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Voltage fluctuations when using ST4000+

arf145

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Nov 4, 2010
430
Beneteau 331 Deale, MD
Our Garmin echoMap 50s chartplotter was occasionally shutting down, and when I finally noticed that it mostly happened when the autopilot (ST4000+) was engaged, I tested the GPS plug voltage and saw voltages in the 11s and even briefly 10s as the autopilot motor operated. I suspect having the refrigerator going at the same time wasn't helping, as the problem stopped once I turned off the fridge while sailing.

Is this kind of momentary low voltage normal for our situation? Wiring issue? Battery issue? Our house bank is two group 27 flooded batts.
 
Nov 26, 2012
2,315
Catalina 250 Bodega Bay CA
As you seem to now realize: the total amp draw is excessive for your battery bank. Add another battery and if you don't have solar, install it. Large self steering units like yours need lots of power to operate efficiently and a constant solar charge while in use is even better. Chief
 
Nov 6, 2006
9,202
Hunter 34 Mandeville Louisiana
Could be bad (dirty, loose) connections at the battery and/or in the wire that feeds the circuit breaker panel. Check the wire size as well. Probably should be a something like a 10 gauge.. if 12 gauge feed to panel, replace with 8 or 10 .. Battery connections would be first suspect, then feed wire connections, then wire size. I am assuming batteries are fully charged and in good condition.
EDIT: you can find the culprit with a voltmeter.. Start at the panel and check voltage there. If the volts on the panel buss don't vary, then the connections to the plotter are suspect.. check the end of the panel feed wire. If the panel varies and the end of the wire doesn't, that is the bad connection. then back to the master switch that feeds the feed wire; again if the post of the switch does not vary, then that connection is bad or the wire is too small. Check wire size by reading the voltage across the wire itself. That should be no more than a half volt but preferably less. One lead of the meter on the master switch post and the other at the panel buss. Check the battery connections by reading the voltage between the terminal clamp and the battery post itself . again the voltage drop across the terminal-to-post should be very small. less than a half volt.
EDIT II : the stuff has to be all running when you take the measurements.
 
Last edited:
Jun 6, 2006
6,991
currently boatless wishing Harrington Harbor North, MD
the only way to tell if it is battery capacity or wire corrosion is to measure the voltage at the batteries with the unit drawing current. My bet is corroded connectors. To test a connection use a volt meter and probe across the terminal/connection. just pierce the insulation on each side WITH CURRENT flowing. Each connection should be under o.25 volts drop across the connection. less is better. If you see anything over 0.5 volts that connector is badly corroded.
 

arf145

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Nov 4, 2010
430
Beneteau 331 Deale, MD
Appreciate the suggestions! I think I'll start by getting a reading at the panel while the autopilot is running. Forces me to wade into the Beneteau WAGO block though!
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
It is one of a few things.

* Low battery voltage
* Undersized circuit or panel feed wiring leading to voltage drop
* A bad connection
* Corroded connection

Ideally the GPS & AP would not shard the same circuit. Often WP AP's need quite large wire, 10GA plus due to the constant starting / in rush loads imposed by the motor and the wire distance in the typical AP circuit..

Check battery voltage, at the physical battery terminals, when this is occurring, and see how much it differs from the AP end.. Then put DVM pos at the batt terminal and DVM neg at the AP end POS wire. Are you getting readings? Now do the same for the neg lead....
 

arf145

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Nov 4, 2010
430
Beneteau 331 Deale, MD
Not sure I understand putting the voltmeter pos on batt pos and neg on AP pos (not to mention that they are far apart). But I can say that there is no way it's using 10 gauge wire right now--might even be 16. No matter what, I'm going to look into putting the AP on its own circuit.

I *think* the batts are not the prob, in that they measure 12.75 after resting overnight, but I don't know their whole history, other than that they were new in 2010, so middle aged I guess.
 
Feb 6, 1998
11,436
Canadian Sailcraft 36T Casco Bay, ME
Not sure I understand putting the voltmeter pos on batt pos and neg on AP pos (not to mention that they are far apart).
You may need a wire to extend one of the leads. Putting the DVM across a wire, end to end, will show you the voltage drop in that wire. Ideally you should have little to no reading. Comparing the neg wire and positive can identify a bad termination.. If both pos & neg give the same drop then it is likely undersized wire...
 
Sep 3, 2013
36
Beneteau 331 Brewer's Warwick
Not sure I understand putting the voltmeter pos on batt pos and neg on AP pos (not to mention that they are far apart). But I can say that there is no way it's using 10 gauge wire right now--might even be 16. No matter what, I'm going to look into putting the AP on its own circuit.

I *think* the batts are not the prob, in that they measure 12.75 after resting overnight, but I don't know their whole history, other than that they were new in 2010, so middle aged I guess.
There is often a large difference in a rested battery compared to one under load. A well rested badly corroded battery can still show good voltage. The corrosion acts to slow down the ability to diffuse the chemicals in the battery and thus reduces the available amperage at a given current. Thus when the current starts to flow the voltage drops. As there is not much electricity transferred in battery at rest it hides the problem of slow diffusion.

If you are re-wiring take a look at this table for chosing correct line size. http://www.engineeringtoolbox.com/amps-wire-gauge-d_730.html

While many of the ideas are well worth doing in any event, it may be the batteries especially if you hadn't had the problem in previous years. Batteries seem to get lighter and crappier all the time. Make sure you have deep cycle batteries. One typically doesn't need a high cold cranking amps (CCA) to start a relatively small sailboat engine. Those starting batteries with the high CCA tend to have higher surface area to minimize that diffusion mentioned above. This extra surface area is often accomplished by (relatively) thin plates or mesh. This is the type of design used in cars. Car batteries typically don't utilize much of their reserve capacity as the generator is running as long as the car is running.
However, after many cycles the plates lose area and have less amp capacity. Problems will occur much quicker in those batteries should you work it hard and require deep charges.
 

arf145

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Nov 4, 2010
430
Beneteau 331 Deale, MD
OK, I get it MS. Thanks!

Barry, we've only had the boat a year, and the problem was there last year, starting in September when I installed the Garmin. But it took me a while to zero in on the circumstances when it occurred because it happens when we're sailing, and I hate to hear the groans when I shift into diagnostic mode while we are having fun!

The batts are not bad, I don't think. And they are deep cycle. I can run everything aboard including the fridge for 24 hours with enough juice to start easily the next day. However, when I replace these batts, I'll be going with 3 group 27s. And maybe get a real battery monitor at that point.

As far as solar goes, I installed a little 20-watt panel in my last boat to keep the batts topped up, but I'd have to get more serious about it if I want to leave the fridge running.
 

arf145

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Nov 4, 2010
430
Beneteau 331 Deale, MD
Here's an update on this. During a recent trip, I noticed that the fridge wasn't working properly under batt alone, its error light suggesting low volts. Which suggested an overall voltage prob and not one necessarily limited to the AP and GPS. I finally took a look at things yesterday, and while I haven't done MS's voltage drop test on the wires, I did go over all connections from the batts and switches to the panel. The volts I was seeing at the panel was 2.5 tenths below the voltage at the batts. The connex at the three main switches were a little loose, plus in a couple of cases they had the big load leads on top of the tiny load leads. I rearranged and tightened all that up (and finally put the non-wing nuts on the batts!) and found the volts at the panel now only a few hundredths off. It did look like Bene used a pretty small gauge wire to the main panel, but I won't work on rectifying that until I understand the overall wiring better. Someone added a second panel (for VHF, small fans and anchor washdown) that is fed power with a nice big 4 AWG wire, but that one bypasses the master switches and is always hot. Working on a diagram.