Measuring Amps and Volts

Submitted: Saturday, Feb 05, 2005 at 09:17
ThreadID: 20130 Views:4577 Replies:8 FollowUps:8
This Thread has been Archived
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..
Back Expand Un-Read 0 Moderator

Reply By: Member - Chrispy (NSW) - Saturday, Feb 05, 2005 at 09:45

Saturday, Feb 05, 2005 at 09:45

You need to measure amperage "between" two points that are in-line with the power source and the appliance. (not "across" them) i.e. you literally need to disconnect the positive (say) lead to the appliance and "insert" your ammeter in-between (using it as a "lead" itself) and then connect the other side of the ammeter to the appliace to complete the circuit.

Only problem with this is that you will be using the ammeter as a real current path, and therefore it must be able to handle the full amperage that you expect to be drawing through the appliance. Otherwise, it's just going to be a "fuse".

AnswerID: 96758

Follow Up By: Member - Crazie (VIC) - Saturday, Feb 05, 2005 at 09:59

Saturday, Feb 05, 2005 at 09:59
Hi Chrispy

Thanks for the reply, so if i understand correctly, there is no real positive or neg, it needs to be inline, so the power for the appliance is supplied through the amp meter??

mmm, so it is not likely to measure the draw off the battery unless I have something that would take the positive battery terminal wire through it....
FollowupID: 355430

Reply By: Shaker - Saturday, Feb 05, 2005 at 09:52

Saturday, Feb 05, 2005 at 09:52
How do you make that LCD display into a voltmeter? Or is it already set up as one?
AnswerID: 96760

Follow Up By: Member - Crazie (VIC) - Saturday, Feb 05, 2005 at 10:03

Saturday, Feb 05, 2005 at 10:03
Hi Shaker
you have to add a 9v (battery) power supply, add a wire to where you would like the decimal point (3 options) at 2 resistors and lead wires. Doesn't take long.
FollowupID: 355431

Reply By: Leroy - Saturday, Feb 05, 2005 at 10:26

Saturday, Feb 05, 2005 at 10:26

What you have purchsed are panel meters. On their own they are not capable of displaying current or voltage. The meter has a maximum input of 199.9mv. You will have to design a current shunt that is capable of handling the large amounts of current you can draw form the battery and these will be extremely expensive as it will go inline with one of the leads from the battery (doesn't matter if +ve or -ve lead).
For voltage measuing you will need to design a voltage divider network. ie when you have 12v at the battery the voltage diverder network outputs 120mv which is within the spec in the input of the display.
Thi s is a little simplistic an explanation but hopefully you get the gist.
AnswerID: 96767

Follow Up By: Member - Crazie (VIC) - Saturday, Feb 05, 2005 at 10:42

Saturday, Feb 05, 2005 at 10:42
Hi Leroy
Thanks for the help.
I have the volt meter up and run for 12v no probs there, I understand how this works. The amps have me buggers. I have made the amp meter, just not sure how to use it properly

How do most people measure the amps draw of a fridge in the car. If you have 6mm wire for the fridge to run properly, how do you have an inline amp meter without losing the power??

getting more confused now hahaha..
FollowupID: 355436

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.

Ian Ritchie
AnswerID: 96775

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.
FollowupID: 355456

Reply By: Member - David C (QLD) - Saturday, Feb 05, 2005 at 12:19

Saturday, Feb 05, 2005 at 12:19
The Dick Smith panel meters don't have an internal shunt and are only designed to measure a dc voltage.

To set one up to measure current you place a shunt inline with the negative terminal of the battery and connect the meter accross the shunt (like it was a resistor). The meter then reads the dc voltage drop accross the shunt.

A shunt is a high current device of very low resistance (typically less than 1 ohm). Current passing through it produces a very small voltage (typically millivolts) that is utilised by the panel meter to indicate current flow. The voltage drop accross the shunt can be positive or negative depending on whether the battery is being charge or current being drawn from it. The panel meter also indicates this as postive (ingoing) current or negative (outgoing current). The shunt will have to be selected according to typical max current for the battery (ie they come in a variety of sizes from under 50 amp to above 500 amp.

Go to this Jaycar site for a very good write up.

AnswerID: 96786

Follow Up By: Member - Crazie (VIC) - Saturday, Feb 05, 2005 at 13:18

Saturday, Feb 05, 2005 at 13:18
Hi David

thanks foir the info, I will have a look a the Jaycar site. I will investigate shunts as well.
FollowupID: 355457

Reply By: AT4WD ADVENTURES - Saturday, Feb 05, 2005 at 16:26

Saturday, Feb 05, 2005 at 16:26
Hi There,

Jaycar sell a combined amp meter & shunt all in one unit that I use. I will try and find the part number. It was under $50 .


AnswerID: 96808

Follow Up By: hl - Saturday, Feb 05, 2005 at 17:14

Saturday, Feb 05, 2005 at 17:14
You can use the negative cable going from the battery to the engine block as a shunt, provided there are no other cables connected directly to the battery negative terminal. You connect one lead from your ameter to chassis ground, and then use a pin probe to tap along the negative lead until the meter reads correctly. You also need to have a known load on the battery while you do this, maybe the headlights if you know what wattage they are.

FollowupID: 355472

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 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?

AnswerID: 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

FollowupID: 355486

Reply By: Wok - Sunday, Feb 06, 2005 at 08:38

Sunday, Feb 06, 2005 at 08:38
If $$$ is not an issue have a look for Battery Monitors, usually associated with Solar systems or the boating crowd. The feature that is useful is ' AmpHour '.They usually have Voltage as standard display.
AnswerID: 96899

Follow Up By: Member - Crazie (VIC) - Sunday, Feb 06, 2005 at 20:43

Sunday, Feb 06, 2005 at 20:43
g'day Wok

Thanks for that, just has a quick look, nice looking panels, around $385. I will have to look it.

FollowupID: 355598

Sponsored Links