Thursday, February 2, 2023

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.