AGM battery fooled me - for a while

[Zilpo55]

I thought I'd share this recent weird experience. Our last trip out in the houseboat was almost cut short by a generator that wouldn't run. Our Westerbeke 12.5 (the most reliable device on our boat) started shutting down immediately after startup. This was out of the blue, it ran great the last time I turned it off. It was obvious the auto shutdown was triggering, but why? I tried the troubleshooting tests in the manual but nothing worked.

Then I noticed something. The starter was cranking slower than normal, even though the engine kicked right over. I discovered the battery was showing only 11 volts open circuit, even though it could still crank the engine. Switched to a spare battery, then started right up!!

Obviously, the auto-shutdown system is affected by low voltage. I had previously learned (the hard way) that AGM batteries do not fail like old school lead acids. They just seem to slowly fade. I did a lot of Googling looking for answers and found no mention of voltage or the battery affecting auto-shutdown.

This was the last of 5 AGM batteries that was on the boat when we bought it. There is no date on it, but I know it's over 8 years old. I don't plan to replace it with AGM.

[desimulacra]

Thanks for posting this! Getting ready to buy boat batteries
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[Zilpo55]

Thanks for posting this! Getting ready to buy boat batteries
.

So, are you considering AGM or old school lead acid? Our boat had AGMs for cranking on both engines and generator. The engine batteries died a few years ago. At the time, I didn't think the cost of AGM was justified. None of them had the install date. But this last one lasted over 8 years. I assume AGM prices might have gone down since I last investigated and it might be worth a second look.