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Hi again everyone. Hope you had a good Christmas and that the new year is good, too.

I have an issue with the (auxillary) 12 volt Battery going flat when the car is standing. It's a nearly new Battery, bit I am aware that a new Battery can be faulty.

I have fitted a Battery isolator switch for when the car is left standing, which resolves things (in a rather unsatisfactory way!).

I detect no parasitic drain on the Battery when the car is resting. I haven't tested it yet with the alarm turned on, though... Which might be tricky to do, but I'm probably going to have to give it a bash.

What I have measured is the voltage of the Battery (12.5v), then left the car for a day with the Battery connected and the alarm on, and measured it again... 12.25v. both measurements taken with the Battery disconnected. According to a handy table I've found, this represents an estimated reduction in charge state from 90% to 50% in just 24 hours😵. Given that it's a 45Ah Battery this is equivalent to a 750mA drain, assuming my calculations are correct.

My hunch is a problematic alarm or a knackered Battery. I will see if I can establish drain again tomorrow, but with the alarm enabled.

Any other car, I would have fitted a more macho Battery, but Lexus haven't provided enough space for one (on the basis that we don't need cranking amps on a hybrid).

Any advice welcome.

 

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Have the same problem with my 4year old RC300h-F Black edition. The 12v Battery is a constant problem. Two solutions  I keep a lithium jump charger in the boot, very light and small, will start the cart no problem as there only needs to be a 12volt current.

open the bonnet, remove the fuse cover and flip open the red protector and connect the red terminal,  clip the neutral to any metal part . Saves messing about in the boot.

Once car is running leave it in the ready state for about an hour and the hybrid Battery will recharge the auxiliary one . The engine will kick in occasionally to keep the hybrid Battery topped up.

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2 minutes ago, derektilley said:

Have the same problem with my 4year old RC300h-F Black edition. The 12v Battery is a constant problem. Two solutions  I keep a lithium jump charger in the boot, very light and small, will start the cart no problem as there only needs to be a 12volt current.

open the bonnet, remove the fuse cover and flip open the red protector and connect the red terminal,  clip the neutral to any metal part . Saves messing about in the boot.

Once car is running leave it in the ready state for about an hour and the hybrid battery will recharge the auxiliary one . The engine will kick in occasionally to keep the hybrid battery topped up.

That is an extremely informative and welcome first posting Derek.

A warm welcome to you and how come you are so well informed?😉😉

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On 12/31/2021 at 3:55 PM, Farnham Dave said:

Any advice welcome

One thing to try would be to use the alarm but turn off the ultrasonic interior sensors in case that part is causing excessive drain.

Even with the alarm fully armed, once the vehicle's electronics have gone to sleep (typically up to 15 mins after closing/locking/arming the vehicle) you shouldn't see a power drain of more than 70 mA, and ideally less than 50 mA.

 

I wonder if the Battery inside the anti-theft siren has gone bad and therefore it is always being charged/consuming power.

I would have thought that it would always be charged, and maybe it is when the ignition is on, but when the ignition is off perhaps it only charges when the alarm is armed. Sadly the wiring diagrams/system descriptions don't go into that level of detail to know - the only way to know would be to take measurements.

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