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pauljohn (Marine/Ocean) (OP)
5 Apr 06 11:48
Can anyone tell me what would be the acceptable voltage drop per day or week on a battery in a car that wasn't being used? If car is new, has factory radio/alarm/etc.

Is there a standard that companies use on how long their car can sit unused before it fails to start?
MacGyverS2000 (Electrical)
5 Apr 06 12:33
You should easily get many weeks of life out of a decent battery and still have enough juice to crank the engine over.  Figure low 10's of mA in current draw for stock components.


ivymike (Mechanical)
5 Apr 06 14:42
one thing that'll mess that up is a battery that's got electrolyte "scum" on the top surface... current sneaks thru the scum from one terminal to another, apparently in large enough quantities to drain a battery over a few days.
Fabrico (Automotive)
5 Apr 06 16:22

It all depends on what is connected to it. Other things that can cause losses are temperature and if a battery is connected to other batteries.

pauljohn (Marine/Ocean) (OP)
5 Apr 06 18:14
The battery runs down after about a week. Obviously there is some component not shutting off and draining it. The dealer is certain he has fixed the problem (this is the fourth time he has said that!).

The only time the problem becomes apparent is when the car is left one week which isn't often. I need some way of checking the drain over the weekend so I can keep returning it until I am sure he has fixed it. I am only able to check voltage to .01v and not amp draw.

Battery is 10 months old, car is one year old with no modifications.
So what do you think the spec is on volt drop?
GregLocock (Automotive)
5 Apr 06 18:52
Buy a 10 dollar multimeter.

I think the airport parking time requirement is 6 weeks, off the top of my head.


Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

pauljohn (Marine/Ocean) (OP)
5 Apr 06 18:55
Thanks for the airport requirement.
What does the 10 dollar multimeter remark mean?
GregLocock (Automotive)
5 Apr 06 19:08
It will measure current up to 200 mA


Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

Helpful Member!  MikeHalloran (Mechanical)
5 Apr 06 19:24
With the ten dollar multimeter, you set it to milliamperes, and connect it between the battery and the car.  Don't try to start the car, just measure the idle current with everything nominally off.

Then start disconnecting harnesses.

Beware of tricky auxiliary connector locks.  You shouldn't have to work particularly hard to separate even a large connector in a car of that vintage, provided you've released all the screws, latches, and cross-pins that prevent them from falling apart in service.

Mike Halloran
Pembroke Pines, FL, USA

pauljohn (Marine/Ocean) (OP)
5 Apr 06 19:26

I should have been more specific. According to the dealer the problem isn't always there.

As far as I know the meters you are talking about need to be connected inline which means disconnecting the battery and resetting the computer and possibly losing the source of the problem. I don't have a clamp-on meter.

If it was as easy as connecting a meter you would think the dealer would of been successful (although who knows, one of the guys at the dealership said that a week of no driving is enough time for the battery to fail so they can't be that bright).

Also if the battery runs down in a week won't the draw be more than 200mA

thanks for the reply.
Warpspeed (Automotive)
5 Apr 06 19:27
I have had a similar problem, there are a few factors to think about.

The first is the age and condition of the battery. Ivymike is right, batteries all suffer from slight self discharge, this can be made much worse by impurities in the electrolyte, either scum at the top, or sediment down below.

The next thing to consider is, the state of charge in the battery. The car may not be doing enough miles to keep the battery fully charged, even if the alternator and regulator are working perfectly. The initial high amperage recharge is only part of the story.  The charging current will quickly fall back to something quite low, and it takes many hours to fully charge a battery back up to full rated capacity.

And lastly, car alarms, clocks, and computer gizmos with constantly energised RAM memory, place a small but steady drain on the battery.

I went through all these dramas myself some time ago.  The car would just not crank over after only a few days of not being driven. The solution was to buy a good quality small (one amp)trickle battery charger and leave it connected for perhaps a week.  Now the car can be used for short trips or remain idle for up to a month and it will still start without any extra recharging.

When I start to have problems again, I give it a good long trickle charge to bring it right back up. Perhaps one week of constant charging every six months is all my infrequently driven car needs to completely solve the problem. For the rest of the time it now works fine.
GregLocock (Automotive)
5 Apr 06 19:30
Sorry I missed the one week bit. You are probably pulling half an amp or just less. Yes, the computer will get reset. Why is that a problem?

First thing to do is disconnect all the after market equipment, especially any power amps and 'hi'fi gear, and anything you can see after dark with the ignition off.


Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

pauljohn (Marine/Ocean) (OP)
5 Apr 06 19:54
Warpspeed - car is new. Battery was changed 10 months ago and is clean. Alternator was replaced by dealer at same time. I am sure the alarm/radio/computer/etc is probably the problem. Car is driven 50 miles a day regularly.  Less on the weekends.

Greg - apparently the computer is supposed to shut down one hour after the doors are locked, if it is staying on this could be the problem. If I disconnect the battery then connect it again won't this reset the comp. and power it back up?

I do not want to do the work the dealer should be doing for nothing, car is under warranty. All I want to do is determine if the fault is still there and hand it back to him and say fix it, seems like the easiest thing was to check the volt drop to determine this.
MikeHalloran (Mechanical)
5 Apr 06 20:08
Two people might manage this:

Connect the meter between the battery terminal and the car's distribution point.  You may need someone to just hold a prod on the battery terminal.  Disconnect the cable while keeping the multimeter connected.  (Impossible on a side- terminal battery, sorry)  

Lift the cable connector up over the prod.  With steady hands, you should be able to measure the idle current without resetting the computer.

Mike Halloran
Pembroke Pines, FL, USA

swall (Materials)
6 Apr 06 10:16
You could have an intermittent short somewhere in the wire harness--say a sheet metal screw has nicked the insulation. As the harness moves slightly, the short comes and goes. An acquaintance of mine was a tech at a GM dealership and found just such a problem--took him 8 hours.Dealers don't want to spend this kind of shop time on a problem--they'd rather change out parts and proclaim that it is fixed. So, it behooves you to try to find the problem and then bring it to dealer's attention.
BiPolarMoment (Mechanical)
6 Apr 06 10:30
I thought for sure you were going to say you had a slightly older Miata. Nevertheless, I agree with MikeHalloran--check the dark current first.
patprimmer (Publican)
6 Apr 06 18:20
It would be a rare circumstance where a dead short like a metal screw into a harness would allow a current path that was so low as to not blow fuses or cause fires.

Only case would be where the switch to activate a device is on the earth. This would then turn that device on. Examples that come to mind are horn buttons and brake light switches. The intermittent operation of the device might then become obvious.

A poor connection between the alternator and the battery can cause low charging rates as the extra resistance tricks the voltage regulator into thinking the battery is charged, or the voltage drop across the resistance overpowers the voltage drop between the alternator and the battery hence severely limiting current flow.

I still think the suggestion made by others that it is an accessory stealing current with the ignition off and the doors locked is most likely.


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Tmoose (Mechanical)
7 Apr 06 18:38
If you disconnect the battery the radio may require re-entering the anti-theft code.
A friend had the tiny screw from a pair of sunglasses secretly fall into an upward facing cigarette lighter socket. The tiny short was not enough to pop the fuse/circuit breaker, but killed the battery over-nite.
I'd  get an ammeter in line and start pulling fuses. Then again I might try to get the zone manager involved first.  Any smoked components would (justifiably) be blamed on my messing around.
evelrod (Automotive)
10 Apr 06 13:15
From an equally painfull dealer experience and fruitless quest on a brand new Ford some years ago...check the trunk/boot or glovebox light switch!  It took several trys but they found it, finally.  Too simple?  I would think the 'simple fixes' would come first...apparantly not in my case.

jbthiel (Mechanical)
28 Jul 06 2:46
Not a solution, but there are little solar trickle charges that you can plug into your cigarette lighter to keep your battery charged over long periods.

"apparently the computer is supposed to shut down one hour after the doors are locked, if it is staying on this could be the problem." I doubt that this would happen on your car as it should be determined by software - that you share with every other owner of this model.

You could have a bad battery cell. I have seen this before. The battery seems to check out OK and the "green eye" is on, but the battery will be dead the next day. If you have the green eye be aware that it only monitors one cell of the battery.

-----And from --------------
Battery life and performance - Average battery life has become shorter as energy requirements have increased. Two phrases I hear most often are "my battery won't take a charge, and my battery won't hold a charge". Only 30% of batteries sold today reach the 48-month mark. In fact 80% of all battery failure is related to sulfation build-up. This build up occurs when the sulfur molecules in the electrolyte (battery acid) become so deeply discharged that they begin to coat the battery's lead plates. Before long the plates become so coated that the battery dies. The causes of sulfation are numerous. Let me list some for you.

    * Batteries sit too long between charges. As little as 24 hours in hot weather and several days in cooler weather.
    * Battery is stored without some type of energy input.
    * "Deep cycling" an engine starting battery. Remember these batteries can't stand deep discharge.
    * Undercharging of a battery, to charge a battery (letÕs say) to 90% of capacity will allow sulfation of the battery using the 10% of battery chemistry not reactivated by the incomplete charging cycle.
    * Heat of 100 plus F., increases internal discharge. As temperatures increase so does internal discharge. A new fully charged battery left sitting 24 hours a day at 110 degrees F for 30 days would most likely not start an engine.
    * Low electrolyte level - battery plates exposed to air will immediately sulfate.
    * Incorrect charging levels and settings. Most cheap battery chargers can do more harm than good. See the section on battery charging.
    * Cold weather is also hard on the battery. The chemistry does not make the same amount of energy as a warm battery. A deeply discharged battery can freeze solid in sub zero weather.
    * Parasitic drain is a load put on a battery with the key off.
jbthiel (Mechanical)
28 Jul 06 2:52
One tech bulletin I found stated that a fully loaded GM vehicle could have an expected parasitic drain of 25-35mA. Seems like a lot, I think that we try to stay in the 10-15mA range. (off road vehicle)
IRstuff (Aerospace)
28 Jul 06 10:20
Sorry to be so late.  The $10 meters that I've bought recently all come with a 10A scale, so there should be no problem with measuring large static currents from a car battery.  If the battery is draining down in a week, the current draw would be on the order of 1 to 2 amps.


pauljohn (Marine/Ocean) (OP)
28 Jul 06 10:47
You could be right. However what I wanted to do was measure volt drop on the battery every day to see if the fault was there not measure amps. As I mentioned in a previous post disconnecting the battery would reset the computer and possibly remove the problem as it was erratic and I did not want to do the mechanics job for him only tell him the fault was still on the car.

I finally got the answer I was looking for from a battery tech. who told me that a 0.02-0.03 volt drop was about average for lead acid batteries although it varied quite a bit with humidity/temp type of vehicle etc. This would roughly agree with greglococks remark that 6 weeks would be a minimum for cars left unused.

Sure enough when I checked volt drop on the battery it was dropping 10 times that fast at about 0.2 volt per day.

Another trip to the dealer and they tell me that the radio was pulling 1 amp when everything was off so they think they finally caught the problem.

I'm still checking the volt drop but so far everything looks good. Thanks for all the replies.
engtwo (Automotive)
11 Sep 06 16:55
GregLocock (Automotive)
11 Sep 06 19:01


Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

patprimmer (Publican)
11 Sep 06 20:50
My old Beetle dropped to 4 volts   winky smile


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Warpspeed (Automotive)
11 Sep 06 21:03
Yes Pat, but they only ever had half a battery to begin with.
patprimmer (Publican)
11 Sep 06 21:56
Ah, but they are 4 cylinder and they do have side lights, so they match the "VERY BROARD" question.


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statorwinder (Automotive)
5 Nov 06 12:45
If one dosen't have a clamp on low amp meter
Here is a procedure that does the job for measureing
1. parallel another battery across the vehicle's battery
2. disconnect the vehicles battery
3. install a 1 ohm 10 watt resistor in series with the vehicles battery
4. disconnect the other battery...with the 1 ohm resistor installed enough current will flow keeping all memory circuits active
5. with a volt meter measure the voltage drop across the 1 ohm resistor
6. the voltage drop reading represents the current draw

gaber2 (Mechanical)
10 Nov 06 18:41
thats a good topic and let me share u it.
i just read the all posts,and just have a comment on what statorwinder said.
i just don't understand why we had to use another battery which is connected parallel to the vehicle battery,what if we connected the resistor in series as u said,and measured the volt drop,and then we would get how much current flow through the resistor?
why should we use the other battery?
MacGyverS2000 (Electrical)
12 Nov 06 16:35
The parallel battery will prevent all of your circuits from resetting while you place a resistor in series with the original battery.

Dan - Owner

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