Battery drain

[windtrader]

Most digital volt meters have an ammeter built in. The upside is you may already have one or can buy one cheap. The downside is that the meter has to be in series with the circuit. This means removing a wire from a terminal and connecting the meter leads in series. It's a lot more time consuming and not always easy to do but if you have a guess at where the problem is, you might be able to prove or rule it out without much investment. If you have questions about how to do it, feel free to PM me or post. (I troubleshoot electrical systems for a living!)

~Andy


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That's why I'm searching for a clamp on. I have a multimeter but it is a real hassle to put it in line for every circuit to test. It seems the clamp on will be a whole lot faster to find the shorting or draw then chase it down.

@John - why 800 amps? More is better? Unless one were testing battery cables during cranking, what else could possibly pull more than 800 amps? Is that some instant peak draw you are trying to capture? I was thinking a couple hundred would be more than enough.

[bevans6]

If you think it's the alternator, just disconnect the alternator over night and see what the drain is.  Or put a DVM in series with the power cable to the alternator.  But I have never once seen an alternator that was working properly draw current beyond the reverse leakage of the diodes, which is usually around 5 mA when I have measured it.  If a diode has failed that might change.

[blue_goose]

The voltage That feeds the alternator will have a diode in the line that should stop a battery drain.  If it goes bad it will put a load on the battery. Be careful if you put your amp meter in line with the load.  Most meters will only take about 10 amps.  If your load is larger that that it will burn your meter up. 
Jack

[Iceni John]

@John - why 800 amps? More is better? Unless one were testing battery cables during cranking, what else could possibly pull more than 800 amps? Is that some instant peak draw you are trying to capture? I was thinking a couple hundred would be more than enough.

That's exactly why!   A starter like the 42MT draws more than 400 amps.   My clamp-on meter still works for lower currents, but I like having the ability to also check the big 'uns if I want.   Actually I bought it primarily for testing my PV system and house batteries, but then I realized I could have one that works for everything.   It does have peak hold.

John

[bevans6]

Both my meters have fuses inside on the amp terminals.  PITA when you go to measure, and find the fuse is blown.  FWIW any diode has a reverse current leakage in the .5 to 5 mA range, depending on different things.  That level of current is usually considered negligible, the battery itself probably loses more than that.  I have never had a standalone diode in the alternator cable on any of my vehicles, again FWIW.  Not to say there isn't one inside an alternator over and above the 6 phase rectifier diodes. 

Brian

[bobofthenorth]

Troubleshooting a power leak is a long process compounded by many years of owners hooking up wires in random locations with little understanding of the effects of their actions.  Start by disconnecting a main battery lead - it doesn't matter if its the positive or negative but I prefer to use the negative side because its usually easier to get a clean disconnect.  Then put a test light between the disconnected lead and the battery.  You can use the ammeter on your VOM in place of the test light but most VOMs have a relatively low capacity ammeter so I find it best to start with the test light and transition to the ammeter once I get rid of the biggest leaks.  If you don't do it that way be prepared to buy a lot of fuses for your VOM. Once you've got a test light in series with the battery lead start disconnecting circuits until your light dims or (ideally) goes out. Be warned though, you've likely got multiple paths so you will need to pull a lot of fuses before your light goes out. You'll probably end up pulling every fuse or disconnecting every circuit and then adding them back one at a time to see which ones turn the light on.  Like I said, its a LONG process.

[windtrader]

Bob, good summary of what I expect. The PO was very detailed, wires are labeled, and documented. Will make things quicker but still quite a project of its own.

I'm hoping the dc clamp meter still make it a lot faster to find the word with current



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[bevans6]

How sensitive and accurate at low current are the DC clamp meters?  Milli-amps or lower?

[bigred]

If your bus has a 12v disconnect ,just shut it off when you park the bus.I do this on my Prevost 'cause if I don't, over a period of time my batteries will drain down to the point of not starting the bus.Can't figure out why when CC converted my bus they did not provide charging to the bus batteries from the generator .

[blue_goose]

A couple of things.
If you have an electronic engine or transmission the power to those should still be on even if you turn the battery disconnect off.
Looks like a good deal on a clamp meter.  Description Item # 03482369000P Model # 82369  Sears, they say 3% accruate.
$59.00
Google DC clamp meter it find it. 
Jack