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Author Topic:   Measuring Battery Voltage at Chartplotter
bigjohn1 posted 11-29-2005 09:12 PM ET (US)   Profile for bigjohn1   Send Email to bigjohn1  
I have the capability of monitoring my voltage via my NAVMAN chartplotter. It ocurred to me though, what am I actually monitoring at the chartplotter - actual battery voltage OR the amount of charging voltage the rectifier/regulator is allowing from the alternator into the battery? I assume these two values ARE NOT one in the same? The engine is a Merc 115efi.

This all came about when I realized earlier in the year that I am replenishing my current battery's water too much.
I am about to switch to an AGM battery but I want to correct the over-charging problem (if there is one) prior to the new battery. I reason that if I don't, I will simply "cook away" a new $200 sealed battery since you can't add water to them.

Here are the voltage values stated in my factory manual:

1. Running voltage: Measured taken at the battery with engine running (but no RPM value given): 14.5-15v

2. Charging voltages:
@750 RPM - 12.5 - 15.5
@1500 RPM - 13.0 - 16.0
@3500 RPM - 13.0 - 16.0
The manual does not state this but I assume charging voltage measurements must also be taken at the battery?

Now I need to correlate these factory values with what's being read on my chartplotter -- BUT, is the chartplotter reading battery voltage or charging voltage?
NOTE - the chartplotter is wired directly to the battery (but it is properly fused).

jimh posted 11-29-2005 09:31 PM ET (US)     Profile for jimh  Send Email to jimh     
The most accurate place to measure the battery terminal voltage is at the terminals of the battery.

Measurement of the voltage at a remote point via connecting cables will include the voltage drop in the cables. The voltage drop in the cables will be proportional to the resistance of the cables and the current flowing in them. The relationship is

E = I x R

E = voltage or Electro-motive force in volts
R = resistance in ohms
I = current in amperes

It would be quite unusual for the voltage drop in the cables connecting an instrument mounted a few feet away in helm to the battery to amount to more than 0.1 volt or so.

Thus, when you measure the voltage at some remote point such as an instrument at the helm of your boat, you are in effect measuring the battery voltage with perhaps a slight drop. I would be more concerned about the accuracy of the fishfinder as a voltmeter than any error induced from voltage drop in the cables, unless there were conductors or terminals with high resistance in the battery distribution wiring.

The charging current is generated in the engine and is connected to the battery by cables. The same situation applies. It is possible that there is a voltage drop in the cables proportional to the current and resistance. The resistance of the cables is made very low (by the large size of their conductors) so that even with substantial current flow there is minimal voltage drop.

Thus, when you measure the terminal voltage of the battery at the battery terminals you are measuring the voltage output of the charging system, minus a small drop in the cables, which, again, would be around 0.1 volts except in cases of the most extreme charging current, say 50-amperes or more.

swist posted 11-30-2005 07:47 AM ET (US)     Profile for swist  Send Email to swist     
You can't easily separate out those two voltages since the charging system includes the battery in the circuit with the regulator.

But if your intrument is getting voltage straight from the battery then it should be a simple matter of looking at the voltage with the engine not running, and then with the engine running at various speeds. A healthy battery should read about 12.7 volts with no load. With the engine running (at high enough speed for the alternator to be working - most don't produce much output at idle), you'll probably see anything from 13 to 15 volts.

Chuck Tribolet posted 11-30-2005 08:44 AM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
The voltmeter in the chartplotter may not be terribly
accurate. I know the one in my fishfinder isn't, compared
to my collection of Fluke 77 DVMs.


bigjohn1 posted 12-01-2005 08:08 PM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Thanks for the input so far. I had the boat out all day yesterday and here are the actual measurements from the chartplotter while underway.

Engine off: 12.3v (not so good)
900 RPM: 12.8 Vdc
1,900 RPM: 13.5 Vdc
3,000 RPM: 14.1 Vdc

The above values were taken at the boat ramp just after launching. I trolled at 2,500 RPM all day and the voltage reading was 14.3 Vdc - 14.6 Vdc all day long. Everything seems to be within factory specs if I believe the chartplotter. Thusfar, the unit's paddle wheel speed and transducer water temperature readings have been extremely accurate so I have no reason to doubt the voltage. I realize it may be off just a tad when compared to a Fluke 77 but those are some of the best meters out there.

I am going to have my battery load tested in the coming days, a practice I have done each year for quite some time now. My suspicion is the battery is marginally o.k. at this point. 12.3 Vdc at rest is not good and I use my boat on a weekly basis.

Chuck Tribolet posted 12-01-2005 09:51 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Fishfinder: 13.8V
Fluke 77: 13.14V

These are on the regulated side of a Humminbird SureVolt
regulator (You put any old DC (6-24V?) in, get 13.2V out.).
Unfortunately, no longer available.

I suspect the voltmeters in marine electronics are pretty
chintzy. But if you know the offset from reality, they
are probably useful. In the above case, the offset would be
+.66V, which is about what I remembered).


bigjohn1 posted 12-03-2005 07:24 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Chuck - I find your comparison interesting to say the least. I have access to a Fluke 77 so I'll measure with that and see what the variance is.
Jerry Townsend posted 12-03-2005 01:42 PM ET (US)     Profile for Jerry Townsend  Send Email to Jerry Townsend     
As Chuck basically points out - any 'included' voltmeter is not necessarily all that accurate - beit in a fishfinder, GPS, boat instrument panel,

Any FLUKE meter that has not been abused will be reliable and accurate - it doesn't have to be a model 77. Chuck likes them - and I would not have anything else - they are the best - but expensive. ---- Jerry/Idaho

bigjohn1 posted 12-04-2005 07:24 AM ET (US)     Profile for bigjohn1  Send Email to bigjohn1     
Test completed - the Navman chartplotter was .2vdc less than the Fluke. I only measured the voltage of the battery at rest. Both intruments though told me the battery was on its way south. I had Napa run a complete load test and it confirmed that - 12.5vdc and 330 cca (and that was fully charged).

Guess my old habit of load testing batteries once a year payed off this time. A brand new Orbitol AGM found a home in my console today. Lets hope she lasts more than a year.

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