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2008 Lexus RX350 Parasitic Battery Drain

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.

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.

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!

I’ll let the experts answer, but in the meantime I’ll point out that a typical car battery should be able to supply that much current for about a month if I did the math correctly, for what that’s worth.

80mA is too high in my opinion. 50 down to as low as 20mA would be more in line.

When you ran the test, the doors were shut or the door switchesheld closed, right?

What was the drop when you pulled only ECU-ACC?

Edit, agree with @lion9car, it should not kill the battery in 2-3 days. Should be good for a lot longer assuming when you drive the car, you are driving long enough to fully charge the battery.

80ma is a little high, I’m happy with 50ma or less. But an 80 or even 100ma draw itself won’t kill a battery over 3 days. You have isolated the fuse where your draw is, I suspect one of the items there may be intermittently “waking up” other modules causing a higher draw.

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Thanks Mustangman, lioncar & asemaster,

I forced the door switch when I was testing.

When I pulled ECU-ACC, the drop was 60-70ma.

I assumed the battery would stay charged longer. Battery was 12.4V when I started checking it. I am thinking of letting it sit without driving and checking voltage each day to see how fast it is dropping.

Thanks, asemaster. I watched the ammeter for a while after powered down to 80 ma, and also left it connected for a couple of hours and checked it periodically. Did not see anything “wake up” but could have missed it. When I remove the ammeter and reconnect it, the system does “wake up”, as the current increases to ~600ma and then reduces back to 80ma after several minutes. Unfortunately, I do not have a meter that records or saves max value.

I did check to see that some of the circuits on these fuses are switched and power down with the ignition switch, and would think that eliminates many of the circuits on these fuses.

Maybe you can borrow one from a chain auto parts store. They have some tools for loan, and maybe a recording VOM is one of them. Does your ammeter have an output? If so, maybe you could get a dongle that connects it to a USB port and record data on your computer or smartphone. If you see a long term average of 80 mA, any increase should scale linearly.

Optima batteries says that 70 or 80 milliamperes is not unheard of when it comes to parasitic draw.

My current and past Lincolns have roughly 700 milliamperes before things go to sleep. About 70 or so afterwards. The battery is 800 amp hour and it will go down if allowed to sit through extended non-use bkut it takes a while. It can sit for a month and start right up.

Don’t know what’s going on with your car but it should not drain in 2 to 3 days. You might define what happens when the no-start exists.
Does that mean no starter motor operation or what? Just trying to determine if there might be a range selector switch problem.

Thanks. No output on my meter. Good idea on the chain stores. Haven’t seen theme on their list of loaner tools, but have not called to verify.

Close the hood latch when measuring the draw, it has a switch that is connected to the body computer.

How many miles is this vehicle driven each month?

When the no-start happens, the car cranks for a couple of seconds and then stopped. Had the battery checked and it checked bad and was replaced. This battery was less than 6 months old.

Thanks, Nevada_545. I checked it with hood opened and closed and had the same current draw for both.

In recent months, the car is driven infrequently. Maybe 20-25 miles/month.

You may have what is called a Sneaky F.R.E.D.

Tester

That is why the battery goes dead. Recharge the battery once a month or use a battery tender.

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That’s about what I drive now.

My battery doesn’t go dead in 2-3 days.

Tester

The Hobbs article is incredibly useful. I had two problems. Chart 1 was not visible. I also wish that Hobbs had given a few more educated guesses about the cause.
I learned one very important thing. If I had this problem I would make sure that my mechanic had even a clue about how to do such an thorough analysis of my car.
I recently had a my 1996 Miata die while driving.My mechanic diagnosed it as the fuel pump not getting a ground from the ECU. I had the ECU rebuilt ($120 online) and reprogrammed. I’m not sure I would have made that diagnosis nearly so quickly.

A battery with a 10% state of charge can go dead in 3 days.

My cars sit for a month or more without the battery discharging completely but my cars don’t have 25 computers.

And exactly who determined the alternator was doing its job properly?

If it was the guys at autozone, I’d take their “diagnosis” with a grain of salt

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Agree with oldnotdeadyet, this is a nice article. Thanks for sharing. Using the RC calculation shown in the article, the battery should start the car for ~2 weeks if not driven. Draw would have to be ~500ma to drain the battery to a level of no start (using calcs from article).

Firestone center checked the alternator.