Wiring Android Head Unit to Coach Batteries

To utilize the Android head unit without starting the engine or depleting the chassis battery (when used with the ignition set to ACC), I rewired the Android head unit to operate off the coach batteries. It's a great way to enjoy streaming media like YouTube, Netflix, Spotify, etc. when parked or relaxing at the campground using the upgraded Android head unit's larger screen. It's also an excellent way to get the navigation started to our destination before starting the engine. A dash switch is installed to toggle the power of the head unit between ignition only and always on. A simple wiring modification and the addition of an automotive relay are all that is required. This is how I did it.

Tool's you'll need:

• Wire stripper and Crimping Tool
• Heat gun (optional when using heat shrink tube) 

Supplies you'll need:

• Automotive 5-pin relay
• NCV3 Dash Switch Adapter (eBay)
• SPST On-Off Switch
• Electrical tape or heat shrink tubing
• Wire butt connectors

Wiring diagram. 

Step-by-step procedures:

Before starting, disconnect the chassis battery and coach battery (not pictured) ground wires. Always wait at least 10-minutes after engine shutdown before disconnecting the chassis ground to allow Sprinter emission control system to complete cycle.

Remove the head unit and locate the main wiring harness. You should find a yellow wire (12-volt constant power), a red wire (ACC- 12-volt switched power), and black wire (ground) in the wire harness bundle.

The yellow wire for 12 VDC coach battery is connected to the under-dash accessory fuse block from this post (click here). 

Above: The wiring diagram using a relay and a SPST switch. During engine start-up, the vehicle ACC power (switched) energizes the relay coil, allowing 12-volt (+) from the coach battery to flow via the relay normally open (NO) pin onto the head unit ACC input, turning on the unit. The dash SPST switch bypasses the relay NC and common pins, allowing the head unit to function even when the ignition is turned off. The coach and chassis batteries remain isolated.

Above: Head unit power override switch. Switch position one only powers the head unit when the ignition key is in the ACC position; switch position two always powers the head unit.

Above: Wider shot of the head unit power override switch.

The YouTube video below shows the Atoto Android Head Unit powering on, automatically activating phone hotspot and connecting to it, and running several Android apps.

Movies and music sound much better with upgraded door speakers and dash tweeters. (on this post).