2008 Lexus RX 400h - Battery issues

car sits in my garage, after being driven more than 25 freeway miles to completely charge battery. Battery is new Lexus dealer installed and about 14 months old. San Jose CA location.
If car sits for more than 4-5 days the battery is dead and car will not start. Lexus dealer check car & battery for 2 days and said " battery is measuring to Lexus specifications". Suggested I buy a battery tender.
There must be something not correct-- a almost new battery (14 months old) should hold a charge longer than 4-5 days.
Is this really “normal” for my 2008 Lexus RX400h hybrid? Anybody had this problem? Any suggestions about what to check that might be producing this problem?
Also, I went to the “Find a great mechanic” site and entered San Jose CA 95117 and got zero results. Strange that a city as large as San Jose CA would have NO reviewed mechanics, much less Lexus mechanics. Am I using the application, right? Is there a way to put in my zip code and do a search for mechanics within 25 miles, 50 miles, 100 miles?
Thanks for any suggestions/ help

Yes, you’re using it correctly.
The listings that do show up (when they do) are usually several years old. The 2 that show up for my location, 1 went out of business 10 years ago, the other hasn’t had any reviews for over 15 years.

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Have your system checked for parasitic draw. A fully charged battery with a properly operating charging system should be able to go for a at least several weeks.

Why would you want to use a shop 100 miles from your home for something that the dealer has said is because you don’t drive enough . That has been a problem for many people since this Virus thing. Do the battery tender first and see if that solves the problem or take a drive more than 25 minutes once a week.

A simple web search of ( Vehicle shops near me ) will give you results and reviews .

Thanks for the reply and info!


Thanks for the reply. The 100 mile number was just a way to see if there was/is a different way to see if I was using the app correctly, No I would not drive 100 miles to a repair shop.

You are referring to the 12V battery, correct?

Correct, the 12 AUX. battery.

You probably have a parasitic drain somewhere. It’s pulling power from the battery when the car’s turned off. It could be something as simple as something plugged into one of the power ports. People have been caught out with that before.

It could also be a light that’s staying on. Or it could be something like the cell phone module - I know Acuras sometimes run into that issue as they age. The cell phone module doesn’t turn off when it’s supposed to.

You have a few choices here:

You can live with the drain, and get a battery tender to keep the battery topped off during the week of no driving.

Or you can try to diagnose it yourself. The good news is that Eric the Car Guy made a video teaching you how to do that. Have a look here:

Or you can pay a shop to diagnose it for you.

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That probably wouldn’t be enough driving to completely charge a near-dead battery. A little arithmetic is in order. A full charge requires about 40 amp-hours. I don’t know how many amps your alternator can produce, but if it could put out 20 amps, it would take 2 hours of driving to fully charge the battery. Sometimes the maximum current output is listed on a tag on the alternator.

I did a little Googling and it appears an alternator typically outputs 100 amps, not 20, so maybe 25 miles would indeed be close to enough to fully charge a dead battery.

The alternator in my wife’s hyundai elantra (small, inexpensive car) puts out 200 amps.

The auxiliary battery is charged via the inverter, this vehicle has no alternator.

Vehicle charging systems charge batteries much slower than battery chargers, the voltage is very low to protect the battery from damage during long trips.

A battery that has been discharged very slowly from the normal parasitic drain can take 10 to 20 hours to recharge at 13.5 volts no matter how much amperage output an alternator is capable of.

BTW, a battery doesn’t know if you are on the highway or idling in your driveway, 25 minutes is insufficient.



Thanks for the reply. This problem is not a “charging” issue. Battery is fully charged after driving. The problem is "rapid " discharge while sitting. 4-5 days in garage and battery will not start car. 14 month old battery that was tested by Lexus dealer and said to be “good” and in specification should not completely self discharge in 4-5 days.

I found this on a Lexus site…

After months of parasitic battery drain, I was told by my mechanic, a former Lexus mechanic, that the multiplex body control module is what’s causing the drain.

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Yep, sounds like something is causing a parasitic drain, like what @weekend-warrior mentions above.


on hybrid vehicles 12V battery powers pretty much only computers and small devices and has nothing to do with starting engine, still if it is low it will make entire car inoperable

now, due to this difference in how battery is used (as to compare to “regular” cars), these 12V batteries are optimized for longevity, not for peak power and if one cares to read the specs, these have strict limitations on charging currents as anything above that can damage them

I agree to other posters that parasitic drain is the most likely culprit and it has to be found, but I do not agree to the advise to “just attach a battery tender if you do not want the hassle”

I’m 99% sure that the 12V battery on this Lexus is AGM type and as such, $10 tender may easily get it damaged via overcharging

CTEK MXS 5.0 – smartercharger.com

OP, to figure out what’s causing this sort of battery drain an auto-electric specialist would measure the total battery current your car used while “off”. Next they would disable the various circuits one by one. One way to do this is by pulling the fuses. Once the circuit causing the problem is discovered it is usually a simple matter from there to the solution.