Voltage drop from battery to instruments

[Razorfish]

Firstly let me put my hands up and state that I am a complete numptie when it comes to 12v electrics, so if I show my ignorance I am not pretending.

The voltage at my instruments (both the chartplotter and GPS) reads about 0.5 v less than the battery voltage. I only noticed when sailing the other day this as the chartplotter has an alarm at 11.5 v.

Today after charging the battery, I checked the voltage at the battery, battery switch and split charge thing and all read the same (12.6 v). Both the plotter and GPS read 12.1v. So somewhere between the battery switch and the instruments I loose 0.5v.

Is this indicative of something I should worry about?

or

Is this normal?

My 1st action has been to clean all of the contacts and reset the plotter alarm to 11v which should correspond to 11.5 at the battery (a 40%ish charged battery or so I am told)

Cheers,

Andy

 

[VicS]

Firstly let me put my hands up and state that I am a complete numptie when it comes to 12v electrics, so if I show my ignorance I am not pretending.

The voltage at my instruments (both the chartplotter and GPS) reads about 0.5 v less than the battery voltage. I only noticed when sailing the other day this as the chartplotter has an alarm at 11.5 v.

Today after charging the battery, I checked the voltage at the battery, battery switch and split charge thing and all read the same (12.6 v). Both the plotter and GPS read 12.1v. So somewhere between the battery switch and the instruments I loose 0.5v.

Is this indicative of something I should worry about?

or

Is this normal?

My 1st action has been to clean all of the contacts and reset the plotter alarm to 11v which should correspond to 11.5 at the battery (a 40%ish charged battery or so I am told)

Cheers,

Andy

0.5 volt seems a lot. Any idea what current you instruments take... or rather were taking when you made your voltage measurements.

I'd measure the volt drop directly along sections of the circuit to find out were it is occurring .. dont forget the negative

 

[elton]

The voltage drop is probably in the cable to the instuments. The more current you draw at the end of a cable run, the greater the drop will be. There will always be some voltage drop along a cable that's carrying a current. It depends on two things: the resistance of the cable, and the current drawn. If the drop is too much, you need to uprate the cable, normally by using a greater diameter conductor. Half a volt isn't really cause for concern, but the more current you draw, the greater will be the drop.

 

[rob2]

It could be the charge splitter is having a significant hand in this. If they are diodes there will be the best part of half a volt drop across them.

Rob.

 

[halcyon]

My 1st action has been to clean all of the contacts and reset the plotter alarm to 11v which should correspond to 11.5 at the battery (a 40%ish charged battery or so I am told)

Get a length of say 1 sq mm wire, strip back one end and twist it round the negative probe on your multimeter, put some tap round to hold in place. The other end fix to the neagtive battery terminal.

You are now measuring voltage relative a fixed point.

By putting the positive probe on your earth connections the display will give you volt drop in the neg wires.

By putting the positive probe on the positive feed wires you have the positive voltage.

By starting at the battery and working away check each connection in the circuit.

If you see a slow drop in positive voltage, or slow rise in negative voltage you need larger size cable.

If you see a sudden change of say 0.5 volt, you have a bad connection, or damaged cable.


Brian

 

[starfire]

Are you measuring at the chartplotter & gps with the multimeter ?

If you are reading the volts off the display on each, they are not known for good calibration.

If it is with the multimeter, do the tests halcyon suggests.

 

[VicS]

It could be the charge splitter is having a significant hand in this. If they are diodes there will be the best part of half a volt drop across them.

Rob.

But the charge splitter wont be between the battery and the instruments. It'll be between the alternator and the battery.

 

[savageseadog]

Yacht instruments generally don't take a great deal of current, the wiring, contacts etc would have to be very poor to drop 0.5V. It may be an indication of corrosion in some cabling or crimps etc. Proceed with advice given by halcyon which is spot on.

 

[Razorfish]

Thanks chaps, all good advice.

I wondered about the charge splitter but that was the same both sides, and as Vic points out its the wrong side of the problem.

No I didn't measure with the multimeter but used the instrument display so will try using the former. If that draws a blankI will try Halcyons (Brians) methodology and report back.

Cheers,

Andy

 

[Captain Haddock]

Get a length of say 1 sq mm wire, strip back one end and twist it round the negative probe on your multimeter, put some tap round to hold in place. The other end fix to the neagtive battery terminal.

You are now measuring voltage relative a fixed point.

By putting the positive probe on your earth connections the display will give you volt drop in the neg wires.

By putting the positive probe on the positive feed wires you have the positive voltage.

By starting at the battery and working away check each connection in the circuit.

If you see a slow drop in positive voltage, or slow rise in negative voltage you need larger size cable.

If you see a sudden change of say 0.5 volt, you have a bad connection, or damaged cable.


Brian

I was wondering how to do this!

Many thanks and a great tip!

 

[Razorfish]

Oh and in response to Vics question, the total current from all the instruments, plotter and VHF is 1.6 amps

 

[VicS]

Oh and in response to Vics question, the total current from all the instruments, plotter and VHF is 1.6 amps

Thats equal to a resistance of 0.3 Ω.
9 metres of 0.5mm² cable would have approx that resistance .

So volts drop in the cable is a possibility only if you have some unexpectedly long cable runs to the instruments or some very light cable.

Do all your measurements with the same volt meter.

Also remember cheap digital voltmeters can over read when the battery start to fail .. or so they say.

 

[blackbeard]

Just as another thing to measure:
try switching on an additional current draw - for instance a 24 watt bulb in a tricolour will draw 2 amps. If the resistance of the wiring is as high as 0.3 ohm then this will give a drop of 0.6 volts. (There will be an additional drop in volts at the battery giving yet another drop in volts at the instrument end.)

and another source of trouble:
if there is a fuse in the positive side of the wiring (and there should be) try the effect of gently cleaning up the metal caps on the end of the fuse.
and, also, check to see if there is a volts drop across the main switch.

Good hunting ...

 

[smeaks]

My chart plotter does just this and I put the voltage drop down to the 12v socket. I know the displayed voltage is lower than the battery and don't let it worry me.

 

[little_roundtop]

So volts drop in the cable is a possibility only if you have some unexpectedly long cable runs to the instruments or some very light cable.

Or a corroded connector somewhere in the cable run. That happened to me, though with the fridge, where the voltage drop had had a more catastrophic effect!

 

[neale]

Could you have a problem in the fuse board. My old boat used to suffer terribly from voltage drop across the fuse board as the builder used something not suitable for the environment and the terminals used to corrode. A regular claen used to help but in the end I had to replace the whole thing with something better designed.

 

[Richard10002]

My 1st action has been to clean all of the contacts and reset the plotter alarm to 11v which should correspond to 11.5 at the battery (a 40%ish charged battery or so I am told)

Cheers,

Andy

Your batteries are about half empty/half full when the voltage is 12.2v..... You need to read them after a period of rest, rather than while in use. They are full at around 12.7v, and as good as empty at 11.7v.

As a general rule, you should aim to not let them fall below half full.

You might be worrying about a problem that doesn't exist Any joy with measuring the voltage at the instruments with a meter, rather than their inbuilt display?

 

[Razorfish]

Thanks chaps!

Not been able to get down today but will try to find and hour or two tomorrow or Tueday.