Low voltage alarm, but batteries check at 12.8 v

[boaterdude1]

On my 1990 Marine Trader trawler, the Low voltage alarm at the upper helm sounds whenever I use the windlass or bow thruster (for even a few seconds), but if I immediately manually check the house batteries below deck they check at 12.8 volts. Usually after running for 2 to 15 minutes the alarm stops sounding. Any ideas what could be causing it? The gauges at the helm station show 11.3 to 11.7 when this is happening, but measuring the house batteries shows these are false readings.

[Bill Seal]

Bad connectiion somewhere, after it gets hot, it either arcs across or the connection grows from the hest and the resistance goes down.
Ever had your car starter go clickity click, and after you leave the key in START position for a few seconds, the lead oxide on the battery terminal burns off, and the car magically starts.

[RossH]

Originally Posted by boaterdude1

On my 1990 Marine Trader trawler, the Low voltage alarm at the upper helm sounds whenever I use the windlass or bow thruster (for even a few seconds), but if I immediately manually check the house batteries below deck they check at 12.8 volts. Usually after running for 2 to 15 minutes the alarm stops sounding. Any ideas what could be causing it? The gauges at the helm station show 11.3 to 11.7 when this is happening, but measuring the house batteries shows these are false readings.

Check all your battery connections, especially the connections at the battery itself. Do the windless and bow thruster operate normally when engaged?

[smac999]

what is the battery voltage when running the bowthruster and windlass? not what it is after wards.

battery should be ~14.4v when the engine is running. 12.0v is a empty battery. 11.3 on the dash is a very low battery. so I would expect the alarm to happen. a battery droping to ~11v with the thruster is normal but only for those few secs.

[PaulCrawhorn]

"Immediately" being more than a second or two, can still mean the voltage under such big current loads did actually drop below the alarm setpoint.

Connections may be the cause, too long a run for that AWG, inaccurate V readings at a remote location

but keep in mind that batteries do wear down and that is one symptom.

A battery monitor with data logging will be helpful.

[rondelais]

Another thing to look at is where the low voltage alarm is being sensed. The closer to the batteries the better. A possible cure is to locate a battery close to the loads (I assume the thruster and windlass are at the bow, and the batteries are much further aft) which would take the load, avoiding voltage drop in the wiring. Best to use an echo-charger to keep that battery up.

[SV__Grace]

Is your engine running when this happens?

Do you have a high output alternator for charging your batteries?

Are the batteries dedicated to your windlass and thruster connected to your high output alternator?

The above posts assume all the answers to these questions are YES, but I didn't see you provide all that info so this is where to start IMHO. A windlass or thruster running without the engine on, or with a low output alternator can challenge any battery bank.

Also, speaking of unanswered questions, how big is your battery bank, what kind of batteries do you have and how old are they?

[Martkimwat]

Have you checked the wire sizing to both devices are correct for the maximum current expected for both devices? (remember the cable run length is out and back length not just one way). Also that your batteries are sized sufficiently to cope with the demands. Sorry if I'm asking the obvious but some times it is the obvious that gets overlooked. Have you had someone look at the battery voltage whilst you are operating either device? Friend of mine had a similar problem with his Eberspacher heater. Would auto shutdown if he tried starting it just on battery power. Had to run his engine to start it. I checked his cable runs and put in a larger gauge wiring and the problem was solved.

[Sailmonkey]

Originally Posted by boaterdude1

On my 1990 Marine Trader trawler, the Low voltage alarm at the upper helm sounds whenever I use the windlass or bow thruster (for even a few seconds), but if I immediately manually check the house batteries below deck they check at 12.8 volts. Usually after running for 2 to 15 minutes the alarm stops sounding. Any ideas what could be causing it? The gauges at the helm station show 11.3 to 11.7 when this is happening, but measuring the house batteries shows these are false readings.

You’ve got a high resistance connection or wire to your upper helm. I’ll bet that your upper helm normal voltage is below 12 all the time, then the extra draw drops you into alarm territory.