Ammeter or Voltmeter?

[silloyd]

Which is the more useful for monitoring battery / electrical status and ammeter or a voltmeter?

BTW I don't have battery issues at present but I have a dynamo and would like to keep an eye on things - I don't see a need to switch to an alternator at the moment.

Thanks in anticipation for your views and guidance.

[MarkyB]

For monitoring battery status, a voltmeter.
For charging system status, an ammeter.

[silloyd]

Thanks for that.

I Was intending to fit one or the other, rather than both (I want to fit oil pressure and water temperature gauges as well and feel that 3 gauges is plenty). Since posting I found the following rather helpful pages:

http://mgaguru.com/mgtech/electric/et220.htm and
http://mgaguru.com/mgtech/electric/et221.htm

I think I'm erring towards a volt meter.

[MarkyB]

Interesting reading, he makes a good case for the voltmeter.

[bmcecosse]

The AMMETER is the best by far - tells you what's going on - not just that the battery is flat! However - it does mean that you have to bring heavy cables into the car to the meter - which carry all the current to/from the battery - and cannot be fused. So - they need to be well insulated and make sure they can't chaff against anything.

[silloyd]

I'm confused now!?!? (Doesn't take much!)

The AMMETER is the best by far - tells you what's going on - not just that the battery is flat!

Is that because you can see the actual charging/discharging current as opposed to seeing the resultant rise/fall effect on the voltage, i.e. viewing the cause rather than the symptom as it were?



In respect of cabling to an ammeter, do you effectively have to use the same size cables as those that connect the battery?

[bmcecosse]

Yes - you see instantly if there is a charge or a discharge. Cables need to be same size as the brown cable at the solenoid (you extend it to the meter and then back to the solenoid) - NOT the big battery cables!

[silloyd]

Thanks.

I've looked at these http://www.autosparks.co.uk/index.php?c ... 536r7c7d23, which look suitable type but what's the current pull at the solenoid?

Also, is a shunt (rather than an in-line) ammeter a better option?

[bmcecosse]

Dynamo can only push about 28 amps max - so that's about the worst possible situation - assuming very flat battery! Even if you switch on ALL the electricals in the car with the engine not running - that would only be ~ 20/24 amps assuming you don't have current greedy heated screens in the car!

[silloyd]

So, about 35amps would keep me on the sane side. Thanks.

Is a shunt (rather than an in-line) ammeter a better option?

[bmcecosse]

Of course that's what's used in Industry, no one measures any worthwhile DC current directly - but haven't heard of it in a car application. I imagine it will lose some of it's accuracy - if there ever was any in the first place......

[silloyd]

This is what prompted the question: http://cgi.ebay.co.uk/Jaguar-Lucas-Shun ... 588840d77d

[bmcecosse]

You can see why - at +/- 80 amps! It won't even flicker on a Minor. It's very expensive - and doesn't seem to include the shunt as far as I can see!
This is more like it! http://cgi.ebay.co.uk/Lucas-30amp-ammet ... 35aa115107

Although I much prefer the Smiths one http://cgi.ebay.co.uk/Smiths-30-Ammeter ... 5ada366c02

[silloyd]

Fair point! Given the points above a +30/0/-30 Amp is indeed more sensible.

[mike.perry]

If you are running a dynamo then go for a 30/30 ammeter, if you have an alternator go for a voltmeter.
You only need two gauges if you also fit a dual gauge temp/oil pressure. Or you could fit a 52mm rev counter in the 3rd hole

[bmcecosse]

The 'dual' gauges cost a fortune - and often as not - the temp side is not working......... And a 52 mmm 'rev counter' - you'll never be able to see it - and in any case - what's the point of 'counting revs'????

[silloyd]

See, I knew there'd be a straightforward answer to this one.

...if you have an alternator go for a voltmeter.

In the words of Maunuel from Fawlty Towers: "Que?"

Why the difference please, given that both a dynamo and an alternator perform the same function, i.e. providing elastic trickery, albeit in different ways?

[silloyd]

...what's the point of 'counting revs'????

Don't be so anti-revs! They do count!

[mike.perry]

Someone is going to disagree but in my opinion the alternator produces a higher amperage than a dynamo so you are unlikely to go into a discharge. With a dynamo it is easy to exceed the output when running mainbeam, wipers, heater etc etc or if the brushes are worn, which would show on an ammeter.
I have an ammeter fitted to my Series MM to monitor the output of the C39 dynamo and a voltmeter in my Traveller which is fitted with an alternator. When I sort out the brackets and wiring I will fit my 52mm rev counter which is in a box so I may as well use it.
Or you could fit one of those economy gauge things that point to the red bit every time you accelerate so that you spend your entire time feathering the throttle and staring at the gauge trying to keep it in the green bit.

[bmcecosse]

My ammeter manges just fine with an alternator - so I say AMMETER every time !
I won't rise the bait on the useless vacuum gauge........