Voltage Drop.

[sailsmanship]

I have been concerned that my charging system was not working properly because my guage at criusing RPM reads 13.5 volts. At idle it reads 12.5 volts.<br />With the igntion on but motor not running it shows below 12.0 I put a digital voltmeter at the batteries and it reads 14.50 at crusing RPM and 13.50 at idle. Also if I put the meter to the back of any dash guages, I produce the same result as the volt guage. I have recently put in a new fuse block, eliminated the jumpering between the guages and ran them individualy to the fuse block, and ran new wire from the block to the batteries. Any thoughts on why the drop ?.

 

[Dunaruna]

Re: Voltage Drop.

What motor?<br /><br />All electrical connections loose some voltage but there are ways to minimise lose. Larger guage wire, soldered joints, clean the batt terminals (wire brush) just to name a few. Don't forget, the return path (the ground wires) need to be just as good as the '+' wires.<br /><br />Your charging system is working o/k.

 

[cp]

Re: Voltage Drop.

If your dash voltage gauge read closer to the voltage at the battery before you installed the new fuse block, then IMHO the symptoms indicate that the new wire you installed from the new fuse block to the battery is too small. A too small wire will have higher resistance over its length, resulting in a higher voltage drop such as you see. If you used a table to determine the appropriate size wire based on the distance from the battery to new fuse block, don't forget that you must double the wire length to account for the return path.<br /><br />If you used a large enough wire, then check for a poor connection which can also cause a voltage drop, but more seriously may be heating up and causing a potentially dangerous situation.<br /><br />Good luck.

 

[Boatin Bob]

Re: Voltage Drop.

I don't see any problems here, without the engine running you just measuring the battery notmally around 12.5, at idle the alternator is putting out some charge and it always increases with the RPM's, looks normal to me?

 

[jlinder]

Re: Voltage Drop.

You show a 1 volt drop to the guages. I agree with Boatin Bob that this does not appear to be a big problem. I would ask for the following information:<br /><br />1. What guage wire are you running from the battery to the dash? Include both the ground wire and the 12 volt wire.<br /><br />2. How long is the wire? If both are about the same length you can just give the one way distance, if not give both, but just indicate which you are giving.<br /><br />3. What and where is the main fuse?

 

[--GQ--]

Re: Voltage Drop.

In-addition to what Boatin Bob said; with the engine running, your volt meter shows the voltage reading from the alternator not your battery. Which is usually higher than the battery voltage. You system is perfectly normal.

 

[bandit86]

Re: Voltage Drop.

bad gauge, trust the multimeter. at least the gauge shows a difference, it's still somewhat useful

 

[byacey]

Re: Voltage Drop.

If your digital meter reads the same as the dash guage, then the guage is fine. If you have multiple fuses, remove fuses one by one until the voltmeter reads the same as the digital meter across the battery. When the voltage bounces up and read the same, you found the "offending" heavy load. I wouldn't worry too much about it, just take a mental note of where the guage reads under normal conditions.

 

[Speedwagon]

Re: Voltage Drop.

If you have nothing turned on on the dash, with the engine running, your volt meter should be reading what the alt is putting out(the 14.5). The gauges in a boat don't draw enough power to make a measurable drop in voltage, unless you have a bad connection somewhere, or far too small of a wire. <br /><br />16-18ga wire would be enough to power the gauges on a dashboard, with nothing else running. And this makes me disagree that all is well, because you might have a small problem somewhere, that could lead to a larger problem later on. Quite possibly corrosion is the culprit, so I'd trace the wires, and check all connections.<br /><br />edit: I'm not saying there is a problem with the charging system, I'm saying there is possibly a problem with the electrical system.

 

[sailsmanship]

Re: Voltage Drop.

Wire from fuse block to batteries is 10 guage. Wire from dash components to fuse block is 16 guage. The boat is 25 feet long so I think the distance from dash to batteries is about 15-17 feet. This is a fishing boat that has pretty basic stuff on it. Nothing to really draw a lot of power, just electronics and livewell pumps.Livewell pump wires are 14 guage. Guage readings were with all pumps off.

 

[Dunaruna]

Re: Voltage Drop.

You don't mention the ground wire size.

 

[jlinder]

Re: Voltage Drop.

OK, get ready for a little math.<br /><br />The formula for voltage, current and resistance is V=IR, where V is voltage, I is current, and R is resistance.<br /><br />You say your cable is 10ga., and I would guess the run is more like 20 feet (take into account turns, etc.)<br /><br />10 ga. wire has .00118 ohms per foot. You have a total of 40 feet, or .0472 ohms.<br /><br />If your 1 volt drop was due purely to wire resistance you could calculate the current in the wire by plugging in the numbers into the formula.<br /> V = I * R<br /> or 1 = I * .0472,<br /> or I = 1/.0472<br /> or I = 21.18 amps.<br /><br />You probably don't have that. If that is the case you are losing some in the connections. This can be splices, fuse block, connections, etc.<br /><br />If you are interested you can measure the voltage at a couple of points, like the voltage into the fuse block, after the fuse where you probably measured, on the wire that connects to the battery (possible dirty battery connections), etc.<br /><br />Keep in mind that 1 volt drop is nothing to worry about, but I would think about measuring again with everything running (lights, radio, etc) just to make sure. If your voltage drop remains low you are fine, and now have a better understanding of what to look out for in future problems.<br /><br />BTW - 10 ga is rated for a maximum of about 30 amps. You should have a fuse on this wire within about 6 inches (or is it 8 inches?) of the battery. In no case should a fuse be rated for more than the minimum current carrying capacity of the circuit it protects, so the fuse should not be for more than 30 amps.

 

[sailsmanship]

Re: Voltage Drop.

Thanks for the info. Ground wire is also 10 guage.