So I decided to run a bunch of tests after an embarrassing and recent experience of losing power on a ferry and waking up one morning to find my battery dead a couple months ago...

I take no responsibility for what you do with this information. Okay now that that's out of the way, we'll start first with some basic facts:

• A 3.5L engine needs 400-500 CCA
• A discharged battery will be at 11.8 volts
• A fully charged battery will be at 12.8 volts
• A battery is measured 20 minutes after the engine, lights, and accessories are turned off
• Tested using a 2012 Napa Legend 75 600CCA battery, Group 24F
• Tested on a 2010 RX350 w/Premium, Tow
• Measured at 72°F

Voltage:

• 12.57V after 20 minutes
• 13.58V with the engine on and AC off
• 13.34V with the engine on and AC on max

Amperage:

• Car locked after 5 seconds - 180ma or a 2.16W draw
• Car locked after two minutes - 20ma or a 0.24W draw
• Car unlocked at 5 seconds - 780ma or a 9.36W draw
• Car unlocked after 30 seconds - 250ma or a 3.00W draw
• Car unlocked and then auto-lock after two minutes - 20ma or a 0.24W draw
• Therefore a trickle charger needs to produce a minimum charge of 0.24W to keep the battery from dying

Also the cigarette sockets produced no power when the engine was off. We'll probably need a OBD-based device for saving the memory when changing the battery.

I'm clueless at how my battery died as I was on accessory mode after having driven 20 miles. I'll add pictures in the next post.

 

Manual



After 20 minutes



With the engine on and AC off



With the engine on and AC on max



Car locked after 5 seconds



Car unlocked at 5 seconds



Car unlocked after 30 seconds

 

I have not checked my RX battery, but going by your voltage readings your battery more than likely have a bad cell.

The voltage should be over 14v with the alternator providing power when the engine is running.

I would have your battery load tested to see if it retaining a proper charge since a bad battery will show a decent voltage reading sooner after a charge, but loses the charge very quickly.

 

This morning I decided to do my monthly battery charge session so I decided to check the voltage on my battery that was sitting overnight with the engine off and then with the engine running.

I may have been wrong about the voltage being over 14v when the engine is running in the Lexus RX since my voltage was only 13.8v at the time.

However, my battery was only charged about halfway according the battery charger, so maybe it read higher after I charge it.

Here is the voltage before charging with engine off.


Here is the voltage before charging with the engine running.


Here is what the battery charger reads.


I will do these tests again when the battery is fully charged and post the readings.

FYI, my AC and lights was on so that might be why the voltage was a little under 14v when the engine was running.

 

Well the battery is fully charged.

My battery voltage right after the charge is 13.87v with the engine off,however the voltage leveled out to about 12.85v with the engine off.

However, even with a full charge the voltage still remained at 13.87v with the engine running with the AC and lights running.

So I am guessing that it would be 14v or more without the load, usually most cars alternators I have check put out about 14.4v without a load(light,AC,etc.).

Battery charged.


Voltage right after charge


Voltage with engine running.


Voltage after shutting off engine.


FYI, I recommend charging a battery with a charger at the least once a month since many car alternators never fully charge a battery completely which ruin the battery cells ability to hold a charge after a few years or even less depending on the quality of the battery.

 

Seems rather high. Usually anything over 50 ma parasitic drain is suspect.

What is the current draw after 5 minutes after all computers go to sleep?

Another thought. I had a LS460 and occasionally in the morning the battery was stone dead. After much troubleshooting the cause was identified as a defective driver door post switch. When the car was in automatic headlight function after exiting the car the door switch starts the countdown timer that turns off the headlights. With the intermittent switch, occasionally the lights remained on until the battery died.

 

Most vehicles will not output over 14v at idle. You need to rev up the engine for the alternator to produce more power.

 

Good point.

However, my voltage with the AC and lights going is 13.87v so I bet that it would hit 14v even while idling if I turned all the accessories off.

To be honest, even the radio was on at the time.

I have checked a few alternators from VW,Mercedes,Nissan that idled at about 14.2-14.4 with the extra turned off like radio,AC,headlights,etc.

But like you mentioned in a way, some cars are different.

I guess it all depends on the amp rating of the alternator.

Some alternators may need more rpms that others to produce a strong voltage output.

Here is an video of how to check an alternator which have a car that idle over 14v with no load.

 

I did a battery check at Napa back in November and found no issues. I opened the battery and checked the cells but everyone looked good there too. I do suspect the battery though as battery dropped to 12.11V overnight. I'll ask Napa to check it out again.

 

Thanks, it would be good to know what draw others are experiencing. I didn't try to measure draw after 5 minutes. The car auto-locks at one minute.

 

At 12.11v your battery is only at a 50% State of Charge.

You either have a weak battery or you have something discharging your battery overnight due to a high parasitic drain.

Once again, anything over 50 ma or so is suspect.

Here's a good YouTube video with explanation and troubleshooting info


Good luck.

 

This guy make some very good automotive videos.

I have learned some good tips from his many videos ranging from brake pad replacements to wheel bearings.

Very good source of information.

 

LOL.

I can not perform an current measurement with my Radio Shack meter which maxes out at .4A.

Hopefully someone else could help provide you with a RX current draw baseline to work with.

 

I replaced my Lexus battery a few months ago. I was looking under the hood, maybe checking fluids (can't remember now) and I saw one of my battery terminals was COVERED in corrosion. I've never seen one that bad. So while trying to remove the cable, we pulled the post out of the battery -- that's how corroded it was (even after doing the baking soda/water thing). At that point there was nothing to do except buy a new battery. The odd thing, to me, is that I've had the Lexus almost two years and it has never not started or even hesitated to start. Just lucky, I guess.

 

^ What, you didn't perform an engine wash for two years?:-)

It's almost spring cleaning time again.