ML-ACR and DC-to-DC Charger Transfer Relay with Time Delay Start

 

If you upgraded from Blue Sea ML-ACR to DC-to-DC charger and wants to keep the ACR functional, this is for you. I added a Renogy 40-Amp DC-to-DC charger (this post) and decided to keep the Blue Sea ML-ACR as a backup alternator charger for our coach batteries.  It will come in handy if the starter battery needs a boost from the coach batteries. When connected to shore power, it can also be used to charge the chassis battery. This mod will also work on battery separator system, although wiring for the separator will be different.

The changeover/transfer relay only activates one charger at a time, either the ACR or the DC-DC, turning them on at the same time could cause some issues down the road. I also added a time delay relay for the DC-to-DC charger D+ trigger so that it does not immediately turn on as soon as the engine is started. I added a 3-minute delay (user selectable) to allow the chassis voltage to stabilize before starting the DC-to-DC charger. Here's how I did it.

Tool's you'll need.

• Wire stripper and Crimping Tool
• Screwdriver
• Drill and step drill bits 

Supplies you'll need.

• 12-Volt Time Delay Relay Module
• 5-pin 12-Volt PCB mount relay
• PCB Prototype Board
• 16-gauge wires 
• 16 AWG Male and Female Spade Connectors
• Project enclosure
• Small cable glands (optional)
• Panel mount fuse holder and fuse
• NCV3 Dash Switch Adapter (eBay)
• SPDT Center Off Switch

Wiring schematics of ACR - DC/DC Charger Transfer Relay

Detailed circuit diagram. I use a 4-pin relay (5-pin is ok) for switching the DC-to-DC time delay module to isolate it, the chassis D+ connection powers the time delay module when vehicle is started.

When the 5-pin relay is inactive, it grounds the ML-ACR remote red wire, which keeps the ACR off. If the relay is energized via a dash-mounted SPDT switch, the ACR remote red wire reconnect to the ARC remote switch, resuming remote switch operation. NOTE: If the 5-pin relay pinout is not correctly wired, it has the potential to short out the ACR remote connections during switching.

Innards of the ACR - DC/DC Charger Transfer Relay Switch

Everything is wired and ready for installation. For a professional appearance, the cover was painted black and the label was laser etched. The relay was installed beneath the driver's side dashboard.

A dash switch adapter purchased on eBay is used to mount the relay switch on the factory switch location. The Renogy DC-DC remote indicator light on the dash uses a fiber optic cable to project the main indicator light from the DC-DC charger enclosure (green=charging, red=fault). The laser etched labels on the switches complete the look.