Ammeter installation

[bigwinn]

Hi all

I've searched the Wiki before you ask

I'm installing an ammeter into the centre console to keep an eye on the drain etc

where should I pull the feed from- guessing I need to put it in series rather than parallel?

What amp rating wire is needed

ref the backlighting, can I pull a feed from the normal dash illumination lights- like the heater panel?

Thanks in advance

Stu

[Speedtouch]

Yes, they go in series, in the main feed between the alternator and the battery.

Use wire rated at least that of the alternator's maximum output, e.g., 55 Amp.

Wire the backlight into the existing dashboard lights feed.

[bigwinn]

cheers

is it best to chop and join the cable?

Is the wire easy enough to identify?

[Speedtouch]

According to my Haynes manual, the main feed cable is a red wire going direct between alternator and starter motor terminal 30, to which a black wire comes in from battery +.

You can chop the cable and splice in the ammeter, or run separate lengths of cable, as required.

[bigwinn]

nice one-

cheers

S

[Brianmoooore]

I don't recommend using this ancient kind of ammeter that goes in series with the wire! If you must fit an ammeter, rather than a voltmeter, you need the type that measures the voltage across a shunt resistor fitted in series with the lead you are trying to measure, or measures the voltage across the battery earth lead.

[bigwinn]

Brian

How is best to install across the battery earth lead?

Stu

[Speedtouch]

I don't recommend using this ancient kind of ammeter that goes in series with the wire! If you must fit an ammeter, rather than a voltmeter, you need the type that measures the voltage across a shunt resistor fitted in series with the lead you are trying to measure, or measures the voltage across the battery earth lead.

An 'ancient kind of ammeter' essentially is a shunt resistor fitted in series with the lead being measured!

[TouringTash]

http://www.ebay.co.uk/itm/Fluke-233-Rem ... 27c763f51a

Only good for up to 10Amps DC though.

And STOP adding things it makes a car slower.

[Brianmoooore]

I don't recommend using this ancient kind of ammeter that goes in series with the wire! If you must fit an ammeter, rather than a voltmeter, you need the type that measures the voltage across a shunt resistor fitted in series with the lead you are trying to measure, or measures the voltage across the battery earth lead.

An 'ancient kind of ammeter' essentially is a shunt resistor fitted in series with the lead being measured!

The movement typically used in car ammeters (thinking back to the days of the Smiths 30-0-30) was a moving iron type, essentially consisting of a single turn of thick copper wire around a pivoted magnet - not a shunt resistor in sight. This was the type where the needle would respond to changes rapidly, but there was also the type where the needle moved much more slowly, and consisted of a bimetallic strip adjacent to a very low value series resistor, and responded to the heat produced by this resistor in proportion to the current flowing through it.
Series connected ammeters these days are almost always actually voltage meters, with the movement being used to measure the voltage developed across a low value resistor in series with the current being measured, and, with a sufficiently sensitive volt meter, the resistor can actually be the battery earth lead. Another alternative, is to use the clamp meter principle, where a sensor is simply placed around the wire carrying the current, and detects the magnetic field produced by this current.
As willnz has already written, it's all a bit pointless these days for automotive applications. Back in the days when the electrical generator on a car consisted of a dynamo, with a maximum output of about 20 amps, it was sometimes necessary to monitor and manage electrical consumption to avoid draining the battery as you drove, and an ammeter was provided for this purpose.

[bigwinn]

Ok thanks all

Had identified the drain but thought an ammeter would be useful in future

As this sounds sledgehammer cracking nut style and potentially dangerous, I'll park it

Cheers

Stu

[Speedtouch]

Regardless of the ammeter construction detail, it will still have a small resistance, hence if placed in series between battery and alternator, is effectively acting as a shunt resistor.

I had one of the Smiths -30 -0- +30A ammeters fitted to my Minor Traveller with Lucas alternator, and found it to be quite useful. It was sensitive enough to register relatively small loads such as extra lights, indicators, etc. being switched on, and you could see how quickly the battery was being charged following a cold start, for instance.

If they're wired in correctly, they're perfectly safe; I had mine on for many years with no problems, but eventually removed it to put an inlet manifold vacuum meter in its place, which is quite entertaining to watch as it acts a bit like the BMW's 'swingometer' economy gauge!