[Samsung Galaxy Tab A7 Lite] tagcart:
Hey there, I have the exact same use case as you. I also used to have a nexus 7 as my head unit and swapped for the A7 Lite within the last few months. Now I'm not sure how savvy or comfortable you are with some of this stuff, but I'll type out what I did here. Sorry if this is long but its a lot of info. Feel free to ask me any questions.

Root/Apps​

First off I rooted the tablet using this same method for the A7 10.5. This is a pretty intensive root method compared to most so I wouldn't advise if you don't feel comfortable. You will also trip KNOX so keep that in mind.

A quick note: Tasker may be able to run all of the commands I use without root so rooting may not be necessary. Greenify is definitely more robust with root but also may be able to run fine without it!

Once rooted, I installed Tasker to handle all the automation, and Greenify to handle blocking background usage. The cool thing about Samsung is they give you the ability natively to block most apps from performing background tasks so battery life should be pretty damn good from the start. Greenify with root just adds an extra layer of killing those pesky tasks that keep your tablet from deep sleep.

Tasker​

For tasker, I have 2 main profiles used to turn this into a proper head unit. Hopefully you have some experience In Tasker so this next part isnt gibberish. I'll write them down here in a readable format but let me know if you want me to clarify anything.

- If any power is connected: Turn on the screen > Set display timeout to 23 hours > Turn on Immersive Mode

As an exit task when power is disconnected: Turn off Immersive mode > set timeout to 30 seconds

- If there is no power: From 8:30pm-5:00am > turn off tablet

When power is supplied, the tablets screen will turn on, set a super long timeout, then enable Immersive mode to hide the top and bottom bars like your watching a movie. You touch anywhere on the screen and they will pop out for use. When power is disconnected, it will revert back to normal.

The second task is set from 8:30pm-5am. If the tablet does not get supplied power between those times, it will shut the tablet off. This works for me because I never really drive between those times but you could adjust to your lifestyle and probably not impact battery much.

The only annoyance is that every morning you will have to turn the tablet on via the power button. There used to be a few methods of turning a tablet on when power is supplied and that is something I used on the nexus 7 but that functionality is currently lacking on the A7 lite. There is one potential way to achieve this, but it involves replacing the batter icon file used when the tablet is off and plugged in, with a script telling the tablet to boot up. Unfortunately, with the changed made to partitioning and the lack of TWRP for this device, I haven't been able to find a method of doing so.

Charging​

The most important part of all of this is making sure you have a good charger/cable that will fastcharge. With all of this done, I have never seen my battery drop below 98%. When it does drop sub 100% it charges back up in a matter of minutes once driving again.

Pics​

As far as the pics below, I 3d printed a slightly angled holder with a slot for a 90 degree charging cable and also a super strong magnet behind the left side. I put a magnet plate on the tablet and is basically does not move with put into place. Looks pretty clean also which is a bonus.

Hope this info helps out and ill update if I find an auto-boot method.

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