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Best way to add a starting battery?
I’m wondering the best way to add a separate starting battery on a new-to-me old (1983) sailboat. The boat has only one ‘house’ battery bank (6 AGM group 27s), which can also get charged with 240 watts of solar panels or shore power. The boat has a Yanmar 3JH2E 35 hp engine with a 125 amp alternator, and I think there is a Balmar charge controller, if I remember correctly. I don’t trust myself enough to not have a separate starting battery, although it didn’t seem to worry the previous owner. Our previous boat had two battery banks. It had the off-1-2-combine rotary switch, and later I installed an ACR, and I just assumed that that is what I’d install in the ‘new’ boat. So, I was super intrigued when I saw Ben Ellison’s recent column on the Victron Tr Smart DC/DC charger, and he mentioned that he was considering changing his alternator configuration such that they would charge his Firefly house bank, which would then charge his starter bank possibly through the Victron Tr Smart. I confess I had never thought of any options other than the rotary or off-on switches and ACR options. Then I read in that column about using the Balmar Digital Duo Charge as an ACR-like device. And I’ve read through the lengthy discussion after that column. There’s at least a couple of people (Grant Jenkens and Dan Corcoran) who also favor the idea of charging a big house bank, and then charging the starting bank either through the Tr Smart like unit or a Xantrex Digital Echo charge unit. So, for you wise ones, if you were to start anew – what would you do? I’m a cruising sailor and I want a relatively bullet proof set up. It seems that having a starting battery with the same chemistry as the house bank simplifies life. Thoughts?
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.
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.
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)
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
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://www.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?