Lexus rx400h parasitic drain.

Siberpir8 · May 19, 2020

Hi all, not sure if it's because I only drive the car once a week if that but the batter has gone flat twice over the last month or so. Battery is good, had it tested as it was relatively new. It is 100% fine. I did a multimeter test for current drawn from the Battery and I measured 0.12amps, mind you I didn't let the car sleep for an hour so some things may have been active. Is this roughly a normal reading or is it still too high?

Herbie · May 19, 2020

I'd personally say it's a bit high but I'm not sure what's an acceptable figure for the RX400h.

As a general rule of thumb, anywhere from about 35-55mA is alright with around 100mA being cause for concern. However, you really must let the car go into sleep mode to get any meaningful readings.

Siberpir8 · May 19, 2020

So, yeah I have just re-tested it and after a few minutes it dropped from 0.12 down to 0.03 - 0.05 so I believe that is more of an acceptable level?

ColinBarber · May 19, 2020

1 hour ago, Siberpir8 said:

So, yeah I have just re-tested it and after a few minutes it dropped from 0.12 down to 0.03 - 0.05 so I believe that is more of an acceptable level?

Yes that is normal.

Every time you let the Battery get flat it will lose some capacity so will take less time to go flat the next time and so on.

You possibly aren't using it enough - if the Battery gets to half capacity you will need to drive it for a few hours to recharge it to full. I'd get a trickle charger if you can, if you can't get mains electricity near the vehicle then maybe a solar charger would help.

welland · May 20, 2020

What is the best way to test the current draw from the Battery? Is it necessary to disconnect one Battery terminal and measure the current between the Battery terminal and the disconnected lead?

Herbie · May 20, 2020

1 hour ago, welland said:

What is the best way to test the current draw from the battery? Is it necessary to disconnect one battery terminal and measure the current between the battery terminal and the disconnected lead?

welland · May 24, 2020

On 5/20/2020 at 7:03 PM, Herbie said:

Thanks Herbie - good video.

welland · May 24, 2020

On 5/19/2020 at 10:03 PM, ColinBarber said:

Yes that is normal.

Every time you let the battery get flat it will lose some capacity so will take less time to go flat the next time and so on.

You possibly aren't using it enough - if the battery gets to half capacity you will need to drive it for a few hours to recharge it to full. I'd get a trickle charger if you can, if you can't get mains electricity near the vehicle then maybe a solar charger would help.

Thanks Colin. And thanks as well for your advice on the other thread I made about the solar charger

welland · May 24, 2020

On 5/19/2020 at 8:06 PM, Herbie said:

I'd personally say it's a bit high but I'm not sure what's an acceptable figure for the RX400h.

As a general rule of thumb, anywhere from about 35-55mA is alright with around 100mA being cause for concern. However, you really must let the car go into sleep mode to get any meaningful readings.

I'd like to know a bit more about the sleep mode in the RX400h. Does anybody know the following:

1) When does it go into sleep mode?
2) Does ignition key have to be off?
3) Does car have to be locked?
4) After how long does it go into sleep mode?
5) if the car is unlocked and a door is opened, will the car be awakened from sleep mode even if the ignition key is not inserted and turned on?
I usually ensure that courtesy lights are off if I anticipate leaving the doors open for a while, but don't know if open doors cause other additional drain on the Battery.
6) if the ignition key is in the ignition, but turned off, will that cause any more Battery drain that if the key is not in the ignition at all?

Herbie · May 24, 2020

4 hours ago, welland said:

I'd like to know a bit more about the sleep mode in the RX400h. Does anybody know the following:

1) Not sure, but I've heard that some cars take up to 2 hours
2) Yes
3) See 5 and 6
4) See 1
5 and 6) I thought this was covered in the Youtube video but maybe not - it's a while since I watched it.

When you open any door the interior lights come on and this is usually achieved by either some form of 'pin switch' that is activated as the door swings open, or by a switch mechanism linked to the mechanical movement of the door latch.

Either way, you need to fool the car into thinking that the doors are shut by either placing a piece of tape over the pin switch to hold it in (if it's present) or by activating the mechanical latch if it's a latch switch.

Make sure everything else is off, keys out and in a Faraday pouch if they are keyless entry type, and wait for at least an hour, or two if you can, then carry out the tests. The aim is to let the car go to sleep and don't open any doors or somehow wake it up once it's sleeping.

ColinBarber · May 24, 2020

It has been a while since I owned my 400h so cannot remember but on newer hybrids if you unlock the door the brake booster pump may start so the vehicle is most certainly woken up before even opening the door.

Full sleep mode is normally achieve within 10 minutes but it doesn't seem to be documented so best to assume 20 minutes to be safe.

Herbie · May 24, 2020

52 minutes ago, ColinBarber said:

but on newer hybrids if you unlock the door the brake booster pump may start so the vehicle is most certainly woken up before even opening the door.

That's why you leave the doors open and make sure the door switches are in the off position.

ColinBarber · May 24, 2020

59 minutes ago, Herbie said:

That's why you leave the doors open and make sure the door switches are in the off position.

I guess it depends what you are trying to achieve. Some Lexus vehicles have the door sensor built into the door lock mechanism which is difficult to trigger. If your door is open you won't want to set the alarm, certainly not the motion sensor, so you won't get an accurate measurement of the normal parasitic load.

Herbie · May 25, 2020

19 hours ago, ColinBarber said:

I guess it depends what you are trying to achieve. Some Lexus vehicles have the door sensor built into the door lock mechanism which is difficult to trigger. If your door is open you won't want to set the alarm, certainly not the motion sensor, so you won't get an accurate measurement of the normal parasitic load.

That is indeed true and it's the reason why you need to fool the car into thinking all doors are closed, locked and alarm activated. The above video does indeed cover this, starting at about 4min 30sec. The motion sensor can usually be turned off with a switch located in the overhead light pod - if not, maybe a piece of masking tape over the sensor would suffice, I don't know.

ColinBarber · May 25, 2020

11 minutes ago, Herbie said:

if not, maybe a piece of masking tape over the sensor would suffice, I don't know.

They tend to be microwave sensors rather than the older ultrasonic type so tape won't help. But again, comes down to what you are trying to achieve, turning something off then alters the standard parasitic state.