Slow Battery Drain While Driving - Cause?

[Ruby911]

Hi all - I got my new project ('71 E with twin-plug 2.7) on the road only a couple of months ago and I am having an issue with a slow drain of the battery. The battery is new and will hold a charge fine when not being used. By that I mean, the car can sit for a couple of weeks when the battery is charged and will start just fine. However, when I am driving it around, the battery slowly loses charge. It happend to me a the first time a few weeks ago when I spent the weekend at Road Atlanta for The Mitty. I put the car on a charger overnight while at the tracks and it ran fine and would start with no problem...until yesterday.

I drove it yesterday to run several errands for a couple of hours. Started and stopped the car probably a half dozen times. I could tell as the day was going on that each time I tried to start it, the battery was losing power and the starter was turning slowly until finally one time it would not turn over.

I jumped it off and drove the car home about 5 miles. I turned it off and it cranked back up again just fine in the garage.

I am rather handy, but electricity is one area that scares me and one are that I do not understand. My assumption is I have a dying alternator that is charging somewhat, but not eough to offset the drain the car creates while driving. How do you test teh alternator to make sure it is douing its job? Or, could it be something simpler like bad connections or bad ground.

Thanks for any troubleshooting advise you can provide.

- Farrell

[setho]

I would start by taking the voltage measurement from the battery while the car is off. (Positive and ground, not the negative off the battery.)

Then do the same while the car is running. You should read somewhere between 13.8v and 14.2 (guessing)..... If you're not, bring the revs up (2000 maybe) and do the same. If you're not at least in the ballpark, you're possibly looking at a faulty alternator or voltage regulator.

While you're looking at the battery, make sure the connections are tight and clean.

It's a start, at the very least, if you're not in your comfort zone.

[Ruby911]

Thanks Setho - I will pick up a volatge tester of some kind this weekend and give it a shot.

- Farrell

[beh911]

Thanks Setho - I will pick up a volatge tester of some kind this weekend and give it a shot.

- Farrell

Your standard Radio Shack multimeter will do. I've had mine for a couple decades now...

[Ruby911]

Thanks Bryce - I looked at a couple at The Home Depot this morning, but had no idea if they were what I needed. Did I mention I don't understand electricity....

[Haasman]

I found a digital one, auto scaling works great. Check this one out, pocket size at Radio Shack- http://www.radioshack.com/product/in...ductId=2104114

[Ruby911]

Found this old dinosaur in some tools that my wife's deceased grandfather left me. If someone could clue me in on how to set the dials and buttons, I will give this a shot. If not, I will go buy a new one.

Thanks - F

[Haasman]

First, check and see if there is a battery that might need to be replaced. See the upper left section that has DC V ? You want to twist the dial to the 50 since you are going to be measuring 12 volts, +/-. If the 50 is too high move down to the 10.

Attach your reg lead to the positive (red or +) on your battery and the black (negative or -) to your battery as described further up in this discussion. Let us know what you get!

[oscillon]

Found this old dinosaur in some tools that my wife's deceased grandfather left me. If someone could clue me in on how to set the dials and buttons, I will give this a shot. If not, I will go buy a new one.

Thanks - F

For that thing there are two things you can do: 1) Check voltage generated by the alternator, and 2) any possible current drain when nothing is running.

For scenario 1)

Put the range on the dial to DC V 50 (that means that 50 volts will be the max of the range of the meter)

Hook the black wire to the negative battery terminal or a very clean, well connected body lug of some sort and the red wire to the positive terminal of the battery.

If at 2000 +/- 200 rpm you read anything near or below 12 volts, you have either a bad voltage regulator or a bad alternator. Each has their own set of secondary tests to isolate the issue. If you are close (13.5v), then you will need to check the effect of lights on etc. So turn on the lights and rev above 2000 rpm and check again what the meter says. If it ever falls below 12 volts at modest revs, you have a problem with one of those items.

For scenario 2) (drain when off), do the following. This isn't what your problem is, but I already had it typed out.

First, make sure the engine is off. What you want to do is set the dial to DC A and probably in the 50 m range to start. Put the wires in series with the battery so that it looks something like this:

-----positive battery cable----()----red wire----(your DC A meter)-------black wire------()(positive batt terminal)

where () represent connections and ---words--- are wires

If you get a weird reading on you device you will have to adjust the range

Needle pegged to full (or overload if digital) then up the range to 500m (m stands for milliamperes)
needle all the way to the left (or 0.0000 for digital) then lower the range by a click and check again.

Then start systematically pulling fuses and note where the value drops to zero. Be sure not to have anything running (including the clock or any lights).


N.B. You could also have a crappy battery. You can have a load test done at any auto-parts store. If you have a bad/dead cell, the battery may not take a charge.

[Ruby911]

oscillon - great clarity, thank you.

here is a pic with the meter set to DC V 50, red to + on battery and black to bolt on chassis......?