Which tablet? All tablets are battery powered and, depending on the tablet, should provide enough juice to stay in standby between starts (I'm guessing 24-48 hrs, at least). If it's an Android tablet, there is a option to "keep screen on when plugged into USB". Just have a relay power on a USB charger with the ignition.
I've never heard of a tablet lasting only 8 hrs in standby, that would equate to only 20-40 mins of actual use. Standby on Android is simply turning the screen off. Unless it's doing a bunch of stuff in the background, the battery should last quite a while. There's no magic box needed to wake up or sleep. Just control it via the USB charger, either it's on or it's off. That could be something as basic as a cig charger (if yours turns off with ign) or add another cig charger that's controlled with a relay to your ign.
I think the harder part is actually figuring out how to start/stop processes/apps with the ign. Fortunately, an app called Tasker can do most of that for you.
It sounds like you don't have much experience with tablets, or Android for that matter. I would suggest just buying the tablet you want to install and familiarize yourself with Android and it's apps before you decide this is the route you want to go. There are many limitations when going with a tablet install (some that may or may not be a concern of yours). If you decide that it won't work, then at least you have a tablet that you can...well, still use as a tablet.
Thank you, very good points. You're right, I've never used an android device. I have been reading a bit and Tasker would probably solve a lot of my concerns. The tablet I'm considering is the a81e / MIDnite.
I've been planning a carPC for a number of years, here's a short version of my minimum goals with this simplified setup.
I basically want a Dashdaq, but I don't want so spend $700 for a tiny screen. I want OBD monitoring, trip computer and fuel economy data, and a GPS molded into my dash to go along with a double din video head unit.
Initially I wanted a PC with two touch screens to do this job, but I think a multimedia head unit and an android tablet would be faster, cheaper and easier.
When the car starts, the tablet will wake. I have mine configured with Tasker so when the usb charger is on, the screen timeout is set to 2 hours, and when the charger is off it sets it back to 30 seconds.
If someone else has issues with the tablet setup, I use the Tasker application on my Andriod Tablet/Phone. When it is "docked", charger powers up, it turns the screen brightness to max, enables WiFi Tether, Bluetooth, disables screen locking, and launches Pandora. This is just my setup, Tasker allows for all types of automation.
Make something idiot proof, and they'll make a better idiot. Car modification is not just a hobby, it is a business.
I have an hp touchpad and my girl has the ipad 2 and they both can stay in standby mode literally for days. It amazes how long they stay that way. I think that car PCs days are numbered. I actually have a the pc, touchscreen, eq, and amp and speakers already installed and I'm seriously contemplated during an android tablet install. The possibilities with android are unlimited. The one thing I don't like is that you can make phone calls via your phone through an android tablet. My hp touchpad allows for this.