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dead battery in rx400h

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Hi  Bought this 2008 rx400h last year.  Don't drive it alot and battery goes dead after 4-5 days.  Had battery and charging system checked, are good.  Terminals clean and tight. Has lexus battery in it.  Lexus report showed battery replaced multiple times in the past, thought because not driven much, now see this is a hybrid problem.  Currently disconnect battery when not going to go anywhere.  Thinking of getting quick disconnect, used one before, worked good.  Anyone ever find an easier fix for this?  Thanks

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Did you take it in to the dealership to have them check it out? Your vehicle should be able to sit for 2 weeks. I'm hoping they didn't tell you otherwise. There could be a light that is not turning off if it is not LED. 

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Hi,  when I bought this from a lexus dealer they told me it wasn't driven enough to keep battery charged.  When I drive 3-4 days in a row and let it sit for 3-4 days still goes dead. Will talk to dealership again but not expecting much help.   thanks for suggestions    mike

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One possible solution for keeping your battery happy is to connect a battery charger when car is not in use for several days. 
I have a 2006 RX400h with over 400k km’s on it without any problems. One time there was a power failure when outside temp was -27C and I took my RX in warm garage after a short drive. All gauges and warning lights went crazy. Battery voltage was dropped to 8 volts. After an hour in warm garage all was well. 
I am really happy with my RX400h. Sunfoof drive belt is only thing that has broken. 

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  • 2 weeks later...

Hi everyone.  I never had a car that had to have a charger on it whenever it sat for 3 days.  Now battery going dead overnight, had checked 3 times and told battery and charging system good. Left it with negative battery cable disconnected and still down to 10 volts overnight, won't start at 10 volts.  I think battery bad no matter what I'm told.   Going to buy new battery and see if fixes problem.  Any suggestions on good batteries for 2008 400h?  

thanks    Mike

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I buy my battery at Costco. And you are correct in that it is not normal for any vehicle to need to be driven every 2-3 days. 

I would've had the dealership keep the vehicle for a few days so they could see for themselves that the vehicle will not start. They should provide you with a free loaner vehicle.

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I had a similar issue where the new battery had a bad cell but showed 12 volts anyway. My old school mechanic (now retired) tested each cell and found two were slightly over volted and one was under volted. He explained to me that there are 6 cells and each should have around 2 volts. 

If I recall correct two were at 2.4 and one was at 1.6 so the tester showed things were fine in total. I took the battery back to the auto store where the guy did a load test and saw that it did have a bad cell so he replaced it. 

When they place a regular tester on the terminals it just checks total voltage. My mechanic had a diagnostic machine that showed all kinds of stuff, but he would then check things the old fashioned way. He was the sort of mechanic where if your car would not start he'd stick a rod into the fuel tank to make sure it had gasoline regardless of what the gauge said it had. 

Anyway, it sounds like it has a bad cell. 

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HI  Just did amp test on battery looking for parasitic draw.  Reads draw of 120 milliamps.  Anyone know what acceptable draw is for 2008 rx 400h?  

thanks mike

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This info is from Herbie in the United Kingdom forum:


Assuming that the DC/DC converter is working alright and you do get about 14.4 - 14.7V when the car is in the READY state, you'll then have to start looking for the source of the parasitic drain.

The car needs to be in 'sleep mode' for any chance of tracing the fault. This means that you may need to wait for a couple of hours after switching off the ignition and removing the key (and keep the key out of wireless range too). If the doors have pin switches to operate the internal lights you'll need to put a piece of tape over them so that the ECU thinks the doors are shut and the lights don't come on.

Once the car is in sleep mode, a normal quiescent current draw will be about 0.035mA or 35-milliamps. This is to keep the settings of such things as the radio presets, seat memory, alarm, GPS and maybe even more things too. If you find that the current draw is slightly higher, say 0.040mA that may be acceptable for the amount of stuff that needs to be kept alive - maybe Lexus publish figures of what's 'normal' for the car, I don't know?

If, on the other hand, the draw is something like 0.080mA or 0.100mA then something is definitely wrong. As an example, if the car has a 50Ah Battery and a draw of 0.085mA, it would go from fully charged to fully discharged in about three weeks (if my maths are correct :rolleyes1:).

It used to be acceptable to disconnect the negative Battery clamp and wire an ammeter in series, then measure the draw and pull and replace each fuse in turn until the value dropped and that would be your faulty circuit, but there's a better way to do it now because disconnecting the Battery could reset the ECUs and make the very fault that you're chasing, disappear.

The preferred method now is to switch your multimeter to a millivolt scale (I know that we've been talking Amps up to now but for this you definitely need it on millivolts) and simply put the test probes across each fuse in turn because you're looking for the voltage drop across the fuse. In very general terms, the higher the voltage drop the more current is flowing, so the highest reading should be your faulty circuit. For a more accurate result you can refer to these charts here. 

Once you have a circuit that's giving a high reading you can pull that fuse and confirm the actual current draw by switching your meter to the Amps scale and placing the probes into the fuse socket.


There's quite a few good videos on Youtube that demonstrate better than I can describe in words, so just search for videos on 'finding parasitic draws' if you want to.

Here is the link to the original posted battery drain issue:

RX400h 12V battery drain overnight - RX 300 / RX 350 / RX 400h / RX 200t / RX 450h Club - Lexus Owners Club


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