How to do battery Amp draw test?

[Natty Bumppo]

On my 2021 ES350 with 2GR FKS V6 how to do a battery amp draw test without loosing any ECU settings? I do not have another source like OBD II to plug into car to keep 12v while doing this Amp draw test.

I have done this Amp draw test many times on other less high tech cars where settings were maintained by simply disconnecting Batt Neg cable and hooking Amp meter from Neg Batt post in series to disconnected Neg cable and reading meter looking for parasite Amp draw  and fix as needed.    

My car has a OEM 2 part Neg cable that can be easily be disconnected a few inches from neg post clamp and leaves the post connector clamp part on the battery post and the other part of the Neg cable free. 

Question - can I do following steps with my high tech electric car and not loose ECU settings ?

1) Car shut down, all accessories off, doors closed, all windows up, glove box closed and hood up. Some Amp draw expected with hood up since there is a dash light on that shows hood up that eventually goes out but probably some Amp draw still there - do not know how to defeat if Amps there?

Car not security locked with key fob and with all above done there is still a red blinking alarm light on dash that never goes out - what is Amp draw of that and how to shut off if there?  

2) Place a short 14 gauge jumper wire on post part of Neg cable and attach to the other part to the Neg cable to be separated,  then disconnect the Neg cable at the separation point a few inches from the post part. So 12v is still in in the system. 

Not going to start engine.

3) Connect my Amp meter probe to the Neg Battery post part of cable and connect in series to the other part of the disconnected Neg cable.

4) Read Amp draw amount as system settles down to sleep mode, hopefully around 50 milliamps or less? 

Appreciate your comments

Thank you all

Pete

 

 

 

 

[RX400h]

I have to ask: Isn't your car under warranty? If there is a parasitic battery drain, couldn't you have a dealership tech perform a drain test?

I ran your proposed scenario by CoPilot and here's how it replied:

Quote

Here are a few additional tips to ensure you don't lose any ECU settings:

1. Hood Light: To avoid the hood light causing an amp draw, you can disconnect the hood light connector or cover it with tape to prevent it from turning on.

2. Red Blinking Alarm Light: The red blinking alarm light might be drawing power as well. Unfortunately, there isn't a straightforward way to shut it off without affecting the car's security system. You might need to consult your owner's manual or a professional mechanic for specific instructions on disabling it temporarily.

3. Performing the Test: Your steps seem correct. Just ensure you're using a reliable amp meter and follow the steps carefully:

   • Step 1: Ensure the car is shut down, all accessories off, doors closed, windows up, and glove box closed.

   • Step 2: Use a short 14-gauge jumper wire to connect the post part of the negative cable to the other part of the disconnected negative cable.

   • Step 3: Disconnect the negative cable at the separation point a few inches from the post part.

   • Step 4: Connect your amp meter probe to the negative battery post part of the cable and connect in series to the other part of the disconnected negative cable.

   • Step 5: Read the amp draw amount as the system settles down to sleep mode.

If you're still concerned about losing ECU settings, you might consider using a battery maintainer or tender to keep the battery connected while performing the test. This way, you can avoid any potential issues with the ECU.

 

[Natty Bumppo]

Dave thanks for the response and confirming my steps to use

Yes the car is still under Basic and Powertrain warranty. I was at Lexus a few days ago on 12/4/2024 for a battery voltage drop issue I noticed over several days parked and asked to check the original battery that is 39 months old. They concluded the battery was defective after a detailed load test. They replaced the battery under Basic warranty since car in service on Sep 4, 2021 and at this service with 22,248 miles and less than 4 year 50k miles on the OEM battery. 

The original battery was replaced with a new one with 780 CCA (the OEN had 603 CCA). They checked the electric system and other things and said all OK. At the time I asked if they did a parasite amp draw test and the answer was no. They suggested to use the car more, it is not a daily driver.    

The reason doing the Amp draw test is to determine what the value is. I tested the new battery fully charged at around 12.8 volts, all closed up and not security locked with key, hood up and no light under hood (that I can see) and the flashing red alarm light on dash operating, there is voltage drop from 12.8 to 12.6 volts over 48 hours not used.

Is that a reasonable voltage drop for this 2021 ES350 with premium option that has additional electric items?   

It could be that the alarm light on dash and hood up are a part of that voltage drop and the other part is normal battery internal chemical voltage drop reaction. 

I have a very good VOM and in next few days I will do the Amp draw test and report results. 

 

 

 

[RX400h]

Yes, the current draw test is typically used to see if there is a parasitic draw. 

For a 2021 Lexus ES350, the typical battery current draw when the vehicle is off (also known as parasitic drain) is usually around 30-40 milliamps (mA). This small amount of current is used to power essential systems like the clock, security system, and memory functions for things like radio presets and seat positions.