Battery drain

thefrustratedbaron · March 19

Need help

2001 Lexus LS430

Brand new Battery fully charged is totally flat after 2 days.

Auto electrician said no drain on Battery and alternator working normally.

After another Battery fail called RAC breakdown they ran several diagnostic checks and confirmed what the electrician had said.

This is the third new Battery in less than a month so not Battery.

Latest attempt was to fully charge Battery 12.7v then removed the DC out fuse which apparently dealers do to preserve Battery charge when transporting.

2 days later Battery charge was 2v without DC out fuse, when fuse was replaced the Battery charge was 12v.

It may be a coincidence but this all started when the CD/Radio crackled loudly and the speakers packed up.

Any thoughts to solve would be much appreciated

BigBoomer · March 20

To test electrical drains you will need an ammeter with ranges of 100mA and 10A.
Disconnect the Battery -ve clamp, connect the -ve wire of the ammeter to the Battery post, connect the +ve wire of the ammeter to the Battery clamp.
DO NOT attempt to start the car like this. That WILL destroy your ammeter.

Wait 10 mins for the car to settle into "parked" mode.
With the ignition switch still off, see how much current is being drawn.
Anything over 5mA is too much but if it is flattening a 70AH Battery in 2 days, then it will probably read over 500mA

Now you need to start pulling fuses and checking to see if the drain drops below 50mA.
It is laborious work but is made easier if someone else can keep an eye on the ammeter for you.
Once you figure out what is drawing the power, you can investigate that.

From your descriptions, chances are it is the Amplifier that blew and that is what is sucking all the power out of your Battery.

RONNIE W HODGEKINSON · March 20

20 hours ago, thefrustratedbaron said:

Need help

2001 Lexus LS430

Brand new battery fully charged is totally flat after 2 days.

Auto electrician said no drain on battery and alternator working normally.

After another battery fail called RAC breakdown they ran several diagnostic checks and confirmed what the electrician had said.

This is the third new battery in less than a month so not battery.

Latest attempt was to fully charge battery 12.7v then removed the DC out fuse which apparently dealers do to preserve battery charge when transporting.

2 days later battery charge was 2v without DC out fuse, when fuse was replaced the battery charge was 12v.

It may be a coincidence but this all started when the CD/Radio crackled loudly and the speakers packed up.

Any thoughts to solve would be much appreciated

Possible short In The cables of the radio/cd Player which is Draining your Battery.. Certainly Something is ..Needs further Investigation ...!!!

thefrustratedbaron · March 20

Big boomer and Ronnie

Many thanks for your input. The auto electrician did exactly that and the “parked” mode reading was 50mA which he said was normal for a car/battery of this size, before this parked reading it measured 1.7A which made me think I had a drain. Both “experts” said there was no drain and car was tested with lights heater fan etc on and the reading was I think 14v which they said confirmed the alternator was working fine.

Would taking out the 20A DC OUT fuse when car not used prevent a drain?
As you both suggest the radio/CD/Amplifier may be the source of my problems.

I know nothing of car electrics but if no joy removing fuses what is the function of relays, are they basically an on/off switch.?

If amplifier is draining the Battery wouldn’t that have shown up in the Battery drain procedure stated by Big Boomer. The auto electrician said with the 50mA draw the Battery in theory should run flat in approx 50 days if not used.

Thanking you both for your help in this matter

BigBoomer · March 21

To flatten a Battery of 70AH in 2 days you would need a constant drain in excess of 1Amp and probably closer to 2Amps.
As a first test I would recommend disconnecting the Amplifier (in the boot) and see if that solves the problem.
If it does, then you have a culprit.

Relays are an electrically controlled switch. They allow the various computers to switch things on/off.
Probably best not to mess with the relays unless you know what you are doing.

thefrustratedbaron · March 21

Thanks BigBoomer I will try that.

In the meantime I will use my hybrid method……keep on trickle charge until I require to use car, at least I will know the Battery is fully charged before I set off. Hopefully electricity price cap reduction will keep costs lower😃

Herbie · March 21

On 3/20/2024 at 8:31 AM, BigBoomer said:

To test electrical drains you will need an ammeter with ranges of 100mA and 10A.
Disconnect the battery -ve clamp, connect the -ve wire of the ammeter to the battery post, connect the +ve wire of the ammeter to the battery clamp.
DO NOT attempt to start the car like this. That WILL destroy your ammeter.

Wait 10 mins for the car to settle into "parked" mode.
With the ignition switch still off, see how much current is being drawn.
Anything over 5mA is too much but if it is flattening a 70AH battery in 2 days, then it will probably read over 500mA

Now you need to start pulling fuses and checking to see if the drain drops below 50mA.
It is laborious work but is made easier if someone else can keep an eye on the ammeter for you.
Once you figure out what is drawing the power, you can investigate that.

From your descriptions, chances are it is the Amplifier that blew and that is what is sucking all the power out of your battery.

With the greatest of respect George, I'm afraid that most of that is incorrect.

For instance, the car needs a good hour or two to go fully to sleep, not just 10 minutes. Also, pulling fuses can cause other (sometimes seemingly random and unrelated) ECUs/modules to wake up and give false readings. The following video shows the best way of doing a parasitic current draw test:

BigBoomer · March 22

On 3/21/2024 at 3:21 PM, Herbie said:

With the greatest of respect George, I'm afraid that most of that is incorrect.

For instance, the car needs a good hour or two to go fully to sleep, not just 10 minutes. Also, pulling fuses can cause other (sometimes seemingly random and unrelated) ECUs/modules to wake up and give false readings. The following video shows the best way of doing a parasitic current draw test:

Agreed. A much better way and one I have never seen before. You learn something new every day. :thumbup:
Most of the cars and motorcycles I have tested my way didn't have much in the way of onboard computers and I had no idea that the volt-drop tables existed for these fuses.