower to run the screen, don't advise to install it.

dimly, would try that app. It makes your screen darker, so your display is consuming less power. You can also activate a blue light filter. Directly in the sun you can't read the display, but in shadow or evening you can. 3days and 12hrs runtime at 40 percent batterycapacity left using dimly. 

Or dimly blue light filter on and pixel screen app, so you don't use all the pixels of your screen.

Installed Facebook, Facebook messenger, Amplify, Greenify, Myandroidtools, Power nap, Xposed installer. Activate Amplify, Greenify and power nap in xposed installer, restart. To make the cube extreme energysaving, do following settings:

Amplify:

Wakelocks:

GCoreFlp: on, 7200 sec
LocationManagerService: on, 40000sec
*net_scheduler*: on, 7200 sec
NlpCollectorWakeLock: on, 40000 sec
NlpWakeLock: on, 40000 sec
wake:com.google.android.gms/.config.ConfigFetchService: on, 40000 sec
SyncManagerHandleSyncAlarm: on, 7200 sec

Alarms:

android.content.syncmanager.SYNC_ALARM: on, 7200 sec
android.app.backup.intent.RUN: on, 84000 sec
com.facebook.push.mqtt.keepalive.KeepaliveManager.ACTION_INEXACT_ALARM.com.facebook.katana: on, 7200 sec
com.oasisfeng.greenify.CLEAN_NOW: on, 7200 sec
com.facebookcommon.executors.WakingExecutorService.ACTION_ALARM.com.facebook.katana.Mqtt_Wakeup: on, 7200 sec
ALARM_WAKEUP_LOCATOR: on, 40000 sec
android.net.ConnectivityService.action.PKT_CNT_SAMPLE_INTERVAL_ELAPSED: on, 7200 sec

Services:

com.google.android.gms/.common.analytics.CoreAnalyticsIntentService: refuse
com.google.android.gms/.ads.jams.NegotiationService: refuse

In Greenify:
working mode: boost
wakeup timer coalescing (4.4+) select
activate at telephonyservices  not select
don't remove notifications not select
abuse of applicationquestion block: select

xposed and payd version
deep hibernation don't select
gcm-pushnotifications ... select
systemapps add... select
hidden sync select

further select which apps to go into hibernation ( not the ones at the top of this page, so amplify, myandroidtools, power nap and greenify).

In Power nap:
select endurance mode, select it again. Now a menu will show up, choose the apps active in standy, so amplify, android system, greenify and xposed installer.
go back, select low battery mode.

that's it, the cube should be very low power using now.

been testing it, however when i reboot a couple of times(for flashing a different kernel), the batterylevel drops like hell. Just amazing how ipad air can keep up for weeks with one full battery in standby.

you can also install Autostarts app, Servicely app and Appops app from marketplace. Add them in powernap app. Testing it, will give results later on.