My first guess would be glove box light/ trunk light staying on. While there is a small drain on the battery on all newer vehicles ie clock, computor keep alive memory, etc that should not drain the battery. To test for a drain on the battery unhook either bat cable and hook a test light between the cable and battery. If the test light is on you've got a drain, it used to be a simple way of finding the problem but with the newer vehicles it's not that simple. Also be aware that the alternator it's self can drain a battery if one of the diodes goes bad yet it still charges when the vehicle is running.

Good pointers above. You can also do a draw test at the battery while pulling 1 fuse at a time or wiggling the ignition switch cylander to see if it hangs partially connected. If it becomes too problematic, you could install one of those battery disconnect switches- they even have them remote controlled. Good for folks that park and fly. The draw back to that is the computer gets to run off memory for a bit.

Easy way is to disconnect your battery cable, go outside in the dark and touch the cable to the battery post it was removed from. If there's anything pulling juice from it, there should be a tiny spark.

Thanks guys. I will try the light test and the spark test and report back.

A test light may work if it has a small bulb. You could have a battery draw that is insufficient to light a test light, but draw a battery down in a matter of a few days.
IF you have a multi-meter, use the AMPS function and see what the actual draw is. I'm not sure, but ISTR that .025A is about normal???

As mentioned above, make sure some "hidden light" isn't on.

Another "basic" method is to simply leave the battery disconnected. If it stays charged that way....

I had a similar problem that drove me nuts.There was no battery drain,the nine month old battery had two dead cells.Installed a new replacement battery,no more battery drain.Who wuda thunk to look there.???tbk

Well, of course I want to believe it was just bad batteries, but now I can't be sure. Today I put the new battery in and before I put on the postive lead I tried the spark test. There was just a tiniest little spark (very hard to see, but it was daylight) when I held the cable very close to the battery post.

I checked all the lights. Found out the overhead dome light is out, but I know that the switch is in the off position now. The trunk light works, and I know it is going off because I folded the rear seat down and checked. The light in the glove compartment is not working so I assume it is a bad bulb also.

I don't have a multimeter. Maybe I ought to get one. :)

Right now, after I took a short trip in the car, I took the ground cable off the battery. I might just leave it off when I'm not using the car.

Yes is good to have some type volt meter when working with battery problems one way most people don't know use them is to hook ground lead up negative post then with positive prog go all around on battery case to see if meter detects any volt leaking even 1 1/2 volts is bad some up to 3 1/2 this voltage is going to ground if reads any so drain on battery a good cleaning off any moisture and recheck volts most dirt are trash has moisture. Most time I use volt meter on auto battery tester its most handy. I bet if check all your batterys at home no matter how old you find battery drain clean check until reading is 0. Taking a voltmeter to battery rack display can be fun.

Put an amp-meter to work. Remove the negative lead from the battery (always the negative lead, so if you touch some metal on the car with your wrench while removing the positive lead there will not be any "sparks"). Place the positive lead, of your amp-meter to the disconnect negative battery lead, with your amp-meter on its max scale, and then touch your negative meter lead to the negative battery terminal. If it does not read, decrease the meter scale until it does read. This is your leakage current. Find out where it is going by disconnecting things. Fix whatever is drawing current or live with it, such as a clock.

When i worked at a Dodge dealers shop, a lady kept calling in, saying the battery was dead in her car, similar to a Plymouth Horizon. So, we installed a new battery and, a week later, same problem. Another battery, although the second one tested good, and a thorough testing of the charging system. All was good!
But, in the rear compartment, which had a drop down cover, sort of like an inner trunk, not separate like the bigger cars, and it had a light in under it. The owner didn't like that cover that came up every time she opened the hatch, so she had cut the strings that lifted it up! Then, whenever she turned on the little light that was under it, she would forget to turn it off, and then that ran the battery down, especially if she didn't use the car for several days! Took a while to find it!

IMO Computer cars are going to have a spark when you reconnect the battery ( your early 90's was the begaining of the computer error now there are several microprocessors in newer cars, my ford ranger has 26. So depending on how new it is several things are fired up intially. So the spark test probably won't be all that effective.

I would leave the new battery connected and see if it goes dead in a couple of weeks. Why? cause you got to know if there is a problem IMO. Then, I if goes dead then I would suspect the alt first or any recent add ons that my be wired wrong drawning power when it shouldn't.

This is not a good idea on a newer fords several computer items can be spiked and burnout. ESPECIALLY on the fly $X$ models. Like the 4X4 module from a simple battery change. The $X$ is a idea on how much it cost to get fixed. I get around the spiking issue by hooking up a battery charger and keep current supplied to the computer car while removing the battery, this will also prevent any memory loss in the computers IMO. Maybe I just over cautious!?!

Mr. rcmoser, I would be a little reluctant to put a battery charger across the electronics in my auto without it having a battery already installed in it, even a bad battery. Some battery chargers (not all, but who knows which ones) have a noisy output, complete with spikes and half wave rectification, which could cause you great grief.

May I suggest if you do this, use an external battery, even a small one, to smooth out the DC voltage. A battery is a TERRIFIC filter for DC current.

Question: Would an interior light draw current if the switch for it is left on even though the bulb is burnt out?

Also, butchs hobby said that an alternator with bad diodes could drain a battery. I never heard that before, but assuming that is true, how could you check to find out?

I just got to wonder why would it be any different that hooking up a battery charger to car when the battery is dead or when jumping from a running car with an 80 amp alt?

Naturally I wouldn't want to use a 50 amp charger set on maximum charge of course the smaller amp the better in this case in case of automaitc overload equipment failure.

A little battery pack maybe good idea? but I still wonder if that would prevent a initial spike when the battery is connected?

If a bulb is "burned out", the circuit is "open" and no current flows, thus "no drain" occurs. You could not make that circuit any more "open" if you removed the bulb from the socket, therefore the position of the switch controlling the interior light would make no difference. But don't think that the "door switch" controls just the interior "courtesy lights" on modern cars/trucks. The door switches typically only provide a "signal current" TO, or a ground path FOR, a relay or an electronic control module nowadays. The signal current arrives at the module (or a ground path is established if grounding method is used) to indicate that a door is open. On some cars, opening a door other than the driver's door, will elicit a different response or result than the switch for the driver's door. The module then reacts to turn on the interior lights, activate the "door ajar" alarm, apply the parking brake (some vehicles) etc. Even if the interior light bulb is burned out or removed, there may be other devices that can drain power if the door is a jar.
Addressing the issue of maintaining electrical power to the ECM (or ECU) while the vehicle battery is out of the vehicle: I have seen "back up battery devices" made for this purpose that can be plugged into the 12V "power supply outlet (formerly known as the "cigarette lighter") that will safely maintain the data stored in the ECM/ECU (and any other sub-systems requiring 12VDC to retain settings or programs). It is not a good idea to use a battery charger for this purpose because the VOLTAGE output of some battery chargers can rise to 18VDC or higher without an adequate size battery in parallel to "buffer" the voltage output.

Lowes has wonderful volt-ohm meter, with a 20 amp meter scale in it for $19.95. Meters with this much amp meter are hard to find.

A little refresher in using this meter may be in order, although it may get me in trouble. Think of the electrical system as a water system. Voltage is comparable to water pressure and current flow is comparable to water low. And resistance is that which impedes or restricts the water flow, such as too small or a stopped up pipe. To measure voltage; measure from ground, or neutral, with the negative lead (assuming you have the meter set to measure voltage and it on the 30v, or greater scale. Put the positive lead on the positive terminal. It should measure 12 to 15 volts. A good battery is 12.6 volts, or slightly more, depending on how long it has been since being charged. If the auto or tractor is running the voltage should be 14 to 16 volts. You should be able to distinguish a difference in your battery voltage with the car running and not running, otherwise your charging system is not working.

To measure current, you must disconnect something and place your ammeter between the things you disconnected. Just as you would in taking a pipe loose to see if water is flowing, and you measure the water in quarts or gallons, not amps.

If you disconnect your negative lead from your battery and place your positive amp meter lead there and the negative lead to the negative terminal of your battery, you are measuring the current being drawing from your battery, assuming the engine is not running. That should be low, assuming nothing is turned on. How low? It depends on the car or tractor, however, it should be in the milliamp range, and a milliamp is one thousand of an amp.

Only way to check for an excessive drain on computer equipped cars is to use a digital ammeter, a test light is not sensitive enough. Most cars I checked were under 35 mA (0.035 A) back when I was a dealer tech in the 90's.

If your drain is significantly higher than 35 mA then disconnect each suspect component (bulb, alternator etc.) until the measured drain falls to a normal level. Wouldn't expect a "burnt" out light to draw a current especially if its filament were broken but there may be a problem in its base.

A car left unattended for four weeks could drain a battery dead even if nothing is wrong with the car. If your measured drain is less than 35 mA & the car sits unused for long periods of time then leave an automatic charger connected.

I wonder if one of those "solar" chargers could overcome the little battery drain? Guess car may have to to be parked outside,in the sunlight....For about nine months,I had a big battery drain on my Bronco,Three days to dead.so I strung an outside extension cord to the vehicle and connected a "Battery Tender" to the battery.That overcame the "drain".As posted before,than I found the "new" battery had two deadcells.Their was really no battery "drain".Installing another new battery cured the "drain" symptoms.It may not always be a battery "drain",simply a "bad"battery..BTDT... tbk

I've seen some solar powered "battery maintainers" that have a long enough wire lead to allow the solar panel to be placed in a sunlit window or to be placed outside the shelter in order for sunlight to strike the panel. I've read that these will not recharge a dead battery but will maintain a state of charge to varying degrees depending on all the factors that affect any other charging (or maintaining) method, plus the added "amount of daily sunlight" factor. I will opine that one of these solar maintainers will perform better with a smaller battery (as in OPE) than with a bigger battery (as in cars/trucks).

I would think a Lexus dealership could check the electrical circuits and tell you if you have a bad circuit.
A good automotive battery should last (maintain a charge 6 months in storage) if not you probably have a problem. I have a '93 Ford Pickup I placed in storage 6 weeks ago, it will be there for 10 or 11 more weeks, I expect it to start up with no assist. If it doesn't I will have the battery tested on a load tester, the battery is a 84 month battery and will be approx 30 months in March. This is a work truck and I can't be down in the summer months when I need it. It may set a week at a job site, but when its time to go, it needs to run.
Many time's I have posted here buying used equipment is a hi risk of a unspected failure because people tend to work around problems rather than fix them, they finally get tired of messing around and sell or trade the problem away. At least this has been my experience in 64 years of living.

I had that problem with my then new 99 pickup. Its first winter outside during the holiday season ( sat for a week in a snowpile) - i go to start it- dead. Keep in mind this truck isnt even 2 years old yet. The dealer takes it in, diagnoses it as a dicharged battery- they charge it- hold it overnite and cant find the drain. So one evening i happen to look behind my seat, where i installed a amplifier for my custom stereo- i noticed the amp was on even tho the truck/stereo was off. Something inside the amp broke, the auto shutoff linked to the stereo wasnt working ( when you turn the radio off- the amp is supposed to turn off)- a quick fix was to pull the amps fuse till i could replace it.

There is no reason that you could not locate the solar charger anywhere you wish it to be by adding extra length to its output leads so they would reach your battery. 16 gauge lamp cord wire would work nicely. The small amount of current flow in these leads would produce an insignificant voltage drop.

Hello, In regards to the Solar rechargers , I have a Coleman ( used for snow mobiles ) mounted on the out side southen side of my shed and leave it hooked up to my tractor battery during the fall & winter months and my battery never died on me. I also have a larger one that Volks Wagon installs on there new cars during shipping, bought on Ebay for maybe 20.00 a couple years ago, I just plug it into my cigarette lighter socket and place the panel on the dash top, I try to face my truck where it will get the most and longest light exposure, I usually do this in the real cold weather and also if I'm not going to drive it for a few days.
Glenn

my stepdad has 3 vehicles he alternates driving. his 2 trucks rarely get used and he often has to boost them off. the problem is that the computer and the clocks slowly drain the battery. look at your cranking amps of your battery. if you have a .25 amp draw, and this is real low for standby draw, then every 4 hours it pulls down 1 amp out of the battery. if your vehicle sits for 3 weeks it has lost over 125 amps. my truck draws around .5 amps with nothing on but the computer and the clock onthe radio. i can leave it uncranked for about a week before i have to crank it or boost it. it won't die completely inthat time, but it will sometimes drag the starter enough that it won't crank until i get more juice.

i recommend simply getting a battery disconnect and pulling the cables when it is not in use. or get a battery tender that automatically disconnects the battery from load if it drops below 10.5 volts. this should leave enough juuice to crank teh vehicle.

Does your Lexus have power seats, a friends ford t bird had them and one was responsible for discharging the battery. You could adjust the seats even with no key and one was always trying to back up even though it had reached it's limit. He fought with a constantly dead battery until one quite night he heard the little motor and giggled the switch, never had a problem after that.

This kind of parasitic drain made me nuts for months. I ended up buying a 1.5 amp charger to maintain the battery until finally the cause of the problem failed.

The Alternator.

Sadly there are many causes and you are better to track it down.

Apart from a small charger you might also consider a 12 v cut off. Screws up radio presets etc but it might work for you.

Here is a link that might be useful: battery cut off

I have a 1974 Nova with the same battery (seems to be draining problem). I drive my car everyday 1hour each way and home from work. The battery seems to be fine. The alarm and security switch works good. If i let my car sit for two days without starting it, the battery would be completely drained by the 3rd day. I made sure all the little lights on the car are off. I just dont know what could be draining the battery. If there is anyone who could shed some light for me it would be greatly appreciated. There is no computer or any special equipment, just a 350 engine.

thanks in advance.

get an DVOM raegular meter set it to amps then disconnect the neg lead on the battery then complete the circuit with the tester. if the current is pulling more than .025 on older vehicles then you have a drainage. On newer ones you have more computer memories and clocks ect... it will be around .125-.15amps. if more check the lamps in glove box and hood first... unplug. then with the help of another person or by yourself (not recommended bc of time) (and if your still over amps) then start pulling fuses under the hood then inside the car until amperage drops.... look up in the owners munual what the fuse controls... Fairly simple process and if somthing still draws current keep going. Thanks, the Hybrid guru. JKSully

A gell cell battery has from 13.2V-13.4V therefore more CCAs. Use one of these in a very hot or cold environment. So your not waisting money. Everything is made to last a certain time including transmissions ect... It just happens to fail out-of-warranty as we call it. In our shop we repair CNG systems, Hybrrids, and EVs (Electric Vehicles). Though fewer problems, when they do happen it is much more expensive to replace than your regular car. A EV or HEV battery pack can cost up to $23,000 for the most expensive (Ex: Fisker Karma Coupe or Mercedes Benz S-400 Hybrid). So love your cheap stuff now and hate the dealers now: Honda, Lexus/Toyota, Kia... ect, Ford, Chevrolet, and Dodge, because they are set to fail after warranty. Your best bet is a foreign car like Fiat, Jaguar, Benz... ect. UK cars in general (Better economy where based). Though Fiat has brought back the 500 with Jeep/Dodge (The companies they own). We needed a small turbo deisel not gasoline powered. With a motor generator and a deisel to charge a pack we would be set. But we are afraid :(. Boo hoo America. We have almost no polution from some deisels now. Lets see your paying an extra dollar to gain a extra 25-35 mpgs its worth it trust me. We are not ready for this innovation, for even i drive a souped up 1994 940 Turbo that uses premeium gasoline.
Thanks, The Hybrid guru. JKSully

Questions about any kinds of batteries I am avalible to answer. NiMH, LIB, PbA or Lead Acid. Please feel free to E-mail me at zohnny@yahoo.com or yahoo messenger if you wish