ammeter to voltmeter issues

[scotdunc]

Hi all, quick question on voltmeters. As my SV runs a modern alternater, I changed the ammeter to a voltmeter .... So now I find that my battery runs down somewhat quicker than it used to! I have it wired to the 4th (spare) terminal on the ignition switch and earth (is this how others have theirs?). Or is there an alternative.

Look forward to your comments guys ...

Toot toot all

 

[jumpinjan]

I'm not sure what better information a voltmeter gives over an ammeter. Its not going to indicate current flow and tell if the alternator is charging the system, or will it?
Does that 4th terminal get switched?; Check with a multimeter.
Voltmeters should have a very high impedance, so its current draw should be very. very low.
I would seem to me, the voltage measurement should be sensed inside the alternator, like across the rectifier diodes, not at the battery terminals?

 

[Nickodell]

This is one of those perpetual debates that has two irreconcilable sides. One says voltmeters are better as they show the state of charge of the battery. The other says that only an ammeter can show total current going into, or coming out of, the battery, and thus if your alternator/generator is matching the electrical load. I incline to the latter. When I flew, the planes had both instruments, which seems to be the ideal.

If your ammeter is connected directly to permanently hot (as opposed to switched hot) line and ground, it will continuously drain the battery, albeit at a slow rate due to its high resistance. ["Impedance" only applies to AC.)

 

[RootesRacer]

["Impedance" only applies to AC.)

Actually impedance applies to both AC and DC.
Reactance or reactive impedance applies only to AC.

 

[scotdunc]

I realised that there was a debate about the virtues of ammeters over voltmeters and vis versa, but I changed because it reduces the risk of electrical mishaps, i.e. shorts and/or fires behind the dash due to the heavy duty wiring needed for the ammeter.
Secondly regards my voltmeter instalation .... I thought the 4th terminal (on ignition switch) was a 'switched' terminal. However, as my voltmeter always shows battery charge, even with ignition off. I guess it is always 'hot', which will obviousley be the cause of my higher battery discharge rate. However, I cant see a way other than using the 4th terminal as a way of getting power to the voltmeter!

So I'm hoping that some of you can answer that one.

Toot toot all

 

[RootesRacer]

I realised that there was a debate about the virtues of ammeters over voltmeters and vis versa, but I changed because it reduces the risk of electrical mishaps, i.e. shorts and/or fires behind the dash due to the heavy duty wiring needed for the ammeter.
Secondly regards my voltmeter instalation .... I thought the 4th terminal (on ignition switch) was a 'switched' terminal. However, as my voltmeter always shows battery charge, even with ignition off. I guess it is always 'hot', which will obviousley be the cause of my higher battery discharge rate. However, I cant see a way other than using the 4th terminal as a way of getting power to the voltmeter!

So I'm hoping that some of you can answer that one.

Toot toot all

You could connect the voltmeter lead to the accessory terminal off the ignition switch (or fuse panel fro that matter).

 

[Nickodell]

Actually impedance applies to both AC and DC. Reactance or reactive impedance applies only to AC.

Hmm. My electronics text book says: "Impedance is the opposition to electrical flow of a single-frequency alternating current, being the combination of resistance and reactance. The inverse term is 'admittance.'"

 

[RootesRacer]

Hmm. My electronics text book says: "Impedance is the opposition to electrical flow of a single-frequency alternating current, being the combination of resistance and reactance. The inverse term is 'admittance.'"

From the wikipedia entry for electrical_impedance:

"When a circuit is driven with direct current (DC), there is no distinction between impedance and resistance; the latter can be thought of as impedance with zero phase angle."

 

[scotdunc]

errr .... I just swapped the voltmeter power wire from the 4th terminal to the acc terminal. Now no more power loss.

Sometimes its better to sit down and actually look at the wiring diagram!!!

Toot toot all