Battery dies in car

Put ammeter on battery. Watch the load. Start pulling fuses out until the load goes away. Check whatever is on that fuse.

If the fuses are all pulled out and the load is still there, start looking at anything on fusible links. Feel around for cabling that is warm.

A clamp-on DC ammeter makes this a lot easier, but you can always just disconnect the ground strap and put an ammeter in series with it. The clamp-on meter, though, makes it much easier once you've started to track it down because you can just clamp it on any line and check the current.

Note that most clamp-on ammeters can't handle DC, but an auto electrical shop will have one right handy.

--scott

does the car even charge the battery? Put a voltmeter on the battery, rev the engine a bit. The voltage should read 14.4V +/- .5V.

Good advice! Thanks much! It worked. And what does the end statement mean?

Good advice! Thanks much! It worked. And what does the end statement mean?

Clamp on DC ammeters are normally used at high loads. They are not so accurate at low current draws.

AC current is much easier to measure with a clamp on meter.

It isn't impossible to measure DC this way, but the equipment tends to be inaccurate, or very expensive.

It's getting better. You can get a meter in the $250 range which are accurate down to 1/10 amp or so. That's not anywhere good enough to find trickle leaks, but it's good enough to find something that discharges the battery in a day.

The things are a lot better than they were five years ago. They are still no replacement for a real shunt ammeter, though.

The neat thing about the new meters is the auto-nulling. You hold the thing in position, press the zero button, then put the wire in and read the display. If you're careful about keeping it in the same position, you're compensating for any magnetic leakage into the thing.

--scott

Measuring a parasitic load might be rather difficult to the beginner with limted electronic experience. A meter to be used for this purpose can be purchased very cheap. It will be somewhat necessary to troubleshoot this prob tho. Let us know if u need help hooking it up. There is an old trick that is just as effective, and doesn't require the use of an ammeter. Obtain a tail light bullb #1157. Disonnect neg bat terminal. Connect one end of light bulb to the cable end, and the other end of the light bulb to the negative battery terminal itself. Key should be in off position. The light bulb should not be lit. More than likely it will be if there is a problem with parasitic draw. If the light is on, proceed as indicated by Scott Dorsey to find the bad circuit. When a fuse is pulled and the light goes out,prob is found. None of these procedures should be tried until,like Az Nomad said, Charging voltage is verified and determined to be good. Good Luck! Jim

Yes, I think it has improved significantly. I was looking for a reasonably accurate clamp on meter about a year ago, and at that time the DC performance just wasn't good enough, at least for the cheaper units.

I finally just rigged an calibrated inline shunt with a millivoltmeter and it worked fine.

Two main possibilities. One- battery is bad. Two, current drain somewhere.

Not likely to be an actual short- a short would create smoke, run battery down in minutes.

To test battery, you need a voltmeter. These are getting very cheap these days. Even a digital voltmeter, usually quite accurate, can be had for less than old analog meters used to cost. With fresh, FULL charge on battery, put voltmeter directly across battery terminals. Should read about 12.5 volts. Now turn on headlights. Voltage should drop no more than a volt. If it drops SEVERAL volts, it is likely you have a bad battery.

If it is not the battery, but a moderate current drain, this is quite hard to diagnose without more equipment. Pulling fuses one at a time- wait a day, repeat, is one low cost option.

And my experience with those meters (at work, where we have a number of different brands) is that they're ONLY accurate to that level if you re-set the zero level every 10 minutes or so (no kidding). Also, if there's ANY audio-frequency to radio-frequency noise in the line you're measuring, such as AC ripple from an alternator, it can throw the reading way off even if you zero carefully just before measuring.

No, no. You have to reset the zero level EVERY time you take a measurement. Don't wait ten minutes. Even if the meter doesn't drift, the field around it might very well be drifting.

Depends on the meter, but it's true. I find the Fluke one at work with true RMS measurement is much better in this regard than the $250 Extech cheapie. Even so, it's very easy to switch over to AC mode and see how much ripple you've got.

I first got one of these things a decade ago to measure projection arc systems that were in the 50 to 200 amp range, and they were very flaky and temperamental with a lot of hand-nulling required and constant drift. They're now good enough to measure smaller currents and a lot faster to null, but they're not perfect. Still faster than disconnecting a cable in most cases.

--scott

Yep, could very well be a bad battery.

I would like to add another senario here even though the OP fixed his problem.

Ciao

Excellent DieInterim. I'll do this test tonight. My problem is my alternator light comes on sometimes, the power to run the heater fan isn't there. Then it goes out and the car comes

*alive* again with full output. I think if there is a bad diode, then this is a permanent situation, not intermittent as my situation is.

My battery checks out OK. My cables seem to be fine. Except that when it isn't getting charged, of course it gets run down. Turning the engine OFF and then restarting corrects the alternator light, temporarily. Then sometimes it comes back on anyhow.

Lg

Excellent DieInterim. I'll do this test tonight. My problem is my alternator light comes on sometimes, the power to run the heater fan isn't there. Then it goes out and the car comes

*alive* again with full output. I think if there is a bad diode, then this is a permanent situation, not intermittent as my situation is.

My battery checks out OK. My cables seem to be fine. Except that when it isn't getting charged, of course it gets run down. Turning the engine OFF and then restarting corrects the alternator light, temporarily. Then sometimes it comes back on anyhow.

Lg ============================================================

leakage between battery and alternator is measured

93 _micro_ amperes. i.e. 0.093 milliamperes.

verdict: acceptable

Another item to check would be the wire to the field. On most autos, power to the start windings comes from the ignition switch if that circuit is faulty the alt cannot work.

So the best thing to do is carry a Volt meter and wait for the light to come on. At that time check for 12v to the field; if not voltage is present then you look at the wires and more specifically, the ignition switch. If these items check out then you need to repair the alt or replace it.

HOWEVER....now we are on to something! The fan will work regardless of a working alt.: I am leaning towards a shorted wire or an intermittent ign. switch.

A shorted wire blew out my cigarette lighter socket. I replaced the fuse, and that blew also, so I left it burned out, but never went after the actual wiring. I evidently pushed too hard on the lighter and this caused the first short. Thereafter, pushing at all caused the second short. I had been running my radar detector from that socket.

I just crawled out of the engine compartment and everything in there seems to check out. So now I have to get behind the dashboard and check in there. THANKS FOR THE TIP !

Will report anything strange/repaired etc.

Lg

A shorted wire blew out my cigarette lighter socket. I replaced the fuse, and that blew also, so I left it burned out, but never went after the actual wiring. I evidently pushed too hard on the lighter and this caused the first short. Thereafter, pushing at all caused the second short. I had been running my radar detector from that socket.

I just crawled out of the engine compartment and everything in there seems to check out. So now I have to get behind the dashboard and check in there. THANKS FOR THE TIP !

Will report anything strange/repaired etc.

Lg

******************************************************************* UPDATE

----------------------------------------------------------------------------- So I went through the engine compartment one more time, took the dash cowls off ( broke one ) and went through the wiring. No fix.

Then I started the engine and watched the alternator light come on.

Put the DVM across the battery terminals.

11.82 volts ( battery only obviously )

Then I started yanking on wires and wiggling connectors. When I got to the voltage feedback PLUG that plugs into the VR on the alt, the voltage at the battery went to 14.2

Although I had a good _mechanical_ connection there, I didn't evidently have a good electrical connection. I twisted the blades about 20 degrees so they would get some *bite* when I put the female plug on again and voila. Fixed.

Instantly the alternator light went off...instantly the voltage at the battery went to 14.x and climbed to 14.2. I suspect if I left the engine running it would have gone to 14.4.

So...THANK YOU FOR HELPING ME !!!!!!!!!!!!!!!!!!!!!

Lg

I am confused as to where this post started from, but you are giving a text book example of a polished up fan belt.

I test them by taking a cold off engine and seeing if I can hand slip the alternator pulley. If I can, the belt is dead. An alternator takes a few HP to spin up under load and if it will hand slip it will not charge properly.

Mike

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