NDR - boat electrical question

John Klingenstein

John Klingenstein

Well-known member
Not a duck boat but.....
Here's a marine electrical question for the boat guys or maybe an electrician.

This past weekend we were moored on a ball in PIB harbor. (Lake Erie)
(no shore power obviously)

2008 Beneteau 37'
Two batteries - 1 cranking and 1 deep cycle
Battery age - <1 year

While on a ball the low voltage alarm went off in the middle of the night at 10.8V

All systems were off - main breaker was off - fridge was off - nothing was on yet the voltage to the house battery (deep cycle) dropped enough to trip the voltage alarm.

I started the engine and ran it for about 15 minutes - All good - voltage on house battery returned to ok.

Cranking battery was fine.

Three hours later.....same thing.

I didn't have a multimeter with me to run additional tests.

Ground problem ?
Suggestions on a troubleshooting starting point ?
 
John,

My first thought is that you have a weak cell in the deep cycle battery. If the entire battery was at 10.8 volts, 15 minutes of charging from the engine alternater, is not going to bring it back to 12 volts. Fifteen minutes may be enough to bring one weak cell back up to 12 volts, but obviusly the weak cell is not staying at 12 volts.

Either that or you have a short somewhere.
 
John,

My first thought is that you have a weak cell in the deep cycle battery. If the entire battery was at 10.8 volts, 15 minutes of charging from the engine alternater, is not going to bring it back to 12 volts. Fifteen minutes may be enough to bring one weak cell back up to 12 volts, but obviusly the weak cell is not staying at 12 volts.

Either that or you have a short somewhere.
Click to expand...


Ditto
 
John,

My first thought is that you have a weak cell in the deep cycle battery. If the entire battery was at 10.8 volts, 15 minutes of charging from the engine alternater, is not going to bring it back to 12 volts. Fifteen minutes may be enough to bring one weak cell back up to 12 volts, but obviusly the weak cell is not staying at 12 volts.

Either that or you have a short somewhere.
Click to expand...


That's what I'm thinking.

I'll take a multimeter with me this weekend and take a reading on the fully charged house battery then try to duplicate the problem.. take a current reading at the battery while the alarm is sounding to confirm it's not a bad alarm.

The battery is only a year old....grrr.....
 
If load test is OK where would you start to find the current drain ?

I guess I would need to check everything directly wired to the house battery with the bildge being first.
 
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Go to the DC electrical panel, most boats of your size have meters for both the AC and DC sides of the panel. If no meters use a hand held meter. Turn off one breaker at a time. One of the breakers will show a difference from the others. That circuit should tell you where the current draw is, or short which ever it is. If your batteries are fully charged you can do the opposite turn off all breakers, read voltage, turn on one at a time and see which one shows a voltage drop, Rich
 
I didn't quite read the responses given and these are probably covered, but a couple of things I would check.

Check for a draw somewhere along the way (little light bulb left on somewhere?). Use a multi meter to measure current draw. Should be darn near zero with all the lights, radios, disco balls etc turned off.

Possibly a bad cell in the battery... Could happen even with a new battery. And this brings me to my final point : Make sure your 2 batteries are isolated when the ignition is off. Use a large solenoid relay so that you can only draw from and charge to the accessory battery when the ignition is on. At the very worst case scenario, if the starting battery dies, it won't take the 2nd battery with it, and you still have the means to start your boat...
 
John, Dont get frustrated yet. It is simply a process of elimination. Thats why with the breakers you can isolate it to at least one circuit. Lets say it is your bilge pump circuit, that narrows it down a bunch. Now it is just a matter of finding out which pump is bad, or which float switch hung up till the pump fried just enough to cause a low voltage drain, yet not enough to trip the breaker. we built a 56 foot sportfish 25 years or so ago. Launched it, everything looked good, after a brief maiden voyage, we tied her up and went home for some much needed rest. Next morning, the zincs on the transom were gone! and I mean gone! Rudders and props were starting to show major pink spots. We hung a milliamp meter off the transom and it went right off the scale. Tried a regular multimeter and found 3 volts leaking out thru the bonding system. Did the old shut off the breakers trick. Isolated it to the main DC engine breaker. Then we disconnected one connection at a time [starters, fuel solinoids, alternators etc.], When we did the port alternator everything went back to normal. Found there was a bad diode in the alernator. Had we left the boat for a week it would have caused big damage. A simple process of elimination saved us a bundle, shouldnt be much harder with your problem, Rich
 
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