Small dedicated start batter Yanmar SB 12

[a64pilot]

350 CCA 1.7 lbs
https://www.batterystuff.com/batteri...xoCDeoQAvD_BwE

[john61ct]

I agree the new starting-point capacity should be much more than you need if lead, unless you're planning on routinely buying a new one on schedule every 8-18 months.

Or go LFP, a third the size per capacity.

Originally Posted by pmagistro

I have a disconnect switch on the lead from the alternator.

Which can of course be located anywhere you like.

[rom]

Originally Posted by pmagistro

Ok folks, please tell me why this is a bad idea, and what you would do instead! Thank you!

I would buy a battery that is capable of starting the engine, possibly AGM, but I will not use it. I would connect it & the house to the system through a 1/2/OFF bluesea selector, and would call it "emergency battery". IMHO it is simpler and more reliable. The drawback is that you will need to manually refresh that battery once in while, could be done with a charger, solar, or alternator depending on their specs.

[hellosailor]

I'm guessing that the alternator can't be putting an incredible load on the engine at starting, since ~750W is just one horsepower, and that's about 50A. Ignoring the friction and mechanical losses (since the alternator is still belted up either way) and figuring that at starting speeds, it can't be putting out more than 50A anyway, so there can't be more pull than that.

If you put a clamp ammeter on the power lead to the starter, and measure the load that the starter normally draws from your normal battery, and then repeat the measurement from the small battery with the field wire disconnected, you will be able to see how close the small battery comes to supplying "enough" power, and how much less power is required when that field wire is disconnected. That at least gives you a number, in plain amps, of what the starter is drawing.

With any luck you'll see something like "200" without the alternator and "300" with, making it fairly likely that you just need another 100 CCA. Perhaps less.

Maybe it can't be that simple, but it would seem like a good starting point?

Or, does your engine have a decompression lever? Starting with a "weak" battery, you can often get an engine started by opening the decompression lever, spinning it up to full speed, and then closing the lever. If all is well the engine will start immediately--and you don't have to mess with the wiring.

[Wotname]

Originally Posted by hellosailor

........

Or, does your engine have a decompression lever? Starting with a "weak" battery, you can often get an engine started by opening the decompression lever, spinning it up to full speed, and then closing the lever. If all is well the engine will start immediately--and you don't have to mess with the wiring.

Yup, as per post #22


If the OP is wanting small (but not cheap), then an SBS30 is the ticket, one these will easily start my 2GM30 even after 10+ years use (only used for starting).

https://thebatterybase.com.au/catego...treme-battery/

[pmagistro]

Originally Posted by a64pilot

If your cutting it that close...
I forget, what is the purpose here? Inexpensive I assume?

I agree completely about cutting it close. That wasn't my intent. Nor is cost a (major) issue. Originally my goal was to quickly and easily retrofit a dedicated starting battery, without rebuilding the entire battery box. I have a small space next to the box that a Group U1 fits, amd they are a size that conveniently have manufactured plastic boxes readily available for....but alas I feel that in the end Inwon't be able to take the easy route on this one.

What I have decided is to scrap the project for now and go cruising. Next winter I will tear out the existing battery box and replace it with a larger, custom box that will hold three group 27s, which will be a major upgrade from the two group 24 I currently have. I'll parallel two as a house bank and run one as a designated starter. The alternator will charge the starting battery, which will be linked via a relay (leaning towards the Battery Doctor) to the house bank. I have a dual solar controller which will route my 160 watts of solar to the battery that needs it most.

In the end, I think this is the "right" solution: clean, elegant, more work and more $$$, but also a massive upgrade in my capacity. Which should allow for a small refrigerator aboard.

Thank you all for your input! Every darn project on this boat has such a steep learning curve for me and it's nice having this community to lean on.

[john61ct]

Yes to Odyssey, Enersys pioneered AGM for deep cycle usage back in the day.

Make a fine product, to the point that even their "dual use" batts are excellent for deep cycling.