Voltmeter

[V8Ian]

I've just purchaced a digital voltmeter. it is a simple red to + and black to -. If I connect it via the acc circuit, so it only displays with the ignition on, will I get an accurate reading? I also would like to fit a digital ammeter (four wire and current shunt) and thought that I may be able to pair the earths and run them through an ignition controlled relay. Wise opinions please.

[slug_burner]

I've just purchaced a digital voltmeter. it is a simple red to + and black to -. If I connect it via the acc circuit, so it only displays with the ignition on, will I get an accurate reading? I also would like to fit a digital ammeter (four wire and current shunt) and thought that I may be able to pair the earths and run them through an ignition controlled relay. Wise opinions please.

The relay method will ensure you know what loads are on the circuit, as long as you tap in to good solid power and earths.

[Blknight.aus]

pretty much for the volts meter, for the amps meter theres a few ways of wiring it up...

do you want to read
the total power going from the alternator
the power flow to and from the battery
the total power being used
?

[V8Ian]

As you know Dave, I have no idiot light to let me know of a lack of charge situation, I suppose all I really need to know is that the alternator is working, so output from it. I went to buy an ammeter but allowed the guy to talk me into the voltmeter, now I will fit both. This particular ammeter does not read a negative value.

[Blknight.aus]

if thats all your after the voltmeter is enough Probably the easiest feed for you to grab is the one behind the ciggy lighter OR the feed for your bluetooth unit.

The amp meter I would suggest putting across the primary earth strap to the battery it will then indicate flow too and from the battery giving a rough idea of the charge condition of the battery and any draw you're making from it. I"f it starts to show draw while the engine is running then you know the alternators not up to speed.

Feel free to book a weekend slot this weekend or in about 4 weeks time as I have some ratting to do on the D1 that you were interested in the rear quater panel from.

[V8Ian]

Not this weekend Dave, Bathurst 12 hr, closely followed by Clipsal in Adelaide. I'll have to go with the 4 week option mate. :D

[Blknight.aus]

done, ATM the schedule has some flexable bookings in it so pick yourself a date at the endish of march and a time and I'll massage it about and see where it slides to.

[DeanoH]

Not this weekend Dave, Bathurst 12 hr, closely followed by Clipsal in Adelaide. I'll have to go with the 4 week option mate. :D

An alternate view.

Back in my racing days (70's & 80's touring cars) we used to put a big red stop light in the middle of the dash. This was an extension of the ign. lamp. Its purpose was to let the driver know if he threw a fan belt so to back off and get back to the pits before the engine cooked (as the water pump ran from the same belt). The lamp illuminated because the alternator stopped. What's changed ?
Ammeters are good and used to be common back in the old days when your 30 Amp generator couldn't keep up with the load when you turned a pair of Super Oscars on at the same time as your aftermarket electric heater fan, not to mention your stick on electric demister. With 'modern' alternators with an output of up to 120 Amps as standard this is no longer an issue.
Modern digital voltmeters give a very accurate reading to either 1/10th or 1/100th of a volt and give a very good indication of whether the battery is charging or not.
So unless there's a specific reason to have an ammeter I wouldn't bother.

Deano:)

[Blknight.aus]

his alternators got a fried reg pack and is running an external unit patched in... (a cheap and effective repair) in an everything on + charging a pair of secondary batteries the system can draw more than the alternator can deliver at full volts.

[DeanoH]

his alternators got a fried reg pack and is running an external unit patched in... (a cheap and effective repair) in an everything on + charging a pair of secondary batteries the system can draw more than the alternator can deliver at full volts.

No problem with the external reg. good idea. Bought a replacement alternator for SWMBO's D2a in Alice Springs a couple of years ago cos the alt lamp was glowing slightly (thought it was one phase out in the diode pack) but replacing it didn't make any difference. Glow disppeared all on its own after a couple of days, never did find out why. The new LR alt cost about $1K from memory. Ouch. Still got the (good) old one in the shed somewhere.

Wouldn't some sort of current limiting device/controller on the aux. batterys solve the problem ? At it's simplest a thinner cable feeding the aux batterys ?

Deano:)

[Blknight.aus]

not really because you then hit the problem of if the alternators being a touch lazy the DBS can back feed amps into the system at times of high demand which might mask the beginnings of failure of the alternator.

[drivesafe]

Modern digital voltmeters give a very accurate reading to either 1/10th or 1/100th of a volt and give a very good indication of whether the battery is charging or not.

Hi DeanoH and volt meters do not tell you if the battery is charging.

This is one of the biggest misconceptions about volt meters, once the motor is running, all they will do is indicate that the alternator is working.

Having a decent alternator voltage reading does not indicate a battery is charging. The battery could be totally stuffed and not drawing any current at all, yet a volt meter will show there is a charging voltage at the battery, it's just not charging.

Amp meters can be very useful in telling you your battery is not charging but again, it indicates that there is already a problem.

The only way you can get advanced info of a battery being on the brink of failing is with a full battery monitoring system, which are expensive ( starting at 4$00 to $500 before fitting cost ) and unrealistic for the average 4x4 use.

[DeanoH]

Having a decent alternator voltage reading does not indicate a battery is charging. The battery could be totally stuffed and not drawing any current at all, yet a volt meter will show there is a charging voltage at the battery, it's just not charging.


You are of course quite correct.

In context my statement was refering to indication of alternator failure/output degradation and its effect on battery voltage, as opposed to physical condition of the battery itself.
In this scenario (alternator failure/low output) battery voltage would drop as it became discharged. With digital voltmeters small changes in voltage are easier to notice than with an analog meter so action can be taken earlier was the point I was making.

Thanks Deano:)