What's draining my car battery?

bad coil in it, i had bought a m2500 murray weedeater from walmart .did same thing i replaced the coil. bad when you buy something from wal mart and have to get it fixxed before you can use it. now it works fine

Hi Robert,

I don't know anything specific about this vehicle. However, vehicles made in the 90's onwards that had factory "security" devices fitted often drew significant current when in "sleep mode" - ie: the car not running. This current drain could be 10-20mA. Over time, this will flatten any battery! Typically, 7000 to 8000 hours = 156 days = 5 months.
Modern car electronics draws significantly lower current whilst in sleep mode (say 1-2mA) so the problem essentially "goes away".

Alternatively, it could be gunk and/or corrosion around door light microswitches causing unwanted drain. You could measure the battery current drain and remove the car fuses in turn to determine which circuit is the culprit. Beware - connecting a meter capable of measuring milliamps in series with a car battery takes a bit of knowledge and skill. Do it incorrectly and (a) your meter goes POOFF and/or (b) you may generate a dangerous arc/spark. Ask an auto-electrician for advice.

Kitski

if u bought it brand new u should check it for dead cells i bought one from wal-mart not to long ago that had a dead cell the car would crank but then run dead

When you say NOTHING on, are you saying no radio or interior light? No light on in the boot (trunk) and no sidelights? If this is the case then a 5 amp drain does sound a little odd.

That said 5 amps should not be a great drain on the battery. If you have a 70 amper hour battery (Obviously, I don't know what car and rating battery you have, so I just took a general stab at a Mitsubishi Charisma which has a 70ah battery) then a 5 amp drain should take 14 hours to kill it stone dead. I imagine that 5 hours would put it in a pretty poor condition and 10 hours would probably have discharged it to the point where all your lights are dim and the vehicle wont turn over on the starter motor.

It does sound like you have something draining moe power than would be expected

3 possibilities are:

• Dead cell in battery; a battery with a dead cell won't accept a charge from the alternator
• Bad alternator; if the car will run on it's own without stalling, the alternator is probably still good; if the car stalls you may have alternator problems
• Bad starter; (assuming you've ruled out alternator problems) a bad starter will start when the car is cold; once warm, it requires more power to engage than the battery has therefore, it needs a jump start

I'd advise bringing the car to a shop (most autoparts chains will do a free electrical system check) and have the electrical system checked.

Find your fuse panel. Take a picture of it with your cell telephone. With the car off, with a volt meter, remove the fuses one at a time. Measure every slot to find out what is hot and what is not. Any Hot circuits should be examined closely to find out what is ON when the car is OFF.

Places to look ... light in the trunk??? is it on or off with the lid closed? Under the hood? Same. Do you have a radio that comes on with the key off??? GPS, you leave plugged in? Cell phone or other gadgetry you may have plugged in to the aux power supply? Any other lights? Parking lights for example? Security system? CB radio?

After you have checked these items, and if you find nothing, you may want to have your alternator examined. If you have a shorted diode in the alternator, the power may be draining back to ground through this shorted diode. You could perform this test yourself by unplugging or disconnecting the alternator during the periods when you are not driving the car. If this is the trouble, on day three, when you reconnect the alternator, two things you will note: 1) there will be a pretty big spark & 2) you will still have charge in the battery (evidenced by the spark)(there will be some spark no matter what ... but a big spark will mean a big drain). I have seen shorted diode problems fixed by inserting a toggle switch in the alternator power circuit. The alternator will still charge the battery when the switch is ON with the car running. When you stop the car, turn the switch off, power will not drain through the alternator to ground. Probably not the solution for your Mercedes. The real solution is to replace the alternator.

BTW - do you have a security system in your car? A thief proof radio? Check your owner's manual for precautions you should take before disconnecting your battery with regard to the anti theft features of your vehicle.

You say your battery is new and dead after three days. If fully charged on day one, and dead on day three, that means your battery has expended between 500 (economy battery) and 1000 (premimum battery) "amp hours" of power (500 amps / 72 hours = about 7 amps per hour going somewhere. (An 'amp hour' is one amp for one hour) Ohm's Law applies.

I hope you find your trouble and I hope I have given you a few leads. Thanks for allowing me to assist you with this problem this morning ... good luck.

Thanks for using FixYa.com today.

Hi, first thing to check is your battery voltage. Use a digital multimenter, or take it to a parts store or wal mart for free testing (can be done while in the car, see if a jump start gets you going.) If low/no voltage, I would say it is a dead battery. After you get it going, check the voltage at your battery with the engine running. It should be between 13.2 - 14 volts DC if not, your alternator could be bad causing the battery drain. Mike Please remember to rate this answer! thanks!

Did you have the battery tested? It should hold 13+ volts at any time. Wal- mart will test it for you. If the battery is good, get the car fixed before you kill the battery, car batteries can only go dead so many times before they are shot. If the alternator is not charging, get that thing to the shop too and have it tested, if it is OK, then you have to have something running like a clock, radio, alarm system, something is draining the battery, you have to find it and get her fixed or carry a spare battery. I have heard of fuel pumps running on, and draining the battery, see if you can hear that thing going when the car is off. Check it out. Hope this helps.

With engine running used a voltmeter and check voltage at battery, you will be checking alternator out put voltage, should have a reading of no less then 13.0 volts, if less bad alternator, the reason it keeps going dead is that the car is using the battery to run it .till the battery has no more power, then you jump it and the battery is charge up again and will run again till it goes dead again.

For one thing get the leak stopped and then run some Dry Gas through your tank. Could be water in there.