Best Way to Charge House Battery Bank

We have 32, 24 and 12V banks.
I can type a book, later..

Let me make sure I read correctly what you typed.
Single 12V battery bank (4 x G31s).
This single bank starts the gen-set and all the ships 12v options
(reads like your gen-set is already charging this bank).

Your looking for a permanent automatic charging method from your mains to charge the 12V bank after the gen-sets alternator and 120Vac mounted battery charger (?).

DCv to DCv converter charger would be my first choice followed by a bolt-on alternator on one of the mains.
I have both installed on our boat.

In my opinion this is a lousy set up... the generator battery should be used for the generator only. Some 24v boats uses the gen battery to run a couple of light 12 v loads, that’s fine. But the gen should be isolated so it can never be run down

Using a main to charge a house bank is fine as a back up but typically boats spend a lot more time anchored than under way so nothing wrong with using the gen to run a charger to charge the house bank. It all depends on how you use the boat like time on the hook vs underway, how good is the ventilation (most modern boats need air con and run the genny most of the time). Usually if you loose the genny it s game over

i d rather have a second charger in case the primary fail. Which has happened to me.

You could split the bank and use one of the G31 for the Gen and the rest for house. House should be deep cycle anyway. The genset alternator will charge its battery

if you run the gen all the time, you can just use the battery charger to charge the house bank. Modern chargers are pretty reliable. Personally I keep a spare on board but that s because we spend a lot of time on the hook in the Bahamas. If the Charger fails, charging from the mains isn’t going to help because we d have to run one main a few hours a day.

curious to know what builder used 12V for house and 24V for mains... and setup a 12V house shared with generator. Lousy set up.

Your gen-set starts from the bank but does not charge it?
How is that possible?

We also carry a spare 40A, portable, 12V charger. And a nice extension cord.
Tenders and boats tied along side have been known to need a jump or quick charge.

Stepped in what?

I like this idea. Plus an ACR between the gen-set battery and the 12v house bank.
This is also on our boat.

Book typing is getting closer.

My boat used to be 32v (engines and house) with the genset 12v

when i repowered, I wanted everything 24v but for some reasons Cummins charged another $1000+ per engine for 24v. So I went with 12v for the mains and converter the house to 24v (all new wiring thruout, new panels etc)

i installed two Pro Mariner chargers, 12 and 24v

The 24v house bank is only charged by the battery charger for now which is fine since we only use the boat for short trips. When we start using the boat to go to the Bahamas I will add a 24 alternator to one of the engines.

I like redundancy and having a separate bank for each main, gen, house and eventually inverter is the way to go.

And when that fails? All your banks are compromised.

About 10 years ago I woke up one morning on charter to a dead boat (Johnson 70) while on shore power The 24v Charger (Dolphin brand) had failed and somehow not only the house bank was drained but neither the mains or the generators would stay running. They would crank, fire up but shut down has soon as key or start switch was released. I was able to get things going by running one of the generators for 15’ while holding the run switch.
Lesson learned, I want had much redundancy as possible when it comes to battery banks.

Before we get carried aweigh in using wrong terms; lets review what these devices are, that have been mentioned above.

A battery isolator or diode pack is an electrical device that divides direct current (DC) into multiple branches and only allows current in one direction in each branch. https://en.wikipedia.org/wiki/Battery_isolator

An ACR or Automatic Charge Relay parallels (combines) batteries during charging, and isolates them when charging has stopped and after battery voltage has fallen. An ACR is intended to keep a load from discharging both of the batteries. https://www.bluesea.com/support/articles/Charging_and_Charge_Management/1366/Automatic_Charging_Relay_[ACR]_Explained

A DC to DC converter is an electronic circuit or electromechanical device that converts a source of direct current (DC) from one voltage level to another. It is a type of electric power converter. Power levels range from very low (small batteries) to very high (high-voltage power transmission). https://en.wikipedia.org/wiki/DC-to-DC_converter

To ad even more complication, I remember a dual voltage alternator on an old Roamer. 12V alternator for the house and bottom batteries with a 24v trickle charge for the mains top starting batteries.
Without the benefit of a current controller (DC to DC converter), this would be the only dual voltage alternator I know of.