parasitic drain

[RandallFlagg]

I've got a 93 roadmaster that I inherited from my dad. He's the original owner, the car's a S. California car and garaged since new. It's in decent shape for it's age. I've got a drain on the battery that I still can't find. I don't drive the car much so it's more of a hobby to fix than a need. Two weeks ago I charged up the battery, drove it around and parked it. I then removed all the fuses and the two relays in the driver side fuse box. I went out today and reinstalled all the fuses and it just clicked, didn't have the juice to start. I put a new alternator in it about 10 months ago when I started messing with it. Since it isn't one of the fused circuits and with the new alternator I doubt that's it, anyone have any ideas what to try next. I took the battery over to Sears and they said it's good. If I drive the car daily it doesn't have time to drain. Not sure how to check the fusible links or if the starter could cause a drain. I've checked the power antenna, glove box, trunk and hood lights, but with the fused circuits not causing the problem that seems to be ruled out. Thanks for any suggestions or info.

 

[945frankenmaster]

I dont mean this to be offensive and i hope u dont take it that way but do you own and know how to operate a multimeter? Or a test light, obviously u can see power with the light but a meter will show you how much and also continuity, to test fusible links you want to test for continuity which is resistance, if its high resistance its bad

 

[buickwagon]

All batteries have a self-discharge rate. AGM types have a lower rate but traditional lead-acid lose around 4%/week at room temperature (increases with temperature). When the battery ages, the percentage lost increases as lead sulphate crystals develop (both insulating the plates and diluting the electrolyte concentration) thereby reducing the battery capacity. Eventually lead deposits accumulate in the bottom of the battery and start shorting out plates which really increases the discharge rate dramatically. Unless the battery receives regular equalization charges (ie short periods of higher voltage), it will deteriorate amazingly fast in storage.

The first thing I would do is disconnect the battery and insert an ammeter in the line to see if there truly is a continuing draw. The computer and clock normally draw around 50-80 milliamps. More than that is probably something else. It can happen: I had a similar issue once that turned out to be the electric brake controller, which was on an in-line fuse that was missed.

If all seems normal, leave the battery disconnected and see how long it lasts all by itself. Your intermittent use and storage habits suggest the battery may be just about done.

 

[frostydevil]

my '93 had an electrical drain. Never found it before I sold it.
GAWD I hate electrical issues!.............

 

[Red RoadMonster]

Mine was a power antenna motor that wouldn't quit running.......pretty quiet too

 

[NFortin198]

I had the same problem with a different vehicle, a Mercury Tracer. I put in a battery disconnect switch under the hood temporarily, till I was able to locate the problem. It was a little worse though, the battery would be totally drained after sitting overnight. I would drive to work, pop the hood and hit the disconnect switch, then I would have to hit it again to go home, then again when I got home. It turned out to be a crack in the male portion of the plug going to the main computer. When there was no significant power draw from other systems, it would arc inside the female part of the connection. I only found it because one day I was sitting in the car with everything turned off, when I suddenly had smoke and the smell of melting plastic coming from behind the dash.

 

[RandallFlagg]

Thanks for the replies. I guess it's going to come down to tracing positive leads to find some broken insulation. I've pulled all the fuses and the car still drains. I stopped at Autozone today and they tested the charging system. He showed 14+ volts with the air running. He said the battery was starting to show signs of age, but it was still charging okay. I've driven it everyday for 10-15 minutes and the car starts right up. Whatever it is, it has to be something direct to battery. I'm going to start with the battery cables then pull the dash panel and check for brittle wires. I got lots of shrink tube.

 

[paart]

I can't imagine that you are going to find much under the dash that's not fused. That sort of approach may cause you eventually to lose your sanity!

You need the use of a fairly good digital multimeter that has the ability to read 10A DC. Disconnect the positive lead from the battery. Wire the meter between the battery terminal and the positive cable. The black or negative lead from the meter should go to the battery, although it really doesn't matter. Be certain everything, including the underhood light is off. Be certain all the doors are closed, and all the delayed interior lights are off. After you have checked that everything is off, one final time, turn the meter on to the 10A DC scale (on most meters, this requires that the leads go into a specially marked jack on the meter). It should read under .1A. (.1A=100ma.). Typically, the reading will be about half that. If it is reading a minus number, it's because you have the meter leads reversed at the battery, and you can ignore the minus.

If you have more than 100ma, then something is drawing too much current. It's very unlikely, however, that you would have more than a 2A drain, as the battery would "die " in an hour or so. Start pulling and replacing the fuses and relays, one at a time, in the underhood box at the rear of the right fender. Something should reduce the drain to less than 100ma. If one of the large fuses marked "BODY" does the trick, then you can pin it down by pulling the fuses on the drivers side of the dash one at time. You will have to figure a way to hold the interior door light switch closed. Good luck!! I used a short 2X4 wedged in the door jam! Turn the meter off while you are setting this up.

If none of the underhood relays or fuses reduces the drain, then it's pretty much down to three possibilities. 1/ the wiring between the battery and the fuse box. 2/ the heavy wire to the starter, or the starter itself (very unlikely, but possible) 3/ the alternator. Sometimes the internal diodes in the alternator can have "reverse leakage". This is also very rare, but I have seen it. You can check for this by disconnecting all the wires on the back of the alternator, except for the ground, then retesting. Again always disconnect the meter or at least turn it off, except when actually measuring current. Good luck!

 

[buickwagon]

I've pulled all the fuses and the car still drains.[\quote]

Yes, but did you disconnect the battery completely and verify it holds a charge?

I stopped at Autozone today and they tested the charging system. He showed 14+ volts with the air running.

Ok, so you know the charging system is probably OK. If they used a scope they can detect diode leakage in the alternator. If they used a multi-meter then it just verifies the voltage output. It doesn't sound like they used a carbon pile to verify the maximum current output, but assuming that not only the air but also all lighting, 4-ways, defog, high fan, electric fans, etc. were running and the alternator still maintained 14 v at idle, that's a good sign.

He said the battery was starting to show signs of age, but it was still charging okay.

Yup, you already know it will take a charge. And it will hold it for 24 hours. The problem is it won't hold it for two weeks.

I'm going to start with the battery cables then pull the dash panel and check for brittle wires. I got lots of shrink tube.

Waste of time. If you had a short, it would blow a fuse or start a fire. Follow Paart's advice and meter for current flow out of the battery with everything shut off. The battery and PCM will draw 50mA or so. More than 100mA and I would try his suggestion about disconnecting the alternator (since you know the battery dies with all the fuses pulled already). Less than 100mA, and you've just proved the battery itself is the problem.

Cold Cranking Amps is an indication of the amount of current a fully-charged battery can deliver in one short burst. This is what gets tested by a battery load test. It doesn't tell you how long the battery can last -- just how much it can put out in a 5 or 10 second burst.

Reserve Capacity is an indication of the amount of power a battery can store. A battery sized for a RM might typically have a RC rating of 125, meaning the fully charged battery can deliver 25 amps for 125 minutes before the voltage drops below 10.5 volts (at that point, you would hear the solenoid click, but the starter probably wouldn't spin the engine).

Self-discharge is the rate a battery loses stored power just by sitting there with nothing attached due to normal chemical processes within the lead-acid battery. The self-discharge rate usually increases as a battery ages.

Another way to express RC is in amp-hours. A 50 amp-hour battery can deliver 1 amp for 50 hours, or 2 amps for 25 hours or 500 milliamps for 100 hours.

In your case, let's assume a normal parasitic load + self-discharge rate of 80mA.

Both CCA and RC drop as a battery ages. Let's say the battery has a 750 CCA and 125 RC rating when new. Let's say the starter draws 300 amps to crank the cold engine. Now let's suppose that both have dropped in half over the life of the battery. Will the engine still start off the "fully" charged battery? Yup: 750/2 = 375. Will the battery last 24 hours between charge cycles with a normal parasitic load? Sure: .080mA x 24 hours = 1.92 amp-hours, or about 4.6 of your available 63 minutes RC.

But it won't last 2 weeks: that's 336 hours times .080 amps = 26.88 amp-hours (or 64.5 minutes, if you prefer).