Why Does My Car Battery Drain Down Over Night

Reader Question

My wife drives a 1991 Honda Accord. Great little car. Still runs great. It has one serious problem which it has had for several years now.

The battery rapidly drains when the car sits for a couple of days. I replaced the alternator and battery each more than once to no avail. I used an amp meter to check drains on various cables while removing fuses and relays, one at a time.

I discovered a drain going to the alternator on a cable that is supposed to provide current to the battery when the alternator is spinning.

I figured the alternator diodes were going bad. Why they kept blowing on new alternators I couldn’t figure out.

I bought a couple of high-amperage, high-voltage diodes and put them in-line on the alternator cable.

The idea was to replace the functionality of the bad alternator diodes and prevent current from draining back through the alternator.

It didn’t help but it didn’t hurt either. They’re still in-line and the car runs fine. The only thing I could find that would stop the battery from completely draining was a device called a “battery brain”. Battery Brain.com It monitors battery voltage and disconnects the battery before it gets too low. There is a key-chain wireless module with a button you press to connect the battery when you want to star the car. Works pretty well.

I noticed one other thing about the car. When I’ve been in the garage I’ve heard a buzzing noise from the Honda’s engine compartment. Checking under the hood indicated the noise was coming from the fuse box.

Opening that, I discovered it was coming from a relay which controls the radiator fans. I think I may have replaced that relay once before but am not sure.

Any thoughts?

Hi there Gregg,

You are on the right track, just keep investigating. I am not sure what you see as a drain going to the alternator, or if that is really a drain at all, but the buzzing noise under the hood is definitely going to be a drain on the electrical system and something you should keep investigating.

An electrical “drain” can be anything that is drawing power (draining) from the battery with the engine/key OFF. Every vehicle will have some amount of electrical drain which is normal. Radio and clock memory and engine computer memory will drain a small amount of battery power with the key off, and that is normal.

Again I am not real sure what the buzzing noise could be coming from, but any electrical noise/buzz/vibration etc. should not be there, and is probably the source of your problem.

If you had told me the battery drains down while the engine is running, I would suspect a problem with the battery or the alternator, but I think you have already ruled them out.

If you get stuck trying to locate the drain I would take the vehicle to the dealership or a qualified electrical repair shop in your area and at least pay them to track down the source of the drain for you. You can always perform the repair yourself to save some money once you know what it is.

Tracking down an electrical drain can sometimes take hours! Make sure the shop you choose to help you has a qualified electrical mechanic so you don’t needlessly pay for inaccurate diagnosis time.

Some causes for an electrical drain

1. Trunk, hood, or interior light or glove box light that is staying on when the engine is OFF

2. Aftermarket radios, CD players, amplifiers, cell phone chargers etc. etc. are notorious for drains

3. Electric seat motors and electric antennae motors that are stuck in the ON position

4. Relays or components under the hood that are ON or running with the engine OFF – for instance electric cooling fan motors, anti lock brake pump motors, air ride suspension pump motors.

Thanks for the link to the batterybrain.com, I was unaware of it until now.


Austin Davis

19 Comments on "Why Does My Car Battery Drain Down Over Night"

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  3. Randy Robertson says:

    Hi I am experiencing the same problem I recently had a battery drain test by an electric shop I really did not trust they said the drain was coming from my driver door or power mirror which does not work can that be right

    • By Austin Davis says:

      It is possible that either the mirror motor is causing the drain or something else inside the door like the door lock actuator motor or the window lift motor. They should be able to easily disconnect those items and see if the drain goes away to prove that is the cause.

      A “drain” is any electrical device that is ON when it should be OFF, thus draining power from the battery when the vehicle is not running. When the engine is on the alternator and a strong battery can over come most drains, because the alternator is recharging the battery. There is always a small drain with the engine off like the onboard computer, the radio and clock, security system etc. These should be relatively small drains and a strong battery should over come them.

      If your electric mirror does not work anyway, I would disconnect the electrical supply and see if the drain goes away. If it does not, you will need to investigate the drain further.

      Common drains are:

      1. Dome lights, trunk or hood or glove box lights ON
      2. Electric seat motors that are ON (sometimes you can listen under the seat for a buzzing noise)
      3. Aftermarket radios, TV’s or amplifiers
      4. An internal problem inside the alternator
      5. Electric radio antenna’s (usually you can hear a buzzing noise around them)

      Drains are sometimes difficult to track down and can take a few hours….which can be expensive. Find a good shop with a knowledgeable mechanic.

  4. Foreign Car Owner says:

    OOPS! Big Typo! I didn’t mean to say we’ve ALWAYS had trouble even opening the door!
    I meant to say, that recently, in the last 5 months, we have HAD trouble trying to get the car’s door on the driver’s side open, because obviously rain got into the door, inside it, and our mechanic look at it, and say rust inside the door!
    Well, that makes sense, since it was pouring rain out, and the cop, literally insisted we keep our car’s windows rolled down to the bottom of the window, even though we pleaded with them to not do that to us!
    Don’t come to southern New England…. it’s a mess in that state!

    • By Austin Davis says:

      Hi there, so to understand your question better. Your battery is draining down over night as you let the car sit? I would do this, if this is the case.

    • 1. Have the battery “load tested” to see if it needs replacement. A good battery will last about 3 1/2 years
      2. Is the alternator charging the battery, check the output of the alternator
      3. Is there a voltage drain somewhere…is something ON, when it should be off. Interior lights, glove box, trunk and hood lights, electric seat motors running etc. etc. There can also be relays for the engine that can get stuck in the ON position and can be checked with a voltage meter at the battery.
      4. Are the battery cables and ground cables at the battery tight and clean
    • I dont think water inside the door has anything to do with it, as long as the window and door lock switch is still working. I would make sure the door is closing all the way and not leaving the interior light ON.

      Now, if the engine turns over, but will not start you have a totally different issue going on and the battery/alternator are not involved in this issue. An ignition coil, fuel pump, sensor etc. would be the culprit in that kind of situation.

  • Foreign Car Owner says:

    We got a new ignition coil put on our car recently, but the young mechanic, who I think was only in his early 20s at the most, refused to do a test drive on the car, right after it was installed. I found that strange, but about a few weeks later, the check engine came on.
    Now, a year ago, this cop who was not helpful at all, insisted we keep our car window rolled down all the way to the base of the window when it was pouring rain out!
    It is a car whose door & window units are powered by electricity and that power unit for the doors/windows is on the driver’s side door & rain was pouring that power unit for several minutes, while the cop, insisted on us keeping the windows rolled down, though we were the ones who called the police for help!
    We’ve always had trouble even opening the car doors on the driver’s side door, and I know there was rust occuring inside the driver’s door, for a mechanic looked at and noticed rust inside about 6 months after that cop did that to us!
    I’m wondering too, if we are getting our battery draining, because, there’s a short in the engine, due to the rain pouring over that electrical unit on the driver’s side door?
    I’ve had at least 3 battery, brand new installed in our car, in the last few months, and I’m afraid every time we get into our car, it won’t even start, because it’s done that before, right after we got a brand new battery installed?

  • Brenda says:

    Anybody heard of a drain on an brand new battery as a result of a remote autostarter being installed? Have a 2005 Honda Accord. Installed a new battery 10 motnhs ago and only started having problems about a month after a remote starter installed and not having used vehicle for a week in cold weather.

    • Austin says:

      ANY electrical device that is drawing power from the car battery can cause a drain on the battery. Your remote starter could be installed incorrectly and not properly grounded, or a switch or relay could be faulty causing the remote start feature to be in the ON position all the time. I see this a lot with aftermarket stereos and amplifiers that have been improperly installed into the main vehicle fuse box on a circuit that is always on. Your mechanic can check for a drain pretty easily, but tracking down the drain to a specific component can sometimes take lots of time. I would check for a drain at the battery, then disable your remote start kit to see if the drain goes away….if it does, you will need to go back to the installer and see what they can do. Most mechanics will not be familiar or interested in spending time trying to figure out how the thing SHOULD be installed.

  • roy says:

    I have had the some problem and my fault is with the alternator, Voltage regulator. It has been throwging out 20 amps to battery and cooking it so drains down very quickly. so will replace reg and get new battery. Hope this gives you something else to look at.

  • craig says:

    I have a 2000 Toyota 4Runner with similar problem. Battery dies after about 3 days. New battery, same problem. Took it to my mechanic who narrowed it down to the circuit that controls power windows, sunroof, etc, but could narrow any further than that (honest in that he said he could keep looking but it may take hours and he didnt want to charge me without asking if that was ok). This car ran PERFECTLY for 10 years…UNTIL I received the car from my folks (it was shipped across the country) Courier mentioned something to me about my “funny alarm”. It is Toyota factory. Mechanic says it’s not the alarm, but as i mentioned ZERO probs until it was shipped. Any ideas??

    • Austin says:

      I would unplugg/remove the power to the circuit board and see if the drain goes away. If not, you might want to try another mechanic with a new set of eyeballs. Tracking these types of problems down takes time and the better the mechanic the less time it could take and less cost past on to you.

      You also might be able to buy a used circuit board from a junk yard to cheaply test/guess at his diagnosis. I have seen electric seat motors stick in the on position and cause a drain. Stick your head under the seats and listen for a humming motor noise

      I don’t think the shipping had anything to do with this

      Not really sure if the shipping has anything to do with it.

  • Coreen says:


    I have a Freelander 2, and my battery is drained every morning. I replaced the battery, after it was recommended, and the very next morning the battery was drained again.
    Is there anything I can look at, anything, I have checked the lights, the hood, the doors, the radio, but nothing seems to be on. Could it be something on the engine itself?

    • Austin says:

      Make sure there is nothing plugged into the AC or cigarette lighter like a phone charger etc. Then check the glove box and vanity mirror lights to make sure they are off. Anything more than that you will need to see a mechanic and have an “electrical drain test” done to see what is draining the battery.

      Should be a pretty big drain to drain down overnight. Could be a relay somewhere, an electric seat motor that is stuck and running (listen under the seats for an electrical noise) or even a drain coming from the alternator or some other electrical component under the hood. This would be best done by the dealer who knows these vehicle well, this could be a complicated diagnosis and I would not trust this vehicle with this repair to just any mechanic.

  • James says:

    I have a 1964 Chevy 2 Nova with a 6 cylinder 194 engine.Pretty basic stuff, right. This is not a regular car that I drive all the time,but I do crank it up every couple of days to keep the juices flowing in it. A few months ago, the alternator light was on all the time. I replaced the original 32 amp alternator with a Napa premium 37 amp rebuilt alternator. I also replaced the original external voltage regulator with a Napa external voltage regulator. After I installed the alternator and voltage regulator,everything seemed to be working fine. The alternator light would come on when turning the ignition on and then turn off when the motor is turned on, like it’s supposed to. I hooked up a multimeter to the battery to check alternator load. Again, everything seemed to be fine. All the normal readings. So everything has been fine the last couple of months. The other day, I went to start the car, and the battery was almost dead. I recharged my battery and when I turned the ignition on,the alternator light did not come on. The car would start but the alternator idiot light was not working. I replaced the alternator lightbulb, still not working. I connected my multi meter to the battery to check alternator load. While car is off, volt reading was 12.35. Turning the car on, it only went up to about 12.73, not the 13.8 or 14.00 volts like it is supposed to. I replaced the old battery just for the hell of it. Again car would start but again the alternator light was not coming on and I was still getting low volt readings with a running engine. After letting the car sit for about four days I went out to start it up, and my new battery was completely dead. Something drained my new battery. There’s no fancy fuses and relays in this engine. I have no trunk light. I do have a glove compartment light, as far as I know turns off, when the door is closed. Dome lights are off when the doors are closed except that the drivers side door does not activate the dome light when it is opened like it’s supposed to. The radio is always left off. I do have an old antenna toggle switch that is connected to wiring under my dash that I mess around with every now and then but the antenna motor has been disconnected and long since been thrown away. So I don’t know whats been draining my battery lately and has my new alternator gone bad? Why isn’t my alternator light coming on when the ignition is in the on position? Could the alternator have something to do with draining the battery while the engine is off? I’m stumped!! I would really appreciate any suggestions. Thank you.

    • Austin says:

      Yes I would be a little suspect of a faulty diode trio inside the alternator as the culprit. You might also want to check for q faulty chassis ground or just add an additional ground to the system. I have seen alternators have an internal drain. You probably should replace the voltage regulator as well because you don’t know which failed first regulator or alternator or did one cause the failure of the other?

  • yodadifu says:

    1st take the bolb out of the trunk, hood, glove conperment
    2nd day if still drain battery unplug the neg of your amp or remove the breaker also find the breaker for your radio
    3rd day if still drain battery make shure that all your door light are of sometimes the switch for the door fucks up so make shour that the light works when door is open
    4th day if still drain battery look for any cables that might be tuching metal (look for missing insulation on cable)

    try this steps to eliminate potential problems hope this helps

  • tad says:

    i have the same problem with my 1992 eclipse. ill let it sit for about 8 hours and my battery will be dead. thought it wss the stater so i replaced it. but still having the same problem. although replacing the started did help a little bit. mabey u could try that

  • floyd says:

    On my 2004 odyssey once in awhile the ac compressor relay sticks closed. I haven’t determined if the relay itself is a fault or the computer is closing it. I think the contacts are probably sticking closed after you shut the van off and draining the battery. When the car is off I can pull the ac fuse or relay out and you can hear the ac clutch click. I did swap the horn relay and the ac relay to see if its the relay. If the relay is at fault someday the horn will stay blowing afetr locking the doors with the remote.

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