On my 86 Vette the battery will go dead overnight. The battery is good and I have unplugged the alternator thinking that it was the problem. It was not. No lights are on, nothing running........ Battery will be grave yard dead in 12 hours...........Anybody got any ideas?? Thanks............

 

You may just have a bad battery. What's your voltage while the car is running?

For starters, take the battery out and have it tested. Not just a load test, but with a test tool that will check your cells as well. Replace the battery if needed or re-charge your battery and re-install if it checks out ok. You can do a parasitic draw test to see if there's a current draw if you have a good multimeter that has a 10A, fused ammeter.

 

Somthing could be shorting out causing the battery to drain overnight. This happened to me and it turned out to be my old Bose head unit. I randomly pulled fuses until I found one that stopped the drain.

 

If you don't have an ammeter, you can disconnect the ground from the battery. Tap the ground cable on the negative terminal on the battery. If you have a large current draw, you will see sparks. Pull fuses until you don't have sparks. That will tell you which circuit is causing the problem (hopefully), then troubleshoot from there.

 

You most likely have excessive leakage current. No shorts, or you'd have a fire or blown fuse or fusible link. GM says leakage current should not exceed 50 milliamps, my 87 draws 27 ma.
First, at night, check for underhood lights, vanity mirror lights, door panel map lights, console light. Do you have a radar detector, aftermarket alarm, aftermarket radio or audio amplifier? check em.
Buy a VOM test meter (voltmeter, ohmeter, milliammeter) so you can measure the leakage current from the battery with everything turned off. You can buy a decent VOM at www.hosfelt.com for $8.00, yes, eight dollars. Disconnect the negative cable from the battery and connect the VOM set up as an ammeter between the neg battery cable and the negative battery terminal. Pull the courtesy light fuse so you don't have to wait 30 seconds every time you pull a fuse for the courtesy lights to time out and you can keep the passenger door open without the courtesy lights on. Observe the current reading, it should be below 50 milliamps, if not, start pulling fuses one at a time and watch the ammeter to find which circuit has the excessive leakage current. If no luck with the fuses, remove the nut on the jump start terminal behind the battery and remove the 8 wires one at a time while watching the ammeter. Unplugging the alternator does not eliminate the alternator because the alternator plug gets power through the ignition switch, and if off, no current is possible. But, the alternator output wire is always connected to the battery and defective rectifier diodes in the alternator can draw current. One of the 8 wires on the jump terminal is the alternator output wire.

 

Kind of off the subject but did anyone ever hear of a MHOS meter? It measures the inverse resistance of a battery.

http://www.elkproducts.com/products/elk-blt.htm

I wonder if anything like this is made for lead acid type batteries. I'm thinking not, because they work on a different principal.

 

The meter you provided a link for is used to measure the internal conductance (mhos) of rechargeable batteries. It could be used to determine if the battery is at fault. But if you charge up the battery and leave it overnight disconnected and it still discharges, then the battery is defective.

 

Thanks guys for all the info...........I feel the battery is OK for I can disconnect it (negative side) for a few days, reconnect it, and the car will start fine. So, I definitely have something that is pulling it down very quickly. I will try removing the fuses one at a time first to isolate hopefully the source...........Thanks again for I have learned something for sure today.........

 

Check the dig dash this is a common source of draw.

 

Fast, in my opinion, follow JFB's advice, but use a good digital multimeter that has a fused, 10Amp ammeter. Most of the cheap multimeters will say right on them that you can't use the 10A ammeter for more than 15 seconds every 20 minutes. You will easily fry that circuit on the meter. The other reason is, if you remove the fuse for the interior lights and you don't find a draw, you'll want to replace that fuse so you can include that circuit in your draw test. The fuse for the interior lights has several other components included on it, including part of the dash cluster.

When you place an ammeter between the battery post and your disconnected battery cable, it will activate the interior lights on the timer. You'll have to keep the ammeter connected for about 20 seconds before the lights go off, which should kill most cheap multimeters. During that time, the lights will draw about 3 amps until they go out, then the current draw will reduce to whatever else is running off that circuit. This is also much easier than pulling light bulbs out from everywhere. And don't forget to unplug your under hood lights.

 

Thanks again guys...............I appreciate all the info.....Im sure Ill find it now.........Thanks..