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2008 RX350 Parasitic Battery Drain

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2008 Lexus RX 350

Looking for help in troubleshooting a problem with battery draining after 2-3 days if car is not started.  New battery and charging system checks OK.  Have had to replace the battery 3 times in last 2 years, and twice in last 5 months.  Car is being driven infrequently these days, 20-25 miles/month.  Had a shop check the charging system - said it was OK.

Troubleshooting completed

Used ammeter between negative battery cable and negative terminal to measure current.  With all fuses in, current starts at ~680ma, drops to ~320ma after 1 minute, drops again to ~110ma after 5 minutes, and finally drops to ~80 ma.  The current stays at 80ma most of the time, and occasionally drops to ~60ma.

Started pulling fuses one at a time.  Only 2 fuses affect the current.  With both of these pulled, current goes to zero.

ECU-B, Fuse 24 (7.5 amp) under hood.  Has several circuits - power window, multiplex communication system, gauge and meters, instrument cluster lights, air conditioning system, garage door opener, illuminated entry system, power back door, driving position memory system, navigation system display, moon roof, tilt and telescopic steering, power seats, outside rear view mirror, windshield wipers.

When ECU-B is pulled, current reduces to ~60ma

ECU-ACC, Fuse 28 (7.5 amp) under driver side dash.  Circuits on this fuse - navigation system display, power rear view mirror control, shift lock control system, multiplex communication system.

With ECU-B and ECU-ACC pulled, current goes to zero, so ~60ma on this fuse.

I exercised all switches and checked to make sure that lights, etc were off when the car is powered down - all looks OK.

1.  What should be the normal current after car systems power down?  Is 80ma normal?

2.  Any ideas on what components are most suspect to cause problem?

Thanks for any help on this!

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Have you verified that the cable clamps are installed all the way down over the battery terminals? Make sure that you cannot rotate either of the cable clamps. Also, make sure no wiring within your engine bay has been damaged by rodents. This is the time of year during which they like to make a home under the hood.
I did find these other suggestions:

So if by chance you have a pre-owned vehicle, make sure it's not a FLEET car or one that has had a GPS or other hidden battery sucking device installed in it.

RX 350 go to page 148 of owners manual tells how to disable communication between key fob and vehicle- which over a number of days can kill battery.

Lastly, if you have no luck, consider taking your vehicle to an Auto Electric Shop. They solve electrical drain problems every day and tend to be very good at this.

Good luck.


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  • 3 months later...

Chris, Did you have any luck fixing this? I am in a similar position having a battery prematurely stop holding a charge, possibly due to parasitic drain combined with driving less.

Anyone have any other tips for reducing the drain? I am definitely going to try turning off the remote.

I will try to make longer trips, but I wonder if you found any ways of reducing the drain.

I  am considering replacing with an AGM battery as those have a lower internal resistance and therefore recharge a little faster, meaning that where if it takes 30 minutes to recharge a flooded cell battery, the AGM might do it in 20? Don't take my word for it.. I'm just learning on the internet. 🙂 Downside is, twice as expensive. They also do not suffer being undercharged or overcharged. So while they may be easier to recharge, they will get ruined just as fast as the flooded cell battery by depletion. Some sites say the AGM batteries are better with extreme temps, but others say they fare worse in extreme heat. Not sure if 100-110F is extreme, but we do see that in the summer.

An Interstate battery (3 year warranty) for this car is $90 at Costco. They do not have AGM in stock. An AGM battery starts around ~$200 from O'Reillys (3 year warranty) or Pepboys (4 year warranty).

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I think that buying a different type of battery is not really fixing the problem. I buy the Costco battery every 5 years or so and have not had any excessive drain issues.

If you are a do-it-yourselfer, take a look at this web page by Popular Mechanics:
Dead Car Battery- How to Test a Car Battery (

If you are not, you should consider taking your vehicle to an Auto Electric shop, as I mentioned above.
Good luck!


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