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Old 14-09-2015, 16:15   #31
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Location: Nova Scotia
Boat: S&S Loki Yawl 38'
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Re: Quick discharge of service battery

Are you reading the battery voltage directly from the battery or is this from your instrumentation? I would try disconnecting the negative lead on the suspected battery with the battery fully charged (turn off your charger before disconnecting) and then read the battery voltage with a portable voltmeter over a period of time just to eliminate the variables. If the voltage is truly dropping per your post then that battery has serious issues and needs to be replaced. The anode problem sounds like you have DC leakage out the prop shaft. Often times I have found this to be an alternator wiring issue but I am sure that there are many other possibilities. You could maybe separate the shaft coupling, read the voltage (there will be some slight voltage due to the anode) with everything turned off and then turn stuff on one system at a time to try and isolate the of luck these things can be tricky to find sometimes.


Originally Posted by Stankut View Post
Dear friend,
I have a Beneteau 373 since 4 years. I changed my batteries 4 years ago. I have one OPTIMA 75 Amp Yellow top battery for engine start and one 140 Amp battery for service. I have serious problem with my service battery. After disconnecting from shore power the service battery drop to 9 - 9,5 volts in 30 minutes.
When I start the engine it takes 30 - 40 min. to recover and the voltage arrive to 11 V then suddenly 13,5 V.
I checked the battery and they said it is OK.
Another problem is the anode. I changed it 4 months ago and it was totally finished laat week. I changed it again.
Do you have any experience with similiar problems?

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Old 14-09-2015, 18:27   #32
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Re: Battery issues

It sounds like the battery is toast, but do test it as Lokiyawl suggests. Four years between charges will kill most batteries, as they self-discharge much faster than that. Charging every few months is a better plan.

Anode problems can come from improper wiring, a non-isolated shore-power ground, neighboring boats with these issues, or a "hot" marina. If you can disconnect shore-power and keep your batteries charged with a solar panel that might greatly improve your anode life. If you must use shore-power, install a ground isolator.

If those suggestions don't work, you have improper wiring, bonding, dissimilar metals in the water, or something similarly difficult to deal with. Get expert help. Quickly disappearing anodes can lead to major problems, including dissolving thru-hulls, props, etc.

Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
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Old 15-09-2015, 13:31   #33

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Re: Battery issues

"I checked the battery and they said it is OK."
They could be wrong.
Or your charging system could be faulty.

In the US, battery and alternator shops are infamous for being incompetent and thieves all too much of the time. Even at the larger brand name locations and chains.

So spend $25 on an inexpensive multimeter (US prices) and another $25 on a basic book like "The 12 volt Bible" or similar from a chandlery, or look on the web for how to test a battery and charging system.

It sounds like your main battery has been chronically undercharged and are partly dead, partly sulphated, but with a small investment you can find that out for CERTAIN.

Otherwise you pay hundreds to a marine electrician and perhaps hundreds more on equipment that many not really be needed. The book reading & learning will take less than one weekend.

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