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Old 03-15-2006
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Battery Charge Level

I just need some basic battery understanding. I have two batteries of unknown age I inherited when I purchased my sailboat 4 years ago. Although they are both labeled as deep cycle, I assume the larger (27 group) is the starter and the smaller (24 group) is the house battery. I regularly charge them during the winter, every six weeks or so, in my workshop. The smaller battery has gradually had a diminished volt reading a day or two after charging to the point where it now reads 12.3v rather than 12.5 or 12.6 as it did last year. My question is whether I'll need a new starter battery for the upcoming season?
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Old 03-15-2006
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I think you will find all you need to know at the two websites:
http://www.trojan-battery.com/ (look under Tech Support)
http://www.uuhome.de/william.darden/

It's interesting that you think your house battery is smaller than the engine starting battery. Normally it's the other way around

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Old 03-15-2006
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Replace it ...

A disconnected battery (at rest) with an OCV of 12.3V is roughly 45% discharged. Doubting that you’ve removed the surface charge, your battery is not actually “at rest”, and would actually have a lower OCV than measured after charging.
The battery is likely dead, tho’ “equalizing” might help.
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Old 03-15-2006
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Oh Oh

Hi, Danapsy,

First, Deep Cycle Batteries should N..E..V..E..R be used as starting batteries. They are designed to deliver relatively "low" current over a long period of time until they have reached 50% discharge. At this point they should be recharged to more tha 85% (100% is better). A well maintained and charge deep cycle can easily give 10+ years of service on a boat.

A starting battery is designed to provide a lot of current over a short period of time and then be recharged rather rapidly. A well maintained starting battery shoud give 4-5 years of service (although the new spirally wound batteries can do substantially better).

A Deep Cycle battery used for starting will self-destruct. The plates are not designed for the "cram a lot of current into a cranky starter trying to turn a cold engine" job.

The voltage readings on the smaller battery do indicate a problem. A properly charged battery of that size should not self-discharge that much in that amount of time. If these are lead acid batteries, check the water level (I'm sure you already did); depletion could cause this. If water level is ok or they are Gel or AGM, the readings you have indicate that starting loads have probably caused heavy sulfation/shorting between the plates, and I would hope my engine is hand crankable.

Trace the cables for each battery back to their next connection. The battery servicing the engine should be removed and replaced with a hefty starting battery, preferably of the Optima design which doesn't care which way is up and can absorb a lot of punishment.

In addition to Valhalla's recommendations, I highly recommend Ample Power as a site to help with battery issues. http://www.amplepower.com/primer/index.html

Good Sailing
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Old 03-16-2006
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Thanks for the information and see that I'll need a new starting battery this season, and also what I assumed was the starting battery was probably the house battery. I haven't been consistent in using one or the other for either purpose, generally leaving my battery switch on Both when starting my engine. In addition, I now realize that using either 1 or 2 for the house battery depending on whether it was an odd or even day of the month was not good for the starting battery either. My next step will be to figure out the location of the 1 or 2 battery so I'll know which position to put my battery switch on.
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Old 03-16-2006
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Nigel Calder, in his Cruising Handbook, suggests you dedicate a battery to each purpose, using a "battery combiner" between the 2 for charging purposes. You don't need a deep-cycle battery for your starting battery, just a regular battery properly sized for your engine. That way, your start battery is used for it's purpose, and your house battery is used for it's. I think you can expect a longer life for each that way.
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