Another ammeter question

[Baltimore Sailor]

I've got Don's wonderful color drawing of the A4 wiring plan, and I've got the instructions for my particular ammeter. And now I've read where it's recommended that the DC distribution panel should be powered either from the common terminal on the battery switch or where the battery lead connects to the starter solenoid (either end of the same battery cable).

My problem is I don't see how this circuit allows the load going to the DC panel to be measured by the ammeter. If the meter has to be in series with the load, wouldn't the run for the DC panel have to go through it as well?

If so, where should the run for the DC panel come from on the ammeter: the I terminal or the S terminal?

Thanks!

[Don Moyer]

Baltimore Sailor,

Being placed where it is in our schematic, between the alternator and
everything else, the ammeter will indicate the total current flow (amps) of
anything downstream, including the DC panel if it is connected to the large
battery terminal on the starter solenoid.

Don

[Baltimore Sailor]

Don,

Thanks for the info. One further question: in your diagram, the orange wire from the alternator goes through the ammeter, but the red wire from the BATT post on the key goes to the starter solenoid does not. In the instructions for my ammeter, it also shows the red wire going to the ammeter, as below:

http://photos.hilltopper.net/ammeter.gif

How does that affect the circuit as shown in your diagram?

Thanks again!

[Don Moyer]

If you always remember that the ammeter can only measure the current flow going downstream of its location, you'll be able to determine what it will be indicating. In the diagram you attached, the ammeter will read the current flow to the big battery terminal and anything else that you have connected to it. It will not read any current flow going in the other direction; e.g. to the ignition switch and anything connected to it.

Don

[Baltimore Sailor]

If you always remember that the ammeter can only measure the current flow going downstream of its location, you'll be able to determine what it will be indicating. In the diagram you attached, the ammeter will read the current flow to the big battery terminal and anything else that you have connected to it. It will not read any current flow going in the other direction; e.g. to the ignition switch and anything connected to it.

Don

I thought that ammeters displayed the current flow in both directions, which is why the gauge had a zero in the middle and the needle was free to travel on either side. Therefore (I thought), any flow out the IGN post on the ignition key (for example) would read as negative flow, while charging the battery would read as positive flow.

[Don Moyer]

It's true that an ammeter measures current flow in both directions, but again, only in the circuit in which it's installed. In your diagram, the ammeter only knows the current that is passing (in either direction) between the alternator and whatever load is connected to the big battery terminal on the starter solenoid (which is only the batteries in your diagram). If you want to know what current is flowing to or from the ignition switch, you'd have to install a second ammeter in that circuit or move the wire to the ignition switch over to the same terminal on the ammeter as the wire is connected to that goes to the solenoid. In the latter case, the existing ammeter will then read total current flow (to and from) everything in you diagram.

In our last boat, I had so much amperage drain from the refrigeration unit that I actually did install a dedicated ammeter just in that circuit so that I could monitor that specific load separately from all the rest. If you do install two ammeters, you would have to add the readings of both to know the total load on the alternator.

Don

[Baltimore Sailor]

Ok, I think I'm getting it. Now, since the alternator and the ignition key switch are on the same side of the ammeter, would the ammeter "see" any current flow across it to something connected to the key, when the alternator was not running and only battery was being used? I can understand that the alternator and the key being on the same side of the ammeter wouldn't make a circuit, but surely something on the key running off the battery alone would, right?

Thanks for putting up with all my questions on this! :o

[Don Moyer]

In your diagram, the ammeter would record (always a negative reading) if the
alternator were not running and there was some measurable load down stream
from the ignition switch. I may have assumed wrongly, but I thought we were
concerned with measuring current flow from the alternator, in which case
everything we said previously should still be OK.

Don

[Baltimore Sailor]

Again, Don, thanks for taking the time to answer my (apparently unending stream of) questions about ammeter hookup.

You are correct that I do want to measure the current flow from the alternator, but my goal (if attainable) is to have an ammeter hookup that would show both the flow from the alternator and drain on the system when the alternator is not running.

Visualize a setup where I switch to the starting battery and turn on the blower. The ammeter reads a discharge. Then I turn the key to start the engine and the ammeter reads a larger discharge, then a positive reading while the engine is charging the batteries. If I turn on cabin lights or other circuits while the alternator is running, the plus flow rate decreases slightly.

Then I turn off the engine and all other circuits and the ammeter falls to the zero position, until I run some lights or whatever off the battery and then it shows the negative flow from the battery again.

Is it possible to do this with the average ammeter from a boating supply store and my circuit diagram, or am I looking at blowing out an ammeter with such a load?

Thanks for taking so much time to help me understand this properly.

[Don Moyer]

Finally understanding your goal, I have to tell you that I think you're
pursuing something of nebulous value. I don't know of anyone who uses amp
readings as a method of controlling electrical usage within the DC system.

In my view, monitoring battery voltage and switching between various
batteries (either manually or automatically) is the most direct way of
making decision regarding what you can afford to turn on and what you'd better
turn off. In this connection, it is convenient to have a general idea of
how much current your individual electrical items consume and use them
accordingly, but you don't really need to keep looking at an ammeter to
remind you how much each device (or groups of devices) consume.

I may have given you a false notion of the importance of amp readings when I
told you I installed an ammeter to see how much power our dated
refrigeration unit drew. What I didn't say was that once I saw that it was
drawing 40 amps every time it came on, I never really had to look at that
ammeter again.

Ultimately, as your batteries deplete between your ability to charge them,
you will use the electrical devices depending on their relative importance
(the stereo being the first to go, and bilge pumps perhaps being one of the
last items to give up).

Most engineers we know of have gotten away from even a single ammeter
in the location shown in our diagram in favor of a voltmeter shown in the
optional location. In their view, as long as the alternator is able to maintain its
rated charging voltage (13.4 for the late model Motorola and 14.2 for more
contemporary alternators) it is assumed to be functioning normally. I still
sort of like the ammeter as a back-up so that I can not only tell that the
alternator is functioning normally (with the volt meter), but I can also
tell how hard it's working (with the ammeter). If I see that the alternator
is producing higher than normal amp flow, I start turning things off to see
which of my electrical goodies may be requiring more current than it should,
but I don't really need to relocate the ammeter to do that.

Don

[Baltimore Sailor]

Don, thanks again for such a comprehensive answer. One of my weaknesses is to over-engineer things (one could probably park an M1 Abrams tank on the deck I built for my house), and sometimes I just need to be told "You really don't need to do that."

Since I do have a functioning voltmeter, I'll just go ahead and put the ammeter in the circuit as you show and quit worrying about it.

Thanks so much for taking the time to explain all this.

Have a great weekend! :D

[Baltimore Sailor]

Don,

Just thought you'd like to know I finished the rewiring job this weekend. I followed your drawing to the letter, and after we buttoned everything up and gave it some power everything worked just like it should. The engine started right up and the ammeter read the charge just fine.

Now on to the carburetor to find out why I have to partially choke the engine to keep it running, but that's another subject. ;)

[Don Moyer]

Good for you, and thanks for the report!

Don

[lhbradley]

Before I changed things, my boat had an ammeter in the A4 control panel. There was a LONG piece of wire from the engine to the ammeter and back again, with two connecitng plugs in the circuit. There was a lot of voltage drop, so that charging batteries took several forevers.

As Don says, the typical auto/boat style ammeter is really only useful for showing you that the battery is charging or discharging - the scale is such that the readings are pretty inaccurate.

If you really want to monitor battery charge/discharge current accurately, and avoid the voltage drop associated with a long cable run to the panel, you should buy a high quality digital (or analog) ammeter that uses a "shunt" rather than going directly in the battery cable. The shunt is installed in the cable close to the battery, and a pair of small wires run from it to the remote meter. But this isn't cheap. And as Don says, if all you really want to know is if the batteries are being charged, and their rought state of charge, a voltmeter is actually better. And cheaper.

To seriously monitor battery state, you need an amp-hour meter, such as the Link 1000 or those made by other companies. These track how much current you have taken from and put into your battery banks, so you know the true state of your batteries. Again, not cheap.