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« on: August 13, 2015, 08:50:33 AM »
I do realize that discussions of batteries is a bit like religion and don't wish to stir the ire of people with a lot more experience than myself and no I am not the PO in this case, but let's look at the facts here...
1) I suspect from reading Ron's post that he is describing the "loaded" voltage on the battery, from which it is difficult to determine a SOC. If he let the battery rest for an hour, I'd bet the voltage would rise above 11.8V.
2) Most of us use our batteries much the way he does while seasonal cruising. That is, we have undersized alternators and rarely put back full charge when running the engine on a cruise. Some of us carry portable generators ($1000) to better top off our batteries, much to the annoyance of our anchorage neighbors. I noticed the other day, after a three-day stop, that my batteries (400 AH GC) were approaching 50%. On my cruise home, even after 2 hours of motoring, my 51 A alternator was unable to register 13V at the panel meter. Am I killing my batteries? Well yes and no. Any time you cycle the batteries, you are taking some of their life, but that is what they are for! Would I have been better off not charging them underway and waiting until I returned to shore where I could use my 100A 3 stage charger- I doubt it. Putting some charge back in a depleted battery has got to be better than letting is sit depleted- which is the worst thing you can do for them.
3) I would argue that you have to look at how you use your batteries is a big part of the system you put together. Your batteries start dying the minute they leave the factory. If you treat them very well, avoiding high temperatures, discharges more than 50%, use ideal charging techniques and never leave them discharged, you might get 10-12 years out of them (for AGMs), maybe 8 if they are wet-cells. It is quite typical for most of us to get 6 years out of our wet cells. How many cycles does a typical summer cruiser put on those batteries? 10/year if you are lucky? That's 60 cycles over a typical battery lifetime, 120 for AGMs. Compare that to the chart from page 3 of the data sheet I've attached from Trojan for cycles vs DOD. As you can see at 80% depletion, you get 500 cycles vs 1000 for 50% discharge. Even if I apply mainesail's very useful rule of thumb of degrading this by a factor of 2, Ron's batteries should last 250 cycles, which should provide him many good years of service. If Ron is lucky enough to cruise more than that in a summer, then he should think about investing in hardware to prolong his battery life.
We can spend thousands of dollars on generators, high output alternators, external smart chargers etc, but we will be saving money 5-10 years down the road, assuming the batteries don't fail for some other reason (shorted or open plates) long after the warranty has expired. That said, it is a good idea to make sure you can provide every reasonable source of charge to your house bank (like an undersized alternator) because the one thing that is certain to kill your battery is leaving it discharged for a long period of time. I hate terms like "battery murderer", because we all take life from our batteries as we use them, but that is what they are for. Some do it more quickly than others.
Sorry batteries.
