I've got a '96 Town and Country minivan that has had repeat occurrences of the battery being dead. It has been very infrequent, and has sometimes been after as short as being parked over night.

The battery is not that old, and I don't want to just shotgun parts at it.

I removed, cleaned, and reinstalled both the chassis and engine grounds, those can be ruled out.

I checked for parasitic draw, I'm reading anywhere between 10mA and 30mA, completely reasonable. I've also checked this several times over a 24hour period with no change.

I figured I'd try attaching an oscilloscope to the alternator to see if anything obvious would show. I'm quite the novice at using an oscilloscope, so please bear with me.

I attached Ch1 probe to B+ on the alternator, Ch2 probe was connected to chassis ground. The ground clips from Ch1 and Ch2 were coupled together and attached to chassis ground as well. The van was started and loaded with the headlights on bright, and rear window defroster turned on. All other loads were turned off(radio, HVAC fan, etc).

What I got was complete chaos... I was certain that I did something wrong. Here's the results:

 

The voltage looks good, but the scope trace is a complete mess. It looks like a field collapsing, like ignition or something. The readings were taken direct at the alternator. I tried putting a lead direct from the alternator case to the battery negative clamp. Didn't change anything.

I thought perhaps I had the oscilloscope setup incorrectly (which I may have...). I tested it on my car, just to be certain.

This is how my car ended up looking. A little more like what I was expecting:

 

Only two causes of a battery not charging.

1) battery knackered.

2) charger knackered.

Qed...

 

Monitor the voltage across the battery with the engine running with a meter. You should get 14.1V to 14.7V if the battery and alternator are good.

You don't say whether the alternator charges the battery correctly but if it does you have either an intermittent fault causing a load on the battery which is discharging it or you have left something switched on when the battery discharges unexpectedly.

http://www.aa1car.com/library/2003/us20310.htm

 

i agree. it may be the charger or the battery itself

 

Monitor the voltage across the battery with the engine running with a meter. You should get 14.1V to 14.7V if the battery and alternator are good.

You don't say whether the alternator charges the battery correctly but if it does you have either an intermittent fault causing a load on the battery which is discharging it or you have left something switched on when the battery discharges unexpectedly.

http://www.aa1car.com/library/2003/us20310.htm

A couple of times Lidl has done a battery/alternator tester - other discount stores like Aldi etc probably have too.

Certain to be something similar always available at auto parts places - at much higher price.

 

Keep monitoring your parasitic draw. The max you want to see is 50mA. In order for it to go dead overnight, that would take a 3-4 Amp draw and the most common thing that can cause that kind of draw, is the alternator. If the regulator stays energized, the rotor will draw that kind of amperage all night long. If you wake up to a low battery, put your hand on the alternator and see if it is warm.
If that is not the cause, I would check your battery as others have said but that seems very unlikely. Usually battery issues will be constant. Have you checked all your voltage drops on your cranking system?

 

Intermittents are always the most fun, or difficult, to find, depending on your POV. To our general frustration and embarrassment, they often turn out to be things we for some reason counted out early on.
-Does your vehicle have a "lighter" socket which stays energized?
-Have you made any wiring repairs or modifications?
-Has the vehicle been damaged in any way, particularly before you noticed the situation?
-Does the vehicle have ANY remote controlled items or functions?
-Are there any auxiliary connections, like trailer hitch, etc?
-What about that "not that old" battery? Really. Is it the battery that the charging system was designed for, or just something "close" that replaced the original? Oh yes, it does matter.

Yes, the alternator/charging system is the single largest cause of battery failures, and old and non-standard batteries are one of the primary causes of charging system failures. This is due to increasing loading of an aging mechanism until a failure occurs.

Have fun.

 

HI,
Just a few thoughts. I'm not an expert, but I an old (!) and not quite senile yet, so I've seen a few things.

Your minivan uses the engine computer to control alternator charging. It doesn't have a standard voltage regulator.

You can get a fair idea of the condition of your battery by charging it fully ( from a little battery charger if possible), and then checking the voltage drop at the battery voltage while someone starts it up. It shouldn't drop to less than 10v or so.

Somebody mentioned 50 ma as the limit for currrent draw with everything off. That's not as true anymore. 100 and even 150 ma is becoming more common.

This does raise the ugly thought that the engine computer could be part of the problem. 96 was the first year for the mopar single board computers. I had one that had an issue because the potting failed and allowed humidity to cause corrosion on the leads of one of the output transistors ( replaced it with one from a junkyard for $100)

The noise you're seeing is not good, and it may be causing some problems. However, you want to be sure of your connections. Try the negative battery lead for your ground pooint. Possibly run an alligator clip from there to the scope ground or even to the ground clip on the probe.

Your van is old enough so that wirong can definitely be an issue. It may not start sometimes because one of the battery connections is bad. Generally it's the ground side. If it happens again you might try
1. Measure the voltage right at the battery
2. Measure the voltage drop from the negative terminal to a good ground while you try to start it.
3. If you can measure the voltage drop from the positive terminal to the starter (nuch more difficult)

Also, you might try getting a connector for the 'power plug' (formerly cigarette lighter) and monitor it with your meter while you're driving around.
- It will only hve power when the ignition is on, and probably NOT when it's turned to start
Check the voltage before you start and observe how it changes as you drive around. It should go up a fair amount to 14+ volts as the batery is charged. It should settle a bit after a while. Note what happens when you're idlilng, when you turn on the lights, etc. It should handle all of those things pretty well,.

Good Luck,

hj