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Doug
Captain

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USA
457 Posts

Initially Posted - 04/18/2004 : 20:55:08

Spent the day in the Port Lazerette (SP?) trying to get the new motor's electric start. Have a question or two for the electro wizzes out there:

1. Does an amp gauge use much power? Noeta has an amp and volt meters. There's a switch that activates the amp meter, but it also opens/ closes the circuit between the battery and the motor. Switch open lets me start the motor and charge the battery, but leaves the meter on. Does the meter rob so much power that charging won't work? Will running with the meter on ruin the meter?

2. Will a fully charged battery show amp increase when charging? I tested the line at a number of places and showed an increase in amps when the engine was running, but at the battery terminal there was no change. Am I charging? Does a full battery simply reject any charge current and test no increase at the terminal?

Doug - #1913 Noeta

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Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 04/18/2004 : 21:51:23

Amperage is usually tested through an inductive field, so there is no load on the circuit. The gauge itself will be trivial. Don't sweat it.

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 04/18/2004 : 22:13:23

1) An ampmeter's power consumption should be completely trivial if not virtually unmeasurable.

They usually work via an induced magnetic field and have no direct connection to the circuit itself. Voltmeters have such a high resistance shunt that there is almost no current flow through them either.

2) "Will a fully charged battery show amp increase... "

Amperage is a measure of current flow through a circuit. Not sure what method you were using to check amperage at the battery terminal itself. The current flowing into a battery under charge is 'sunk' into electro-chemical changes inside the battery.

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dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 04/18/2004 : 22:51:12

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Doug
 

2. Will a fully charged battery show amp increase when charging? I tested the line at a number of places and showed an increase in amps when the engine was running, but at the battery terminal there was no change. Am I charging? Does a full battery simply reject any charge current and test no increase at the terminal?

<hr height="1" noshade id="quote"></blockquote id="quote">

If the battery is full, it should draw very little amps.

Think of the charger as a water pump, the electrical wire going to the battery as a hose, and the battery as a square water tank.

With the water tank (battery) empty, you start the water pump (charger) and it starts pumping water (electricity) through the hose (wires) to the empty tank (battery). Since the water tank (battery) is empty, water (electricity) is free to flow and fill it. The water flows through the hose at a certain rate (current/amperage) and pressure (volts). When the tank (battery) is full and can't take anymore water, water flow (current) through the hose stops, but since the water pump (charger) is still churning, water pressure (volts) is maintained.

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Doug
Captain

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USA
457 Posts

Response Posted - 04/18/2004 : 23:18:39

I sure do love this group.....

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Earl Landers
Navigator

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USA
157 Posts

Response Posted - 04/19/2004 : 11:41:03

Doug,
I started a reply last night, but it got lost before I was finished.

Too many browser pages open at the same time.

Answers;
1. Power consumption by your ammeter can be calculated fairly easily. P(watts)= I (current in amps) squared times resistance (in Ohms). A typical meter such as would be found in this application requires 1 milliamp for full scale deflection and has an internal resistance of about 500 Ohms, so P = .001 amps X .001 amps X 500 Ohms which, if my math is correct, is .0005 watts. Not a significant load.
Below is a link to Blue Sea Systems Installation Manual for their ammeters. The reason for including the link is that despite the replies that you got from Frank and Clam, inductive coupling isn't how typical DC ammeters work. DC ammeters actually measure the voltage drop across a precision resistor called a meter shunt. In the Blue Sea line of meters, the shunt is internal to the meter in those that measure between 0 to 50 amps and external on those that measure greater than 50 amps. A meter shunt might look something like this;

This is a large example

The meter shunt is wired in series with the load, (big holes on the shunt) and the meter measues across the shunt (small screw connections) http://www.bluesea.com/Instruction/9322.pdf On the second page of the link is a schematic drawing of the connections to an external shunt meter. Again, the shunt may be internal to the meter, depending on the brand.

I suspect that the switch that "Activates the ammeter" also energizes the electrical system which allows you to start the motor.

The only thing that will "Ruin the meter" is excess current as indicated by a pegged meter or blown meter fuses or smoke.

2. I don't understand how you "tested the line in a number of places". Were you moving the location of the ammeter shunt, or were you actually testing voltage?

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