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I failed at my solar panel setup – while my boat was in the yard on St. Kitts for 18 months, something happened in the electrical system and my panels stopped float-charging the batteries. So my nice, expensive Lifeline batteries were dead after only one year of use! Improper charging isn’t covered by the warranty so here I am, out shopping for a new battery bank…
Multiple Sailboat Battery Bank Solutions
The battery compartment in my boat is very small. It’s like this on all small cruising sailboats. Previous owners had a Group 27 100 amp-hour house battery and an engine starting battery. One hundred amps wasn’t going to cut it for cruising; I needed at least 200 if I was going to be living aboard.
So I managed to cram two Lifeline GPL-4CT 6V batteries AND a starting battery in this space with no room left to spare. Combining these two 6V batteries in series gave me a 12V house system with 220 amp-hours (still on the small side for amperage). They worked pretty well though I had a hard time keeping them topped off with my renewable energy setup (see Windpower).
As a replacement now I want something a little more robust, or I should say cheaper.
My options of getting sailboat batteries in St. Kitts are fairly limited. My only choices are Lifelines, as I had previously, and the Exide Edge. Replacing the two Lifeline house batteries would run me $975 in St. Kitts, plus $520.30 for a starting battery – almost $1,500 (though slightly cheaper in the U.S.). What?!?
What about Exide? They make a dual-purpose deep cycle AGM battery. They’re 12 volts, 75 amp-hours, and smaller than the Lifelines, and cost $250.35 a piece in the Caribbean (around $195 in the U.S.). I’d need three of them to get a 225 amp-hour house bank. The three of them would only cost $750, but because they’re advertised as “dual purpose”, I could, in theory, use them as a boat starting battery. A solution half as expensive as my previous setup. Too good to be true?
Deep-Cycle vs. Starting Battery vs. Dual Purpose Construction
Deep-cycle batteries are designed to handle heavy loads for long periods before needing to be recharged. They can be repeatedly discharged to half of their capacity without shortening their life. They’re great for sailboats, especially for cruising sailors, who are living aboard and operating many DC systems like lights, refrigeration, a stereo, etc.
Deep-cycle batteries have thick plates inside that can withstand heavy discharges and are excellent at regaining their full charge if utilized properly.
Starting batteries are like what you have in your car. They have much thinner plates, and more of them. The increase in surface area provides a higher amperage to turn the starter, but because of the thin construction they are discharged quickly and will not fully recharge if left in a low charge state. They won’t last if used on house circuits in a sailboat because of their low tolerance for deep discharges.
Dual purpose batteries are just what you would think – a compromise between a dedicated deep-cycle battery and a starting battery. The plates are thicker than a starter battery to withstand deep discharges, but will also provide a higher burst of amperage for starting than a typical deep-cycle battery. The drawback is that for the size of battery you get fewer amp-hours than a normal deep-cycle battery.
Using House Batteries as a Starter Battery
People generally will not recommend this, and the reason is that all engines are not created equal. Some engines will start with a normal deep-cycle battery while others will only start with a proper starting battery. This is why you’ll always hear “you should have a house battery and a starting battery”. A starting battery will guarantee enough amperage to start any engine found in a sailboat (there’s another catch I mention later).
Let’s look at the specs of my previous starter battery and the Exide Edge Dual Purpose.
Cold Cranking Amps (CCA) is the maximum load the battery can handle at 0 degrees F for 30 seconds without falling below 7.2 volts. Marine Cranking Amps (MCA) is the same test conducted at 32 degrees F and is usually somewhere around 125% the CCA rating.
Lifeline Starter GPL-2400T: 650 CCA (MCA 790)
Exide Edge Dual Purpose: 775 CCA (MCA 970)
We see that the Exide already has more CCA than the Lifeline, and since I’ll have three batteries connected in parallel, the amperage is tripled, giving us roughly 2,325 CCA.
Starter motors usually have an initial inrush current of around 1,000 amps for 1/4 second to get the motor going, and is then at its normal load.
My Yanmar 2GM13 has a 1.2kW starter motor, which draws 100 amps on a 12V system (running the starter for five seconds will consume 0.14 amps). Yanmar recommends using a battery with a 200 CCA rating for the 2GM series engines – our CCA is tenfold.
These small engines don’t take much at all to get started, and can therefore be recharged quickly by the engine’s alternator.
Even larger engines like a 20-25 HP diesel, at the bigger end of engines found in small cruising sailboats, recommend around a 500 CCA rating.
There’s Always a Catch
The other reason people don’t recommend using your house bank as your starter also is because you must properly care for your batteries! If you let your battery bank get too low, your engine will not start!
Use good practices of never letting your batteries dip below their 50% capacity (112.5 amps in my case). This will prolong their life and leave some juice for starting when you need it, like when you’re dragging anchor on a lee shore.
This configuration requires special attention. Always turn your battery switch to OFF when the boat is unattended (I have my bilge pump on a separate circuit so it’s always powered).
If you were one of those people who would drain their house bank, then use the starter battery to start the engine and recharge the house bank, you need to get out of this practice. Your house bank is your starter bank too – you can’t drain your house batteries down and then use the same batteries to start!
Your batteries should also be within arm’s reach of the engine (as recommended by Yanmar). If the batteries are any further away, the current loss in the wiring will be too much and it’ll take more to get the engine started. However…
You should regularly test your battery capacity to identify batteries that might be going bad (I never did this). A cheap ($43.40 with Amazon Prime) battery tester is the SOLAR BA5. You tell it what your battery’s CCA rating should be, then it tells you what it actually is, with a simple LED indication of Good/Bad.
What about just using regular old AGM deep-cycle batteries to start?
Yes, you could do this also. A Lifeline Group 27 battery will deliver 575 CCA – lower than the Exide Dual Purpose, but still enough to start a small diesel. And if you have two of them to get a 200 amp-hour bank, that will double your CCA.
I just don’t have this option where I’m at. Being limited to either the Lifelines or the Exide, I can get more amperage for the space in my battery compartment, and at a cheaper price, by using the Exide. Your situation may be different in your boat and if you have more battery brand options.
I also like the redundancy of having three 12V batteries. If one should go bad, I can still manage with two until I can get a replacement. I didn’t have this option with my two-6V battery setup.