How to add a 4th battery

[lacoa1]

I have done a lot of searching and reading on here but seem to be unable to find a scenario like mine. I apologize if this has already been covered but here we go: I currently have a 3 battery setup (One for the port engine, one for the starboard engine and genny and one house battery) I would like to add a second house battery and although I am proficent with 12v I am a bit uncertain about this particular project. Ideally I would like to create a setup where I do not have to use a manual switch for the batteries and that will properly charge through the on board charger (Noco Genius Gen4). I have researched battery isolators but still would really like to talk to someone who can point me in the right direction. Thanks in advance!

 

[sam am I]

I've used this product (single engine,3 batt type but kicker motor's charger sys wired in) now for I think it is 3 years running, works flawless so far. They make several two engine four battery units that would work nicely for your app.The two engine units will even charge/isolate such that if engine #1 is down, engine #2 can charge batt. #3 and #4 or vice versa.....engine #2 can charge batt #3 and #4.

The units use a alternator management technique such that the battery's don't have to be paralleled in order for each to be charged, they are individually, if required, charged as needed. This prevents a situation knows as "battery back charging"........The alternator output voltage can sag (i.e. 13.799V) below the full battery's voltage (i.e. 13.8V) when paralleling a full battery with low battery.

At this point, the full battery begins to dis-charge back into a low battery defeating the idea of "keep the full battery full desire of isolation". This can happens with the more limited output alternators (40A,60A, 80A etc types), overly dis-charged batteries (drank too much, left stereo and lights on all night) in conjunction with VSR relay/both switch paralleling type systems .

I don't often see this on my setup (40A) but every once in a while when I take a house (either one) battery down too far with like making a few too many brews of coffee off my inverter.......It'll switch into the "priority management" mode and the single low battery is isolated from the rest to the of the batts and receives all 40A till it comes up far enough then it pops the full battery(engine and/or other house) back into the charge circuit...........works very nicely actually and is more efficient method obviously.

With the genny, I think I would use a transfer switch setup that would allow either engine's input to the proIso unit to be substituted out for the gen-set

 

[sam am I]

Edit: first paragraph, last sentence ........"The two engine units will even charge/isolate such that if engine #1 is down, engine #2 can charge batt. #3 and #4 or vice versa.....engine # 2 down, engine #1 can charge batt #3 and #4."