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  #1  
Old 09-27-2013
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How big of a starting battery do I really need?

My current battery setup involves three identical group 27's (flooded), two as a house bank, and one as a starting battery, with an ACR to charge both. It occurs to me that my investment in the starting battery (both in terms of dollars, and in space) is going largely to waste, since very little of the capacity is needed to start the engine.

If I purchased a smaller starting battery, I could add my current starting battery into the house bank, which would substantially increase my capacity. But to do that, the new starting battery would need to be pretty small, because I don't have much spare room in the battery compartment (starboard lazarette) (I'm envisioning glassing a small battery shelf partway up the inside of the hull).

So what's the smallest battery I could get away with? (Engine is a Yanmar 2QM15. No glow plugs, so a bit of cranking is needed, particularly in the winter). I've seen comments on SN about using a small lawn mower battery as a starting battery -- would a battery that small really be adequate? Is there a way to figure out what I really need, short of trial and error?

(I don't have a clamp on ammeter, but I'm planning to install a battery monitor in the near future -- might be able to do some experiments with that).
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Last edited by Rusty123; 09-27-2013 at 12:49 PM.
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Old 09-27-2013
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Re: How big of a starting battery do I really need?

Even the smallest car starring battery you can find will start that motor. With three group 27's as house there will never be a need to use the "starting battery" unless you are not practicing good battery management. At 50% DOD your house bank will still have gobs more cranking capacity than a single group 27 at 100% SOC... You might even look into some of the motorcycle or small tractor batteries.

A group 26 auto battery is about the smallest you'll find. You may also do okay with a U1 lawn and garden battery if you buy the highest CCA or MCA version you can find.... Some of them have CCA's (0F) of 350 and at 32F at 400CA. This means that at 70F you'll be well over 450 cranking amps..

There are also the Odyssey AGM's such as the PC925 or PC545. These are small batteries. I install a fair amount of the PC925 and they have a 380CCA.

I have one that has been starring a 3YM now, as a dedicated starting battery, not just reserve, for about three years. They take near identical charge voltages as the flooded batts so no charging issues there.
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Old 09-27-2013
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Re: How big of a starting battery do I really need?

Consider that the 15 hp engine you use is a 'marinized' garden tractor engine. Like MS says above, a U1 will do it.
Google this : 15 hp diesel engine starting battery
it will take you to a link (too long to post) that is for marine starting battery calculations.
Bottom line - 15 hp - 420 CCA is sufficient to start you 6 times with no charging per The Marine Electrical and Electronics Bible
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Re: How big of a starting battery do I really need?

Great feedback, thanks! This gives me a good target for battery shopping.

As an aside, I should have mentioned that the "start" battery is configured as a dedicated start battery, rather than as a reserve (using one 1-2-B-O switch for the engine, and another for everything else, with charging sources hard wired to house bank). This is because my previous electronics didn't deal with the starting voltage dip very well. My new electronics don't seem to have this problem, but I rather like the added flexibility.
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Old 09-28-2013
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Re: How big of a starting battery do I really need?

I did some shopping and research today - here's what I found:

There seems to be two general capacities of U1 batteries available: around 350 CCA for about $55, and around 250 CCA for about $35.

The smallest automotive battery at Costco was a group 35: around 600 CCA for about $65. While the price and capacity compares favorably to the U1, I don't think I have the room for it. So I'm leaning towards a larger capacity U1.

(BTW, my Yanmar manual calls for a battery with a capacity of 70 AH (no rate mentioned). These small batteries don't seem to list AH capacity, so that's not much use).

One concern I have about all of these small batteries: they seem to invariably be maintenance free (or even sealed), which concerns me a bit since the start battery will be paralleled with my house bank during charging. I'm still learning about the dynamics of batteries and charging systems, but something tells me that paralleling a small, almost always fully charged battery that cannot be topped off; to a large, sometimes substantially discharged battery bank; using the same charger for both, might be a mistake.

Is my concern warranted?
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Re: How big of a starting battery do I really need?

I checked out the Odyssey batteries on line also. They seem ideal for my application, but quite expensive. I'll certainly keep them in mind also.
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Old 12-23-2013
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Re: How big of a starting battery do I really need?

Quote:
Originally Posted by Maine Sail View Post
There are also the Odyssey AGM's such as the PC925 or PC545. These are small batteries. I install a fair amount of the PC925 and they have a 380CCA.

I have one that has been starring a 3YM now, as a dedicated starting battery, not just reserve, for about three years. They take near identical charge voltages as the flooded batts so no charging issues there.
I am curious what the tradeoff is between using a PC925 as the starting battery vs. using the house bank as the starting battery and the PC925 is a emergency starting battery. Say, for example, a 4xT-105 house bank.

My inclination would be to always use the house bank and keep the 925 always charged (via ACR) but never used. My assumption is this would make the 925 last much longer, but not impact the house bank much.
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Old 12-23-2013
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Re: How big of a starting battery do I really need?

Check out my Post #8 in this thread: Starting battery recommendations - Cruisers & Sailing Forums

Bill
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