How to wire two house banks plus starting battery shared between engine and genset

[Mhorowitz]

I want to have two banks of deep cycle agm's plus a starting battery as a backup. I usually use the agm batts to start the engine they have plenty of cranking amps. The PO had a single bank of agm and a dedicated start battery for the engine and another for the genset. The gents batt never got charged so it. Was hot or miss whether it would start when needed.


Ideas as to wiring and switches needed

Thanks

[Stu Jackson]

Why would you want two separate house banks?

Largest House Bank 101 (by Nigel Calder)

stereo-battery connection

For the same amount of lead... It also greatly simplifies the wiring and switching, allowing the alternator output to go to the house bank.

That said, perhaps I misunderstood your question and you meant you have two batteries making up the house bank.

[Mhorowitz]

The way it is set up now. There are 3 agm's in one house bank, a single flooded grp 24 in a second bank on a 0,1,2,both switch. These run both house loads and starting. Then there is an isolated batt grp 24 to start the genset.

What I really want is to increase the agm count to 4 and have a small battery of some sort like an agm grp 34 that can start either the main engine or genset. Unfortunately there isn't room for the 4 th house batt in the location where the house bank is located. I figure I can hook the fourth batt to the existing switch or I can run wires from the fourth battery back to the existing 3. But I really want the house and start batteries to be able to start and be charged from either the main engine or genset.

I hope that made sense.

[Reefmagnet]

Parallel all the house batteries together and use a voltage sensing relay (VSR) between them and the start battery. Your engine and genset can connect directly to the start battery. The charging voltage automatically closes the VSR contacts and allows both the house and start batteries to charge. Rewire the 0-1-2 switch to allow using the house battery for starting / independent charging if needed (another function of the VSR is that it allows the start battery to contribute to the house amp hours, so the starting battery can be partially included in house bank capacity).

[AnchorageGuy]

Ours is set up with the house batteries combined to make up one large bank. Start battery is charged via an echo charger connected to the house bank. Generator is started off the house bank, we don't need a separate battery for that. The genny can be started via the start battery using jumper cables if necessary. All chargers, 120 volt, alternator or whatever ONLY charges the house bank directly. Simple efficient and never failed us. Chuck

[Mhorowitz]

Thanks sounds like the relay will fix my issues. Pretty similar to Blue Sea's suggestion although they opted for more redundant paths

[Stu Jackson]

Like Chuck said:

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
1/BOTH/2/OFF Switches Thoughts & Musings - SailboatOwners.com

[mlibkind]

Originally Posted by Mhorowitz

Unfortunately there isn't room for the 4 th house batt in the location where the house bank is located.

I just happen to be reading Nigel Calder's book on the subject of batteries. He points out that all the batteries should be in the same place (or at least in places that the temperature will be the same) because a temperature difference will cause them to charge and discharge at different rates.

[mlibkind]

Nigel Calder says that all batteries should be the same make, type and age. Let's assume that one has a start battery and a house bank. Obviously all the house batteries should meet his criteria. But what about the house battery?

If the wiring does not allow the house battery to be connected to the house bank, then might one use a different type, e.g. AGM for house and a more conventional start battery for the start? Obviously if one wants to be able to connect the start to the house for added capacity, then they all need to be the same. But it seems that is problematic because you never want to connect two batteries with one fully charged and one partially discharged. Or so I think.