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2008 Beneteau Oceanis - Engine battery alarm on while battery appears to be appropriately charged.

Jun 24, 2019
Beneteau Oceanis 40 Rock Hall
Hi all,

We recently purchased our first bigger sailboat, a Beneteau Oceanis 40 from 2008. The boat was in very good condition and appeared only lightly used.
The survey did not discover any significant problems, and most of the issues mentioned by the surveyor were fixed by the owner bevor the deal was finalized.
So far, so good.
To get comfortable with the boat, I did spend almost two days crawling through the boat together with a knowledgeable person. We named every part, checked and explained function, he explained problems that could come up, and solutions to solve them. While doing that, we also opened every box with "spare parts" we could find, and one box was holding a complete electric panel. So obviously the electric panel had been replaced.
Anyhow, during our second trip with the boat after about 35 minutes of going under motor, the engine battery alarm came on. The small display in the electric panel showed the engine battery with 6.5 volts, obviously way too low, literally dead. We went back to the marina and connected the boat to the shore power. Because I had no voltmeter, I asked the marina for help. They came the next morning checked the batteries and found that everything was OK. Also no battery alarm on the panel anymore. However, they also mentioned that the house batteries are lead-acid while the motor battery is AGM, and that might cause problems. The charger is a Xantres Truecharge 40+, and it's stated in the manual that mixing battery types should be avoided.

I also looked in the surveyor report, and there the house batteries are reported as AGM batteries. So I have to investigate further who is right here, the surveyor or the marina technician.

The next day we went over this again, now I had a voltmeter, and we did some simple checks on the batteries with the engine (alternator) running and another with the charger on. It seems that the batteries being charged either way and without any load on them or charging all batteries had normal voltage.
During our testing, when the engine was running, the engine battery alarm cam on again and showed about 7.7 volts. We checked the battery directly, and it showed 14. 2 volts with the engine running and without about 12.5.
So long story short, from our simple way of testing things, I would carefully assume that battery and charger/alternator are running as expected, and the problem seems to be the panel giving false alarms, but I am not sure.

Are other people here having a similar experience?
Any thoughts about what else could be wrong?

Thank you,
Sep 25, 2008
Alden 50 Sarasota, Florida
Follow the wiring back from the alarm to the batteries -somewhere along that path, there is a bad/corroded/loose connection.
Feb 26, 2004
Catalina 34 224 Maple Bay, BC, Canada
Congratulations on your new boat.
So obviously the electric panel had been replaced.
Not necessarily. Could it be that this one is the new one he never got around to installing?
So I have to investigate further who is right here, the surveyor or the marina technician.
You look at the batteries, they usually have labels on them.
and it's stated in the manual that mixing battery types should be avoided.
Except in this case the charging voltages are very similar, and shouldn't be a big issue. Read up on this some more.

And, follow Don's advice.
May 17, 2004
Beneteau Oceanis 37 LE Havre de Grace
If you got 14+ volts on the battery itself when charging then I agree your charging system is probably working fine, and it could just be a panel issue. To confirm whether it’s the panel or a connection between the battery and the panel you can check the voltage on the wire that goes into the panel. Attached is the manual for what should be your panel, or similar to it. The engine battery wire should be pin 10 on the 12 pin plug. See if you can get a multimeter probe on that wire when the alarm is on, or unplug the plug and measure it that way.


May 24, 2004
CC 30 South Florida
It is not unusual for panel voltmeters to give inaquarate readings. There is likely a voltage drop due to old wiring or bad connections. I have never had a Battery Voltage Alarm so if it was me I would disconnect any anoying buzzer if that is the case.
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Jun 24, 2019
Beneteau Oceanis 40 Rock Hall
Hi all,

Thank you very much for your help and advice!

With the information, I will try to narrow down the problem in the next few days.
It's just not comfortable when your panel tells you that your engine battery is literally dead.
Anyhow, meanwhile, I got one of those handy small starter batteries and will carry a jumper cable, just in case.

Nov 21, 2007
Beneteau Oceanis 34 Tacoma, WA
Are there electrical components/systems on the boat that ONLY operate when the engine is running (windlass, bow thruster)? Do they function, and do they function ONLY when the engine is running? Does the engine compartment blower work? I had a problem with an alternator alarm last year, which ended up being a broken connector on a relay near the alternator (Hate your engine blower?). My symptom was an engine control panel alarm, not an electrical panel alarm, but what @Don S/V ILLusion said sounds reasonable. Your bad connection may be in a different portion of the system than mine was... good luck!


May 3, 2010
Beneteau 40 LI Sound
For what it’s worth I also have a B40 and randomly - maybe once a month- the low voltage alarm goes off for no reason. The voltage reads ok on the panel.