How long does your battery last when the car is not used?
With a modern car, there is, of course, a parasitic current draw on the battery even when everything is turned off and the car is just sitting there. Hence, the battery connected to an unused car will discharge much faster than a disconnected battery. The question is, how much faster.

On my 2020 RX (non-hybrid), a battery tester shows the battery going from 98% charge to 68% charge in just three days. This indicates likely failure in about a week. This seems unreasonable. What experience do people have with this?
Has anyone measured the parasitic current draw? I believe it's supposed to be no more than about 40mA.

 

That's a problem -- I'd make a warranty claim, ASAP.

 

If you have LED interior lights, be careful they turn on at the slightest touch, you may have one on somewhere you cannot see.

 

Don't ask the dealer, they'll tell you it's normal, and then sell you a battery.

 

They don't turn off after a set time? Even if not, isn't the draw from LEDs pretty minimal?

 

Some do turn off, like the one by the rear view mirror it turns off when you lock the vehicle UNLESS you touch it to turn it on and it takes but a thought to do that.
It happened to me when I put sun glasses in the holder. I don't know about the trunk but I think those have a switch.
You're right the draw is minimal, but I don't know what it is specifically. With the OP there must be more than just a parasitic draw for that discharge/time.

 

I thought about an LED being on but looked carefully after dark and didn't see any light.
BTW, in my 6 months service, I complained about the battery. They said my battery tested bad and replaced it under warranty. However, I am now measuring the discharge with the new battery. Hence, the previous battery was probably damaged by being discharged too much.
I am trying to gather testimony from other owners about how long their battery lasts when the car is unused since I expect the dealer to tell me that "this is normal" and "you need to drive more". I expect that some people have left their RX unused for 6-8 weeks without a problem. If you have any relevant experience, please let me know.
Thanks!

 

Your dealer should have done a whole check of the charging system, if he has not my advice is go to a competent indie mechanic that specializes in electrics, pay him do a check (I know it hurts) but it would not be more than 1/2hr labour. Then based on this I'd decide how to proceed. If you dealer has not done a total electrical test Go to another dealer you car is under warranty and any dealer can fix it.

 

The charging system seems to work fine -- the battery state of charge increases significantly when I drive even short distances (e.g., ~12 miles got it from 55% to 91%). I think getting some evidence from other owners would help with discussions with the dealer. In any case, I am planning to try another dealer and, if necessary, an independent. I have also ordered a clamp DC ammeter with which I may be able to measure the parasitic draw.

 

I also own an ES and in that forum, there are several complaints about bad batteries on only the 2020 models. Perhaps issue is related? Although you say yours was already replaced. Maybe with the same brand/batch? Guessing at this point.

 

IMO, the ONLY way to find your problem is to do a parasitic load test on your car. Basically they just disconnect the battery and put a special kind of ammeter between the battery cable and the terminal. Every car has some electrical drain and the amount is specified by the manufacturer. If yours exceeds it by a significant amount, that's your problem. Then the tech will know how to find the problem. Otherwise, you're just throwing parts at it, like a new battery.
I leave my RX in the garage for 2 weeks or so, when I take it out for a spin.

 

Yes, that seems to be the case. There is a relevant calculation that can be done. The battery I have now has a "reserve capacity" of 133. Assuming that you need 40% of capacity to start the car, this translates to about 33 AH that can be used before there may be starting problems. A parasitic draw of 50mA would be in the high-normal range for current models. This translates to about 27 days before I should have a problem. This draw should be about 2.2% capacity per day.

Does anyone have the Lexus spec for the expected parasitic drain current?

 

From Lexus Drivers web site:
The primary battery in your Lexus vehicle needs to remain fully charged to start and drive your vehicle. If not used regularly, it may reduce its performance and life. In the worst-case scenario, it can even leave you stranded. There are conditions including how long it is stored without being driven, as well as parasitic draw that can impact the battery’s available charge. Because of these factors, battery maintenance is required to ensure proper operation and optimal battery life. Under the current COVID-19 environment, some people may not be driving their vehicles for long periods of time. There are three methods of battery maintenance:

1. Driving the Vehicle/Starting the Engine This is the easiest method. The vehicle is used weekly after non-use. Driving the vehicle for 30 minutes with all accessories off for at least 10 miles above 30 MPH will maintain the battery’s charge. If you are unable to drive the vehicle, start and run the engine in an open-air environment for 30 minutes with all the accessories off.

2. External Battery Charging Connect an external battery charger (also known as a battery “tender” or “trickle charger”). This device hooks up to your car battery on one end and plugs into a wall outlet on the other. It delivers enough electrical power to prevent the battery from discharging. For instructions on how to safely connect a battery charger, contact your Lexus dealership. Your local Lexus dealer may have an external battery charger available for purchase at a nominal cost. Alternately, you can purchase an automotive battery charger online.

3. Disconnect the Battery Cable Parasitic draw or parasitic drain is a term for an electrical component consuming electricity when the vehicle is not used, even after the vehicle has been shut off. The average vehicle can have as many as 50 computers that all use small amounts of electricity. Over time, this can cause the vehicle's electrical system to function improperly and the vehicle may exhibit the following symptom: a persistently dead battery.

To reduce parasitic drain on vehicles in storage for a week or longer, the negative (-) black colored battery cable may be disconnected to eliminate the battery discharge. Note, when the negative (-) battery cable is reconnected, you will need to check and reset electrical components such as the clock, radio settings, seat memory, etc.

 

Thanks. I actually found this document several weeks ago.
Unfortunately, it does not provide information about how long the battery can be expected to be usable if you start with a fully-charged battery. This is relevant not only for the current (COVID) situation, but also for the situation during normal times if you go out of town and want to know whether you can expect to come back to a working car. As I wrote in a previous message, a simple calculation, based on assumptions, indicates that the answer should be something on the order of 27 days. The actual spec *must be* there somewhere. Of course, that does not mean that it is easy for mortals to find it.

 

There may not be a published spec for the vehicle -- the parasitic drain may vary with the options packages. Possibly they just selected a battery that will keep the worst case options package able to start after sitting for a couple of weeks (when new), and that's it.