How to hook up ammeter?

[Baltimore Sailor]

I know I've covered this ground before, but I'm still not getting it right. What I want to do is hook up my ammeter so that it will show me when the battery is getting amps from the alternator, and also show when I'm using amps when running stuff only off the batteries.

It seems to me the connection for this scenario would be for the main battery lead from the selector switch to go to one side of the ammeter, and from there to the terminal on the starter solenoid. This doesn't seem plausible, however.

The wonderful diagram that Don has posted before shows the orange lead from the alternator going to the ammeter, and thence to the IGN terminal on the ignition switch, with the red lead going from the IGN terminal to the terminal on the starter solenoid.

This would show all the current going from the alternator into the battery, but not from the battery out into the other devices. How can I make the connection so that I can see both usages?

[High Hopes]

Hello Baltimore!

You have posted a questions about one of my favorite (perhaps over labored) topics.

One end of the ammeter goes to the output of the battery switch, that is correct. Every other 12 volt device (lamps, solenoids, ignition switches, depthsounders, radios) must attach, ultimately, to the other terminal of the ammeter.

The circuit is depicted below.

Batteries to BatterySwitch to Ammeter to EverythingElse

Everything else includes the alternator. The only thing that bypasses (goes around) the ammeter is the B+ for the starter motor.

Here is the relevant clip from a diagram I posted earlier.

With this arrangement, you will see the difference between the Battery charge (from the alternator) and the battery draw to equipment. You will need a meter that reads both plus and minus.

I hope this helps.

Steve M

[Baltimore Sailor]

A couple of questions:

How is that the 8 ga. wire in your diagram can handle the current brought in by the 4 ga. wire from the battery? Shouldn't there be 4 ga. all the way from the battery to the ammeter and then to the bus?

All the figures marked xxA are fuses, right? The "BUS 80A" to the ammeter, the "ALT 60A", etc...?

Looks good, though. When I relocate my instrument panel later on I'll use this.

One other question about my setup: right now I have the orange line from the alternator going to the ammeter, and then on to the ignition BAT terminal. However, the ammeter reads nada. Just sits -- all the time, whether the engine is running or the batteries are ON -- with the needle a little the right of 0 (it reads + and -). Sound like a dead ammeter to you? Everything electrical works OK, I just don't get an ammeter reading at all.

Thanks!

[High Hopes]

In two parts. . .

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A couple of questions:

How is that the 8 ga. wire in your diagram can handle the current brought in by the 4 ga. wire from the battery?

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An excellent question. It took me a while to get this, too. The key in the fusing is preventing wire fires. Most fires are started by wires which get hot when they bump into the wrong thing and get shorted. This may involve a device failure which shorts, but usually a wire get damaged or gets loose and touches the wrong thing. So look at it from the wires perspective.

A #4 wire will handle a current of about 150 amps without getting hot.

[[[ Fuses in this range are sort of pointless for several reasons. (1) You have to get off the battery with a wire to get to a fuse, so there will be at least one wire un-fused. (2) The starter needs about 100 amps, about the capacity of the battery. (3) If the starter motor cable shorts to anything, it will be hard to tell the difference between the starter motor current draw (which is very large) and a short current draw (which is also very large). When push comes to shove out at sea, you want to be able to start your engine. A fuse in the starter motor circuit introduces a potential failure point. You don’t want to be fishing around for starter fuses at sea. Bilge circuits are usually over-engineered for the same reason. When you want the bilge to run, you really want it to run. ]]]

Now, on to your question specifically. The #8 wire can carry 60 amps without getting hot. The fuse limits the current to 60 amps max. If the 12 volt bus is shorted, the 60 amp fuse will blow. The total amperage of the #4 wire can not be fed to the #8 wire. Nothing over 60 amps can reach the #8 wire because the fuse is in the way.

Here is another example. The blower motor is on a 7 1/2 amp circuit. If the blower circuit tries to draw too much current (say it has a stalled motor), up to 60 amps available from the 12 volt bus will start to flow across the 7.5 amp fuse which will melt and open the circuit.

So, as fuses value decreases, wire sizes can decrease. There are ampacity charts which identify wire ratings.

Also, notice the advantage to fusing down circuits. The lower value fuse will blow first. This keeps the rest of the devices before the fuse in operation. For example, if the alternator wire shorts to ground, the 60 amp fuse will blow, not the 80. The 60 is weaker than the 80 and it will blow first. The rest of the devices on the boat remain powered up by the 80 amp fuse which remains intact.

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One other question about my setup: right now I have the orange line from the alternator going to the ammeter, and then on to the ignition BAT terminal. However, the ammeter reads nada. Just sits -- all the time, whether the engine is running or the batteries are ON -- with the needle a little the right of 0 (it reads + and -). Sound like a dead ammeter to you? Everything electrical works OK, I just don't get an ammeter reading at all.

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Several things…

(1) The ammeter will never read negative. When the alternator is off, there is no electrical path through the alternator back to ground. This has to do with the diode output of the alternator and is a requirement of the design. Otherwise the alternator would draw the batteries down when not running.

(2) If your batteries are charged, there is little current flowing into the battery. If you have a 60 amp gauge, it might be hard to see 1 amp or less.

(3) Is there another path around the ammeter? Does the alternator output go anywhere else? There should only be the one wire from the output of the alternator and it should go to the meter. No other wires should be connected to the meter side either.

(4) Yeah, it could be a dead meter. Take it out and test it with a 12 volt lantern battery. (It could be a dead alternator? wince...)

You might want to start your engine and then disconnect the batteries at the positive terminal. The engine should still run and your ammeter should show current as the alternator should now be powering the engine and the house circuits. Turn on some lights to make sure you have a decent load.

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Let me know what you find.

Happy sails,

Steve M

[Baltimore Sailor]

This is great stuff; I really like understanding my electricals. Thanks for all the info.

A couple more questions:

You said that "The #8 wire can carry 60 amps without getting hot. The fuse limits the current to 60 amps max," but your diagram shows the BUS fuse at 80A, not 60A. Did you mean to say a #8 wire can carry 80 amps, or did you put 80A in the diagram where you meant 60A?

For your main bus (the one that the lead from the ammeter powers), what size is it? I don't know much about sizing buses; do they come in amp readings as well? If so, what size should I get?

As for how my ammeter currently reads (or I should say, doesn't read), when I'm running the engine the voltmeter shows a strong 13.5 volts going to the batteries, so I'm pretty sure the alternator is working.

Did I mention the ammeter always reads a slight positive current, even with the engine off and the battery switch set to None? I get the feeling that means a dead ammeter -- though it is passing the current, since everything works. And now that I think of it, there is a ground wire leading to one of the ammeter contacts; should that be so? I'm sure I read the instructions for hooking it up, but since I managed to incorrectly connect other leads that I had expressly labeled, I have little confidence in wirings that I did previously.

Thanks again for all your help!

[High Hopes]

Balt,

Oops, I meant to say 80, not 60 in the text. The drawing is correct.

On the "bus,". . . this may or may not be an actual terminal strip. On my boat it will probably be a few terminal strips with some interconnecting #8 wire and at least one distribution panel (which has a bus in it). The bus is more of a concept here than a physical item. Yes, terminal strips and switch panels are rated by amperage capacity.

How big to get depends on what you want to power. I bought a bunch of #8wire for a bilge circuit so I settled on 80 amps because I had the wire for it. I would probably never use more that 25 amps while at sea even at night with all the lights on and the radio, instruments running, etc. Normal usage with LED lamps will probably be less than 100 watts (8 amps).

I just picked 60 amps as a "design point" because I had a 60 amp ammeter and this will allow me to plug in a high intensity 500 Watt light if needs be in an emergency. Be advised that you want to choose a fuse (80) that is 1.4 times the planned max steady state current (60).

Actually, my ultimate dream is buy a DC to AC converter and play my electric guitar out on the water using my 50 Watt Peavey.

My ammeter needle is about 2 degrees to the left sitting in a box. That doesn't mean that the meter is bad. Mine works. It just means it is not exactly zeroed. Boat meters aren't high quality finely calibrated instruments.

Pull the meter (or take the wires off it) and wire it across a 6 volt lantern battery. It should show about 3 amps. Switch polarity, and the needle should move again the same distance, but in the opposite direction.

Steve