Dual Engines and small house battery, ACR or DC to DC Charger

[Arthurc]

Not a trawler specific question but I’ve come to really value this groups input. I am configuring a smaller boat (large tender) that has twin engines and 3 batteries, P Start, S Start and House. Today there is a selector switch for which engine to charge off of and then a single ACR.
The boat previously had no other charging so I’m adding a 3 bank Victron AC/DC charger and a solar Victron charger that will only charge the house bank.

My first thought was to eliminate the charging engine selector switch by adding a second ACR but am realizing that there is a risk that if it’s super sunny or I’m still plugged in then the ACRs will bridge the batteries because of the higher voltage and a start will affect the house bank. Also I don’t like that the start and house banks may be in different states when charging starts from the alternator. Today all 3 batteries are the same (Bluetop optimas) but I may add more to the house over time.

It got me to thinking, why not use two Victron DC/DC 30a chargers and remove the switch and ACR? Any downsides to doing this other than cost? The units can stack and also can be controlled to only kick on when the engines are running. I’m making an assumption they will still stack if technically connected to different power sources, the manual is very light on details.

Any thoughts or advice would be appreciated.
AC

 

[Mischief Managed]

I think a second ACR with an automatic start isolation is a much more elegant solution. Blue Seas ML series has inputs that connect to the starter relays on both engines and open the ACR contacts during an engine start on either engine. They also have an override function so you can deliberately use them as a battery combiner (from the helm with a switch or directly on the ACR with a lever) for emergency start power. I have one on my boat and it works great.