After battery charging isses the other week whilst out rallying we've decided to fit either an ammeter or a voltmeter into the mini to keep an eye on things rather than the problem just appearing (original issue was with the alternator).

Now I'm of the opinion that in a competition car an ammeter would be a bit dodgy on the safety side as we'd need nice big thick unfused cables running to it carrying silly high currents that could potentially cause an electrical fire if things went breast up.

I think a voltmeter will tell us what we need but with a lower risk of death if things go pear shaped.

Am I right in my reasoning?

I'd have thought a voltmeter would be better...AFAIK most tests on a suspected flat battery just check it's potential...

Rob.

Voltmeter across the terminals with a fuse & on/off toggle is deffo easiest, extremely low current flow through it, very easy to install. Measuring current as you say would be rather a lot more of a pain.

An ammeter would only tell you if the battery's charging or discharging AFAIK (from what I remember from my dad's old MG Midget) rather than then giving an indication of alternator / battery performance. As you identified, ammeters have nominally-zero resistance and must be installed in series, thereby requiring you to have beefy cables carrying fairly high currents past your dashboard. Voltmeters have nominally-infinite resistance and are installed in parallel, therefore allowing you to have much thinner cables carrying virtually no current to the meter. A voltmeter's therefore MUCH safer!

HTH

Which is all well reasoned, but is still complete ballcocks! Has no-one else ever seen a multimeter with voltage and amperage scales on it? They're parallel devices, and measuring amperage.

Voltage goes across, current goes through. If you want to measure 'in parallel', then generally a very, very small resistance is put in series, over which voltage is measured.

Which is all well reasoned, but is still complete ballcocks! Has no-one else ever seen a multimeter with voltage and amperage scales on it? They're parallel devices, and measuring amperage.

Fair point.

The main point was that I didn't fancy a very high current coming through the bulkhead. If we had an accident that caused a short to the ammeter wires it would be like arc welding.

I've ordered a voltmeter to go in so my driver has no choice now:D

Voltage goes across, current goes through. If you want to measure 'in parallel', then generally a very, very small resistance is put in series, over which voltage is measured.

Ok, but the point that the ammeter power at the panel is instument power, not full-on alternator power, still stands.

Which is all well reasoned, but is still complete ballcocks! Has no-one else ever seen a multimeter with voltage and amperage scales on it? They're parallel devices, and measuring amperage.

Hubris, Ken, hubris. As Dan Aykroyd had it: "We mock what we don't understand!"

Digital multimeters are able to measure current when connected in parallel because they use algorithms to infer the current from the voltage across a known internal resistance, and the voltage across the item being measured. There's no way you could do this with a moving coil device, such as you would fit on your dashboard. But then again a digital multimeter held on with gaffer tape is probably cheaper...

IIRC you can get an ammeter that moves the shunt resistance near the battery then just measures voltage accross it.

Another point: since this is a mini, you've already got high capacity cables running about as the battery is in the boot....

I'd still fit a voltmeter though. It's likely to be easier and cheaper.

Hubris, Ken, hubris. As Dan Aykroyd had it: "We mock what we don't understand!"

Digital multimeters are able to measure current when connected in parallel because they use algorithms to infer the current from the voltage across a known internal resistance, and the voltage across the item being measured. There's no way you could do this with a moving coil device, such as you would fit on your dashboard. But then again a digital multimeter held on with gaffer tape is probably cheaper...

Backatcha - we also mock what we don't know.

I've used an analogue multimeter with amperage scales. What you do is use the voltage to drive a needle with an amperage scale rather than a voltage one behind it.

I've used an analogue multimeter with amperage scales.

In the classroom, with LR20 cells perhaps - I bet you it couldn't measure anything above about 250mA, though!

I've used an analogue multimeter with amperage scales. What you do is use the voltage to drive a needle with an amperage scale rather than a voltage one behind it.

Correct, but you move the meter in series with the current. Setting the multimeter (analogue or digital) to a current range switches a (very low, but known) shunt resistance in line, then the meter measures the voltage dropped across the shunt. Try connecting a multimeter on a current range across a car battery, and you'll either blow a fuse in the meter that protects the shunt, or you'll send the shunt up in smoke.

In the classroom, with LR20 cells perhaps - I bet you it couldn't measure anything above about 250mA, though!

Nah, a 10A scale is quite common even on an analogue meter. It's usually unfused though, so there's a real risk of smoking the meter.

^ So we're both wrong then, Ken!

I checked the DT catalogue; there are 12v +/-30A and +/-60A meters in it. I presume the makers are bright enough to spec the shunts to cope with the 850W or so that the 0 to 60A range implies!

IIRC you can get an ammeter that moves the shunt resistance near the battery then just measures voltage accross it.

Another point: since this is a mini, you've already got high capacity cables running about as the battery is in the boot....

I'd still fit a voltmeter though. It's likely to be easier and cheaper.

True we've got one battery cable running through the inside of the car already but that is well held down and well grommetted as it passes through the bulkhead.

I just don't fancy having another couple running around the back of the dash which won't be able to be clamped down as well due to their location.

Got a nice TIM voltmeter to fit and a TIM oil pressure gauge to go with it as the current gauge is leaking.

I actually think a voltmeter is more use, because it does battery condition with the engine off, and alternator condition with the engine running.

I really only got involved because I objected to the bad science and engineering that was being used to argue that you needed high power cable behind the dash to run an ammeter.

I checked the DT catalogue; there are 12v +/-30A and +/-60A meters in it. I presume the makers are bright enough to spec the shunts to cope with the 850W or so that the 0 to 60A range implies!

Well OK, so you can get them, and I stand corrected on that point. But I still question why you'd want to use one instead of a voltmeter. After all why on Earth would you want an instrument containing a resistor with a heat output almost as high as the element on a one-bar electric fire sitting inside your dashboard when an intrinsically-safe, non-interpolating voltmeter would do?! OK, so as cjb says, you could always move the shunt so that it's either across the battery or the alternator, but nevertheless the 850W thermal output of the power resistor is well over 1bhp! That'd be like saying "I know how to improve this rally car's performance - let's add air con and run it whilst we're driving!!!"

Anyway, the meter's been ordered, so it doesn't matter anymore...