I’m definitely curious to hear what others think.  You note the simplicity — though maybe it’s really more options — from having the same chemistry.  I believe with any sort of charge relay that will close when charging you’re going to need the same chemistry but if you go with a dc-dc converter hung off one bank you gain some flexibility about dissimilar chemistries.  

My own boat has 24v starting and 12v house systems so there’s no question about the need for separate banks.  But, even if I didn’t need separate banks I’d sleep better at night with them,   I like the idea of nothing I do to the hour bank having the ability to take down an engine.  

I think that if I were in your shoes I’d look awfully hard at the dc-dc converter approach. I personally prefer the separation it affords during charging.   Otherwise, especially with different ages on the house bank and starter you’re going to have to consider things like the older battery taking down the newer.  

Good luck 

Ben

Thanks for those thoughts Ben. I just realized, the dc-dc converter approach would not have a "combine" option - correct? I guess a set of jumper cables or switches/cables could be a workaround.

@peterh

You’re correct that it doesn’t have an inbuilt combine option, but equally correct that you could accomplish the same in a number of ways.  I’m tempted to believe the simplest might be a 1/2/off batter switch from the motor to the two banks.  

Ben

What type of solar pv do you use?

I am very glad you found value in what we all wrote in the comments section, and happy to answer your question.

Yes, I favor idiot proofing the electrical system, having a big house bank, a tiny starter battery (Odyssey PC950, 20 lbs, for my Yanmar 3YM30), alternator output direct to the house battery (bypass battery switch), and to charge the starter battery with a DC-DC converter. The Victron product is absolutely the top choice right now.  I would have used it this year if I was aware of it.

Note the PC950 has a slightly higher voltage requirement than other AGM's, don't miss this detail, the sweet spot for high battery life is higher than the bottom of the printed range. Set absorption 14.7 vdc, float 13.6 vdc. (source Enersys). My house battery is the PC2250 so I have the same chemistry for both house and starter. 

For battery switch, easy choice, Blue Sea systems e-series, 5511e (On / Off / Combine)
https://www.bluesea.com/products/5511e/e-Series_Dual_Circuit_Plus_Battery_Switch

While you are on this project fix any lack of power fuses, likely you should have more.  

Also on your new starter battery install one of these
https://www.bluesea.com/products/5023/ST_Blade_Battery_Terminal_Mount_Fuse_Block

Consider battery monitors, so you have the information to take good care of them.

When adding a starter battery first look at the existing heavy battery wiring and next consider where all your components will fit. You may need additional heavy battery wire between battery compartment and engine. On my J/109 for example I found the engine starter and alternator shared the same 2/0 wire, for your project you will need 2 separate positive wires so the alternator output can go to the house battery while the starter connects to the starter battery. 

Dan - sorry I didn't see your comment until now. Thanks for your additional thoughts and encouragement to check out other aspects of the system - I'll do that for sure. In the mean time, I've run across FET battery isolators. It took me a bit to understand how they differ from battery combiners. I found this article helpful: https://panbo.com/marineelectronicsforum/projects/best-way-to-add-a-starting-battery/

How would you compare your approach to the one shown at the top of that page? I wonder if this is a better solution? 

Thanks, Peter

I didn't understand your question. For the helpful article, did you intend to link to something else?

@dan-corcoran

Argh - thanks Dan - yeah, I mean to link to this: https://www.pysystems.ca/resources/tech-talk/keep-charged-charging-scenarios-using-battery-combiners-and-battery-isolators/

And here's a better link to the battery isolator diagram. 

https://www.pysystems.ca/site/assets/files/2532/pys_battery_isolator.pdf

Reading the FET article, I am thinking your use case does not fit for the Argofet. The product Ben wrote about recently is far superior. 
https://panbo.com/dc-dc-chargers-victron-introduces-orion-tr-smart-series/

At $263 street it is a bit above double the price over the ArgoFet, yes, but your situation is an exactly perfect fit for the orion-tr-smart product (just take your alternator output to the house battery instead of the starter battery)

The Orion-Tr-smart is perfect for example in your primary use case. Your two battery banks are very different in size, and your starter bank hardly gets discharged. If your house battery gets significantly discharged, at the point you start your engine the alternator would end up exposing (through the Argo battery isolator) both battery banks to a bulk charge for very long periods of time, which for your starter battery having only been discharged 1%, that would be too much. (As Victron writes in their description of the Argo product "Battery isolators may not be the best way to go if you are designing a new system."). With the orion-smart-tr, your alternator connected to the house batteries and the orion-smart output to the starter battery, your engine would give your house batteries a sustained bulk charge, while your starter battery gets a brief bulk charge and then gets topped off in float mode as it should. The orion-smart-tr if I remember right has a feature to recognize when it should not even bulk charge the starter battery, which would be useful if your starter battery is an AGM.

When installing, be sure to use that built-in bluetooth interface to refine the settings. If you're not careful, it is probably possible to configure your orion-tr incorrectly to accidentally not charge your starter bank when your large house bank is deeply discharged. I set-up my similar but different Balmar Duo Charger to start charging my starter battery at 13.7 volts (recommended minimum charging voltage for the starter battery) and key switch on, and found that my starter bank wasn't initially getting charged when I started the engine. lowering to 13.3, 13.2, or 13.1 wasn't good enough either. In fact, my starter battery got further discharged while the engine was running as the engine panel drew 1 amp of power from my starter battery while the engine was running. I experimented and found 13.0 vdc was the correct value in my case for the Duo Charger to kick in and consider (other parameters applied) charging the starter battery, as in the case of a deeply discharged house bank the 13.7, 13.3, 13.2, and 13.1 wasn't enough as the alternator was unable to get the voltage much over 13.0 when the house was significantly discharged.

Dan - Thanks for that. I guess I hadn't 't thought about the downside to the starter battery getting the bulk charge for a long time, while the house bank is deeply discharged. Good point. So, you've convinced me. I'll go with the tr-smart option. Thanks too for the thoughts on the voltage going to the starter battery. 

I did really appreciate the point that Dan Cote made in that article about the problems with ACRs in battery banks of vastly different sizes. I had not realized that the 'inrush' current could blow the fuse on the ACR, which you might not realize for a while, or which would be difficult to fix quickly. 

Thanks again - I really appreciate it! 

-Peter

I happened to see this article on RC Collin's website regarding the Sterling DC-DC charger. This is a good compliment to Ben's original article. Some might be interested in why he really like's Sterling's charger. 

https://marinehowto.com/understanding-the-sterling-power-pro-batt-ultra-battery-to-battery-charger/  

Peter - Yes, this Sterling product adds to this starting battery thread, especially with the "Buck or Boost" feature / multistage charging done independently of the voltage level of the source battery. Impressive on the Sterling that when sleeping the parasitic load is so low, drawing just 0.168 Ahr / week (Useful when the ignition excitation option, which has no parasitic load, isn't the best choice) 

So many applications where less than 5A, or even 3A, is all that is needed and at least so far I have not seen "Buck or Boost" available in that size, have you?

I would like to see a much lighter weight / low power battery to battery "Buck or Boost" charger with similar features, smaller packaging, and a Victron like feature to program via Bluetooth (or) three such chargers packaged together in a similar case, maybe with a 5A / 5A / 5A  setup. I could see myself hooking up a starter battery, a small emergency battery for my navigation lights (required for some categories of racing), and the third shared by a USB charger station (for bluetooth speakers) and my fridge. Fridge and USB charging stations would run whenever the engine or shore charger is energized only, not putting a load on the house otherwise. Others might choose 5A / 5A / 5A for dual starter batteries and a bow thruster, maybe for charging electric trolling motors, etc.