Wednesday, 6 January 2016

Ammeter wiring - why doesn't it work?

I promised to show you how I connected my 100A ammeter. Perhaps someone might be able to point out why it doesn't appear to work. Please excuse the scrappy (and dark) nature of this wiring diagram:

The current shunt is in the negative path immediately before the battery bank (four 12V batteries in parallel, not four cells in series!). The actual negative cables attached to the non-battery side of the shunt are one to the engine block "earth" and one to the domestic negative busbar next to the circuit breakers.

The thin wires to the display unit are connected to battery + and either side of the shunt.

On my diagram I added the connections to the inverter as an afterthought. Having done so I am now wondering whether the negative cable to the inverter could actually be earthed there, thus bypassing the shunt.

Whatever, the result of this wiring is that the boat electrics function normally but the current shown on the display unit never exceeds 0.4A, whichever way round I connect the thin wires to the shunt.

Any ideas, anyone?

11 comments:

ditchcrawler said...

Just looked at your drawing and the photo from a couple of days ago. Can you scan the wiring instructions for the ammeter. As for load is the inverter the only thing connected to the left of the batteries?
If you put a digital volt meter across the shunt where the blue and green wire are with a load flowing through the shunt you should be able to measure a very low voltage, a few millivolts.

Tom and Jan said...

You should have only one big fat negative cable between the left end of the shunt and the domestic battery negative terminal. All the negative cables that used to connect to the battery negative terminal should be on the other side of the shunt (except for the inverter negative cable). Because you don't want to measure the amps used by the inverter the positive and negative cables from the inverter must connect directly to the domestic battery bank terminals.

Halfie said...

Brian, I'll post the wiring for the display unit later. The red wire from it is positive and that is connected correctly to the battery positive. The only other wires connect to either side of the shunt, so one is "negative" and the other is what I call the "current sense" wire, on the slightly more positive side which detects the minute voltage drop across the shunt. I reckon this should be the battery negative side, but I have tried connecting the two non-red wires both ways round to try to get a reading with no success. As you suggest, the only load which doesn't go through the shunt is the inverter. My next job, when I get back on board (in three weeks) is to measure the voltage across the shunt as you suggest.

Tom, the wiring is exactly as you say (except for a small temperature sense lead going to the alternator controller, which should not carry any current. If, when I have measured the voltage across the shunt myself and find this relates to a decent current, I shall have to conclude that the new display unit is faulty. For info, the unit should indicate the maximum 100A when there is 75mV across the shunt, making the shunt's resistance .00075 ohms; for this maximum current condition the power dissipated by the shunt would be 7.5W.

John Lomax said...

Can you get hold of a suitably rated dc clamp on ammeter? If so you can test the current on both the positive and negative sides of the charge circuit. Assuming that the alternator is OK it is possible that the return charging path is via a combination of earth and the inverter negative cable, so test all cables to and from the batteries.

Halfie said...

John, unfortunately I do not have access to a clamp ammeter. I'm not convinced the alternator/alternator to battery charge controller is charging correctly - that's why I wanted to install an ammeter! Aside from measuring the voltage myself across the shunt I could disconnect the inverter negative cable and see if the ammeter springs into life. Then I'll know what's going on, even if I'd then wonder how I'd get an ammeter up and running with the inverter connected.

John Lomax said...

You could also temporarily install the shunt in the inverter negative with a modest load applied so that you have a known current ( 100w bulb in an inspection lamp?) With all other connections to the battery negative disconnected and no landline connected this should show if the ammeter is working

Halfie said...

John, yes, I could try that. Thanks. I'll be able to say how I get on towards the end of this month when we're next on board.

Dave Ward said...

My first thought was the inverter wires weren't included in the circuit, but as you explained the size of inverter, you are quite right that this would cause problems! According to your diagram you have the negative & sense wires reversed. These units will operate as a simple voltmeter if you only connect the positive & negative (which I think you said works) directly to the battery, and when the sense is connected to the "load" side of the shunt it should then indicate current flow in either direction. But I note that you've already tried swapping these two round, which suggests something wrong with the display unit.

When you are able to test with a digital meter, you should see voltage drops across the shunt in proportion to the quoted figures. i.e. if you put a 10amp load on, the drop should be 7.5mV, but note that at such low values it's likely there will be some discrepancies. DVM's will normally read DC voltages in either direction, so watch the display whilst you first apply a load (several filament lights, for instance), then start the engine and see if the meter reads in reverse.

HOWEVER, the load taken while cranking a diesel engine (particularly when cold) can easily exceed 500amps, so I think you will have problems with such a low rated shunt, and the voltage drop whilst doing so may have damaged the display. For your purposes you really need a 500amp shunt and matching display. Failing that you would have to take the alternator negative back to the shunt separately, and this can only be done if your alternator is an "insulated return" unit. This would allow the display to show charge into the battery and load going out (excluding the inverter), but is not the most satisfactory method...

Halfie said...

Dave, thanks for taking the trouble to write. The shunt is only in the "domestic" battery circuit; the engine starter is supplied from a separate battery so there is no way any excess current could have flowed through the shunt. I'm looking forward to putting a DVM across it in a couple of weeks time - I will post the results!

Meanwhile here at home I have found an old centre-zero analogue panel meter marked 10 (volts) fsd. I have calculated the meter's coil resistance to be 833 ohms; fsd is obtained with 0.103mA through it, corresponding to about 86mV across it. This is remarkably close to being able to be connected across the shunt and reading off the current by multiplying the displayed reading by ten. If I were to do this fsd would correspond to 114A through the shunt. One huge advantage would be that it would work with current flowing in either direction, i.e. charging or discharging. The digital display unit which came with the shunt will work with current in one direction only (but I have a cunning plan to be able to make it work both ways ...!)

ditchcrawler said...

I think you will need a volt meter with a much higher impedance than 800 ohms across your shunt to get an accurate reading

Halfie said...

Brian, yes, when I calculated the resistance of the voltmeter I was surprised to find it as low as 800 ohms. But the shunt resistance across which I am connecting the voltmeter is orders of magnitude less than 800 ohms, being 0.75 milliohms. I'm back at home now, but when I return to the boat later this week I'll double check with my DVM taking the analogue voltmeter out of the circuit.

Incidentally, I've had a response from the supplier of the shunt and faulty display unit offering me a half-refund! They told me that their supplier is on vacation!