Fitting an ammeter

[davidej]

Sorry - I meant ammeter

Would something like this show charge as well as discharge? It had a voltmeter as well

http://www.ebay.co.uk/itm/100A-5-Wi..._Measurement_Equipment_ET&hash=item3cd2c27b56

 

[VicMallows]

I can't see a circuit board on mine. Just a lot of wires coming from the start switch. .

The circuit board is the rear of the tagstrip in the middle of the panel. It is notorious for suffering corrosion which often manifests as false buzzer alarms. It is basic diode-resistor logic, but as pampas says is best got rid of and the alarms rewired in a simple straightforward manner. Incidentally, the warning lamps are all ordinary incandescent, not LED.

FWIW although I have a fancy battery monitor, I still find an old-fashioned directly connected ammeter in the alternator circuit useful. (mine is ancient moving-iron rather than moving-coil). As long as the connecting wire is adequate and not too long its effect will be negligible ... or zero if you have a battery sensed smart regulator.

 

[William_H]

Yiannis,

You seem to be getting there.

The alternator red light is connected between a feed taken from the output of the alternator, and the 12v. battery positive. It is a very low current, high resistance, bulb. When the power to the engine is switched on, there is no output from the alternator, so this will be near zero volts. As the charging voltage builds and rises to something over around 13.5 volts, the difference in voltage driving the lamp becomes negligible, so the light goes out, thus indicating that all is well.

What you do NOT want is a low resistance ammeter stuck in circuit instead of the bulb: as you have found this does not work, as the circuit feeding it is relatively high voltage, and in extremis could possibly be damaged.

What you really need is one of the normally shunt-driven battery state meters available on the market. Votronic do a nice one. The shunt is inserted in the common negative lead to the battery. It is a very high current resistor capable of taking e.g. 100A and is tapped off with leads from each side of it, so that the instrument can detect the very small voltage drop across it. The greater the current, the greater the voltage drop (Ohms Law). The instrument is also connected to the battery positive, so that it can measure battery voltage. You now have the ability to read the voltage, the current, and most importantly, the ampere-hours (AH) used, so that you can monitor the state of your battery.

Using your cheap ammeter in circuit would not achieve the same. The shunt built into it (they all have one) is probably only built to withstand 30A, so you will have a possibly high insertion resistance, and hence voltage loss. It may well not be able to withstand starter motor currents, or if the batteries are down, higher charge rates from your alternator.

I hope this helps..

Just to correct that detail in first line. The red light incandescant bulb connects between the battery supply via the key 'ign switch" to the regulator and alternator field coild.
The output wire of the alternator is essentially connected to the battery so is always around 12v. The small current through the lamp on start up is enough to put some current through the field coils (the rotating coils) to get the laternator to geenerate some voltage. The alternator usually has another set of 3 diodes to provide its own supply to feed the regulator and field coils. It is this supply that makes the feed from the lamp redundant so lamp goes out. The supply through the lamp is always via an ign or engine switch so that with engine stopped this small current does not flatten the battery. it will if you leave the switch on.
If an LED is used as an indicator then one would expect there to be a resistor of the order of 50 ohms to provide but limit this current from the battery. An LED with its own resistor would sense the volt drop across the resistor as so give the same indication. or it could sense alternator function in other ways.
OP s ampmeter would normally be inserted in the positive wire that uns from battery positive to the alternator. This will sense then charge amps. Always good. If the ships other power circuits are taken from the alternator end of that amp meter it will also indicate total current either charge or discharge of other gear. it is Ok to run a heavy wire to and from the ampmeter or mount the amp meter somewhere closer to the lateernator or battery. Be mindful that all your electrics (except the starter) run through this wire so it needs to be heavy and robsut. Hence it is better to use a meter with a remote shunt and light wires to the panel meter. good luck olewill

 

[GreekYiannis]

Brian. I'm getting a bit lost here. On the one hand, some people seem to be saying that I need the bulb to 'excite' the alternator into action and charge my battery. On the other hand, AFTER my bulb blew, when I measured the battery voltage - across the terminals - before starting the engine I got 12.8. After starting I got 14.4. So it seems my system doesn't need the bulb to 'excite' the alternator.
Q. Am I right?

 

[GreekYiannis]

On this model, in the ebay picture, I can't see a space for a minus sign. Only three digits. Therefore, by logic, I'd say it probably doesn't show discharge. Only charge.

 

[pelissima]

On this model, in the ebay picture, I can't see a space for a minus sign. Only three digits. Therefore, by logic, I'd say it probably doesn't show discharge. Only charge.

Vre Yiannaki mou, for the small apparatus to show you charge voltage and current you will probably have to put it close to battery but along the charging plus line from alternator.
Discharging currents usually travel a different path, typically from battery to electric board so to monitor discharge you will need a second ammeter. A fancy electronic monitor can probably do both jobs and more, but then again this is a completely different animal.

 

[VicMallows]

So it seems my system doesn't need the bulb to 'excite' the alternator.
Q. Am I right?

Your alternator was obviously charging when you measured it.. Quite often there will be enough residual magnetism for the alternator to self-excite without the help of the additional feed via the warning lamp. The problem is this can not be guaranteed every time, and can even vary from occasion to occasion Hence the specific excitation feed..

 

[alan17]

I use a simple LED type digital voltmeter from Ebay cost about £3. Cannot remember exact price! It is simply put into a cigarette lighter socket and shows the voltage at the socket. Glance at it before starting engine. Look again with engine running. First reading should be a little over 12 and the second reading about 14. If this is the case the your batteries are being charged.
I leave this digital voltmeter in circuit but when I leave the boat I turn the main battery switch off and check that there is no displayed voltage.
Not the most sophisticated system but it does what I want.

 

[GreekYiannis]

Vic - Ah ha! That makes perfect sense to me. I've now had great help from Tony Brookes [fantastic!] and I think I'm on the verge of a solution. Because I have an engine temp gauge, I don't use the yellow temp warning lamp. So if he says the yellow lamp's cold resistance of 17 Ohms is the same as the kaput red lamp, then I'll simply use the yellow one as my charging 'excitor' and indicator.

 

[GreekYiannis]

Thanks to Tony Brookes and all the others who helped me fix my charging problem. I have gone from zero understanding to enough knowledge and skill to fix the problem in just three days. Basically, all I've done is to utilise the water temp light in place of the failed charging light - which I could not replace. I have a temp gauge, so using the 'spare' temp light was a neat solution. But more importantly, I now know the function of the charging indicator light is to 1) draw a little current which excites the alternator before starting the engine so that the battery energy taken by cranking the engine begins to be replaced immediately the engine revs are high enough ie. tick-over or above. and 2) to go out after starting, indicating that the balance between the lamp circuit and the charging circuit has tipped in favour of the charging circuit. Great stuff. I now feel confident in going fishing this afternoon. Many thanks guys. What a great place this is.