# Ammeter Shunt Change

I’m attempting to retrofit an external shunt style ammeter into the charge circuit in my car but am far from an electrical engineer, so I figured this would be the best place to ask.

The ammeter I’m looking at, originally, used a length of wire as a shunt, but the wiring has been completely changed. Originally the shunt wire was 8AWG (don’t know the length, since it’s gone), and the wires going from the shunt in the engine bay to the ammeter in the dash were 20AWG.

The wire I’ll be using as a shunt is now 0AWG. This would of course make the meter less sensitive, but since it’s the main battery wire I wouldn’t want to decrease its size.

How would I be able to get the meter back to its correct reading? Would I be able to to lower the resistance in the wire going to the ammeter by going to a bigger size? Or would I have to do the opposite and add a resistor?

As I said, I’m not much of an electrical expert, so I’m completely out of my depth here and would really appreciate some help.

• You should be asking this in mechanics, not electrical engineering, with any luck you'll find someone with actual experience with this type of meter. Commented Feb 26 at 11:03
• Is this an electronic meter (digital) or magnetic (moving pointer) Commented Feb 28 at 0:41
• Its a moving pointer Commented Feb 28 at 3:24

Changing the resistance of the shunt cannot change the scale of the meter, you need to match the shunt to the meter and if the scale isn't suitable, you need a new shunt/meter combination entirely.

The solution for your current meter is actually easy: look at the meter and see what its full scale current is and get a shunt resistor that matches the full scale voltage and the ammeter reading at full scale e.g. 50mV = 200A. Both of these values should be printed on the meter or listed in its datasheet. If the shunts you are looking at only have a resistance listed, match it to your meter by dividing the full scale voltage by the full scale current reading.

Wire or non-shunt resistors are not suitable since the resistance changes with temperature and with wire, accurately measuring resistances in the range you will need is difficult without specialized meters.

• Exactly, using #8 wire as a shunt resistor was dopey, and probably was never accurate in the first place. Use a proper made-for-purpose shunt. Commented Feb 26 at 3:38
• My hesitation to use a shunt is that it’s on the main battery charge wire. So in a worst case scenario with higher than normal charging current, the shunt would be a weak point since it’s only rated up to a certain amperage. Would it be safe to have a shunt on this wire? Commented Feb 27 at 15:46

As I understand it (from talking to a mechanic many years ago) this type of meter is installed with the alternator output cable as the shunt. The meter will not read starter current.

Now I'm guessing.

The connections to the alternator cable should be made by splicing to the cable and not connecting to the terminals at either end of the cable.

As the shunt is copper the temperature will perturb the reading, but these meters never had high precision anyway. the meter's response can be adjusted (reduced) by adding series or parallel resistance (to the 20ga wires), or by moving the taps closer together on the alternator cable.

If you car connects any loads other than the starter to the battery terminals those load will have to be moved to the alternator's output terminal, else they will show as a charging current.

• I appreciate the response. I do have all of the loads on what would be the alternator side of the “shunt”. I haven’t measured the meter yet, but let’s say the full scale reading is 100mV. If, with the bigger wiring, the voltage drop is only 50mV, do I just use ohm’s law to do the math for what size resistor to put on each 20ga wire? For example, if the meter draws .5 amps, what resistor is used on each wire? Commented Feb 27 at 15:39
• If it reads high measure the resistance of the meter and add series resistance sufficient to increase the meters resistance by the same ration that it read high by. if it reads high by 50% (10A reads as 15A) add 50% of the meter's resistance in series. (if there is not an adjustment on the back of the meter to compensate for this) if the meter reads low and there is no adjustment there is nothing you can do. (except relabel the scale perhaps) Commented Feb 28 at 0:36
• Since it reads current flow in both directions, I would assume if I had to add 50% resistance, it would be 25% on each wire, correct? And if the meter reads low, would reducing the resistance by swapping the 20ga wires for a bigger size be able to fix that? Commented Feb 28 at 2:52
• Adding it all in one wire or spreading it across in both will give the same result, but by spreading it you give short-circuit protection to both wires, so yes spreading it is better. Commented Feb 28 at 3:30
• If the meter reads low (which I think is the most likely case, since the “shunt” is bigger than original) would replacing the 20ga with bigger wires be able to help with that? Commented Feb 29 at 4:55