Hunty, personal experience from loooong ago taught me to connect a vehicle ammeter differently (also my background is Electronic Engineering, but have never played with one of these ammeters). If it were me, I would find the main incoming lead from the alternator to the battery and leave it connected directly to the battery, then run everything else through the shunt from the positive terminal of the battery. This way the ammeter is reading only the draw from the battery and not getting confused with the charge going back in.
The setup you have is simply a very low voltage volt meter measuring the voltage across the shunt (FB resistor). If the resistor (shunt) is 0.001 ohm for example, then 100amps through it would give a voltage drop across it of 0.1 volt (Volts = Current x Resistance). This readout is simply a voltmeter showing this 0.1 volt as 100 (and we interpret that as amps).
However, this is presuming the ammeter has 4 connections, positive and negative to run the display and electronics, plus another positive and negative to run across the shunt.
My reasoning is the alternator is connected directly to the chassis for its earth and directly to the batt positive for charging, so your ammeter will detect this and should show it as a negative draw assuming your battery was nearly flat and you have no lights running (battery taking a good charge with no heavy draw from it). If you turn your lights on, they will be drawing from the across the shunt for their earth, but at the same time that the alternator is feeding into the shunt to charge, so you will be getting the draw from the lights minus the charge going into the battery showing on the meter. So if your battery was charging at 10 amps and your total electrics draw was 10 amps, you would have the meter (when wired up like yours) showing 0 amps
Am I making sense or am I confusing the carp out of everyone!!
Did they end up supplying a connection diagram or is this what you worked out yourself?
EDIT: Scrap that - was thinking the negative terminal was still hooked directly to the alternator.... will have to rethink my madness!!! But will leave the above writtin with my erronious thoughts so anyone else thinking about it doesnt fall for my mistake!!!
EDIT #2: Ok, went and drew out a circuit diagram of it and should not have doubted myself! My reasoning above was not quite right, but the result is the same. The alternator is charging the battery through the shunt and should read a negative current, the battery is supplying the loads (lights) though the shunt and will show a positive current, but any current from the alternator powering the load directly will bypass the shunt and not be reading. So in other words if you compare the meter reading with motor off and with motor on (and at around 2000rpm to get the alternator pumpin hard) the meter will read less as some of the current will be coming from the battery through the shunt and the rest will be coming directly from the alternator and not going through the shunt.
Have I lost everyone now!
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Last edited by SeaComms; 24-11-2010 at 06:01 PM.
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