Measuring Amps and Volts
Submitted: Saturday, Feb 05, 2005 at 09:17
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Member - Crazie (VIC)
With my battery dying the other day, I thought it would be a good Idea to keep a
check of the new one, so I found Amp/volt meter in dick smith. This little panel can be made into a voltmeter or ampmeter.
I have brought 2 and made them up, 1 volt and 1 amp meter.
Now the question. Am i better to measure the amp drain off the battery or off a specific applicance. I was thinking straight of the battery.
If I go with the battery, is it as easy as positve wire onto + battery terminal and earth on to - battery terminal, or earth it on the truck, or am i completely wrong.
its too early to be at work....... and i am working in between this post..
Reply By: Swanning it - Saturday, Feb 05, 2005 at 11:16
Saturday, Feb 05, 2005 at 11:16
Hi Crazie, A DC ammeter works by measuring a predifined smaller percentage (something that the meter movement or electronics can handle without frying) than the maximum load current. To do this, it is necessary to have an item of equipment called a "shunt" which is a fixed resistance which, as the name suggests, allows the majority of the full load current to pass through to the load (fridge, light, starter motor etc) whilst shunting a proportional but conciderably smaller current through the meter movement or electronics of the ammeter.
Most DC ammeters have a rating on them which indicates "full scale deflection" of the meter. ie: an ammeter may be marked 50A which indictes that a load current of up to 50A can be passed "through" this meter without causing damage and will accurately read the current flowing between 0 and 50 amps.
I hope this is not too confusing, but if you have an LCD display with a few backup resistors etc I would suggest it probable won't do the job you want. Phone dick smith and ask if it has an internal shunt (and if so whats the maximum amperage it can handle) or if you need to add an external shunt (which one would assume they'd sell). Bear in mind that a DC ammeter does need to be wired with correct polarity (+ to the battery pos, - to the load pos, NOT to the battery neg or chassis). If the polarity is opposite, the meter will read backwards.
Regards
Ian Ritchie
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Follow Up By: Member - Crazie (VIC) - Saturday, Feb 05, 2005 at 13:15
Saturday, Feb 05, 2005 at 13:15
Thanks Ian
Learning new things here.
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Reply By: StormyKnight - Saturday, Feb 05, 2005 at 18:17
Saturday, Feb 05, 2005 at 18:17
Simplistically, if possible...
A voltmeter measures voltage difference between two points. Usually the + & negative terminals of a battery for example.
An ampmeter measures the current flowing in the circuit. The normal method is to have a precise resister in the circuit of very small value. A shunt. As the current increases so does the voltage drop across this resister.
The formula is V=IR which is Voltage(V) = Current(I) times Resistance(R).
Since the resistance is fixed, the voltage will vary proportionately with the current...so an ampmeter actualy measures this voltage but is calibrated to show the equivalent current on the screen that would be generated by this voltage.
Remember that your start current may be 100's of amps, but your general load may only be perhaps 50 at most with
driving lights & a fridge.
A 100W driving lamp uses 7.25Amps assuming the battery/alternator is outputing 13.8V.
From P=IV or Power(P) = Current(I) times Voltage(V)
So with at most 4 100W loads & a fridge that at most would be 8amps, that gives a total of 37amps.
I know on my vehicle, the starter motor has its own dedicated cable, with the other 12V components having their own separtely. If I was you I would insert the shunt into these wires & ignore thie actual starter motor current as this will effect the range of the ammeter by having to accomodate such a high current when in general the max reading would be 50 odd amps.
Also with the alternator working, it will match any load up to it's rating, so I suspect the meter won't give you an indication of the actual load being drawn from the battery, but only the difference between these to currents. The load being drawn & the current being put back by the alternator. If you want to measure the true load, you will need to bypass the alternator circuit as
well.
Unfortunately, just measuring the voltage & current doesn't always give you a good indication of battery life. How many times have you started your car OK, only to find the next time its dead?
Cheers
Richard
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96830
Follow Up By: Member - Crazie (VIC) - Saturday, Feb 05, 2005 at 19:48
Saturday, Feb 05, 2005 at 19:48
Hi Richard
thanks for the in depth info. Gives me a better understanding in to how and where to measure or if I really am going to gain anything from doing so.
Thanks mate
Adam
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