Basic electrical question

[cseger1]

I think that my battery is draining slowly when parked. When I pull the negative wire and put the voltmeter between the negative wire and the body I get battery voltage. That indicates a closed circuit somewhere right?

[philip]

I think that my battery is drining slowly when parked. When I pull the negative wire and put the voltmeter between the negative wire and the body I get battery voltage. That indicates a closed circuit somewhere right?

"Cables" connect to the battery posts. Their purpose is to handle several hundred amperes for a limited time.

"Wires" are all those smaller gauge insulated conductors you find everywhere else.

So for clarity, exactly what cable or wire did you pull and from where?

[cseger1]

Sorry, I unplugged the main cable on the negative terminal, removing that side of the battery completely from the system. Then testing from the neg. terminal to the body my meter reads 12.5V. Which is what the battery reads post to post.

[philip]

I unplugged the main cable on the negative terminal, removing that side of the battery completely from the system. Then testing from the neg. terminal to the body my meter reads 12.5V. Which is what the battery reads post to post.

As it should, as does mine

The truck's wiring harnass grounds to a bolt below the battery as does the truck's OEM battery ground cable ground to this same bolt before it grounds to the engine block. Don't use a voltmeter as an unshunted ammeter.

[asavage]

I unplugged the main cable on the negative terminal, removing that side of the battery completely from the system. Then testing from the neg. terminal to the body my meter reads 12.5V.

While several things can draw current ("parasitic loads"), the main "normal" culprits are the dome light (if you did this test with a door open, for example: don't laugh, I've done it), and the radio's clock.

If you can eliminate those, then put your VM on the windshield, close the doors and start pulling fuses. If you still get 12v+ reading on the VM with all fuses pulled, disconnect the alternator's plug. Still 12v+ on VM? Remove the large wire from the alternator's + terminal (or is there a harness connector for it? I can't recall . . . actually, there are two harness connectors between the charge wire at the back of the alternator and the battery. One is the fusible link, so pull that).

At some point the VM should drop to near zero.

Abnormal culprits include the alternator's voltage regulator, and one of the alternator's positive diodes. Both of those are fairly common failure items that can cause the battery to run down, both from undercharging and from electrical leakage of the semiconductors involved, draining the battery.

While a VM is an essential tool for electrical troubleshooting, at some point you might need an ammeter. There's a problem with using an ammeter alone when troubleshooting this kind of problem: you want total current draw with everything off to be less than 0.1a (a co-worker likes it to be less than 0.05a; I'm less picky.). A tenth of an ampere. An ammeter sensitive enough to show you current under an ampere will also likely have a full-scale capacity of one to ten amperes, and a fuse to protect it against current higher than its capacity.

In practise, using such a "delicate" or sensitive ammeter is going to cost you a lot of fuses . So go ahead and start off using the VM, recognizing that if most any non-trivial device is on, the VM is going to show battery voltage. It does not require much of a load in an electrical system to make a modern, high-impedance VM, wired in series with the load, to read battery voltage.

For that reason, if you at any time get a VM reading of about, oh, say three volts less than battery voltage during this testing, you probably do not have a significant current leak, and switching to an ammeter is necessary to ascertain the leak's magnitude.

Make sense?

[philip]

For that reason, if you at any time get a VM reading of about, oh, say three volts less than battery voltage during this testing, you probably do not have a significant current leak, and switching to an ammeter is necessary to ascertain the leak's magnitude.

Make sense?

Learned something handy here ... a guideline.

My truck has an aftermarket CD player/radio/clock that has a memory and of course a seemingly good alternator. Battery voltage is 12.8v while inline (battery post to disconnected ground cable) is 12.5v .

I had to laugh about leaving the dome light ON while hunting down parasitic losses. Guilty too!

[cseger1]

Makes perfect sense thanks for the detailed reply. This truck has no dome light radio or clock. I'll take a closer look and report back. Napa is bringing a Raylock starter to my town so I should be back on the road on Tues. Thanks again for all the help. This site has saved me $$$$$.