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Rx450h dead... suspect dead 12v battery

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We found our 2016 Rx was dead today, not enough juice to light up the dashboard, assume the 12v is dead... no warning (grrrr)

Trouble is, we cannot open the boot, to check the batter.... is there any way to open the boot from inside?

Also, is there any 12v charging points in the engine bay, so I can try to trickle some charge into it?

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Posted (edited)

I have the same issue with my GS300h. I don't think there is any way to open the boot until the Battery is charged. However, I have discovered that there should be a Positive terminal in the Fuse box  under the bonnet- but there isn't. If you can find it connect the Negative to an earthing point/bolt and jump start. It says that you should continue to receive the charge from the second motor for some time in order for the Battery to top-up.. The Battery fires up the computer, without which you can,t  start the engine. I'm looking for someone to tell me where the Terminal is for my car, as I haven't yet found it.

Edited by Walburghian

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there is a special jump/charge point within the engine bay - +ve within the fuse box which I think is located in the top right looking at engine bay from the front and a ground location on a metal part of the engine. This should be documented within the owners manual.

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I've looked 3 times for mine and it isn't in the Fuse box, which only contains fuses.

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8 minutes ago, Walburghian said:

I've looked 3 times for mine and it isn't in the Fuse box, which only contains fuses.

do you have a picture? Definitely there on the IS300h, open the fuse box, there is a red cover with + on it, open cover and you have a metal terminal.

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found the boot release lever... my wife was flexible enough to get in there from the back seats.... the Battery was down to 6v, so taken it out to charge on my bench

We drove the car a week ago, so this is a bit shocking it's lost so much charge in that short time 😞 .... I'll get it charged, but given it lost charge so quick, and the car is 4yr old, I'll order a new one just as piece of mind.

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8 minutes ago, ColinBarber said:

do you have a picture? Definitely there on the IS300h, open the fuse box, there is a red cover with + on it, open cover and you have a metal terminal.

I've seen pictures of a Lexus Fuse box with a Red plastic cover for the Terminal but there is absolutely nothing in the Main Fuse Box other than small fuses.

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one further question... once I have the Battery charged enough ... is there any special procedures I need to do once re-connected?

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One or other (perhaps even both) of the two items arrowed should be the positive terminal - it just looks like the red cover is missing. If it's not marked as such, check with a multimeter. If you haven't got a multimeter then just a 12V bulb will do as a test lamp.


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There is definitely a special jump point in the main fuse box on a 4th generation RX450h.This is on the nearside of the engine bay. 

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28 minutes ago, olliesgrandad said:

There is definitely a special jump point in the main fuse box on a 4th generation RX450h.This is on the nearside of the engine bay. 

And on the nearside of my 17 year old MB too !

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Realy surprised there is no key slot tucked up where the reg plate sits i thought i was locked out of my boot but i found lock beside one of the reg plate lights

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Oh just seen ur car year these are the power operated bootlids so no manual key function

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popped the Battery back in after a quick charge and seems to be ok .  Car feeds 14v to it, and when ignition is off, it seems to be holding around 12.3, suggesting it is a bit old.

I couldn't see any internal lights left on, so assume it must be an ageing Battery and a week of no driving pushed it over the edge. 

My wife did say she's noticed the engine always fires when she first starts is ... where a couple of years ago it was always silent until we pulled away.... that might be the telling sign of an old Battery.. just a shame it doesnt throw a warning message to replace before it lets you down.

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On a related point, on a 4th gen 450h, I have the red cover over the positive as shown above, together with an uncovered similar point, which is presumably the negative - Can these points be used for trickle charging when the car is laid up? It is a right pain to have the boot partially open to have a trickle charge feed direct to the Battery.

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35 minutes ago, luckyduck said:

On a related point, on a 4th gen 450h, I have the red cover over the positive as shown above, together with an uncovered similar point, which is presumably the negative - Can these points be used for trickle charging when the car is laid up? It is a right pain to have the boot partially open to have a trickle charge feed direct to the battery.

Yes, I believe it is a direct connection to the Battery. Use any metal part in the engine bay for the -ve.

Double check by connecting your charger and then a meter on the Battery to make sure you see an increase in voltage.

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If your Battery was down to 6 volts then i fear that it may be beyond redemption and that new Battery may be required

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Yes, I decided the same... it is an AGM, so can take low charge levels, but after a re-charge it didn't hold it, so bitten the bullet and ordered a new one.  

Until that arrives,  I'm starting the car daily which is keeping it breathing until then. 

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AGM batteries can operate in partial state of charge but what kills them is being left very flat or on a small parasitic load. As soon as you try to recharge them from this state, lead metal is plated out onto the surfaces of the glass fibre mat separator. This causes a high resistance short internally in the Battery and stops the Battery holding a charge as normal.

From the description above, this is what has happened.

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A situation many motorists are finding due to infrequent use at present.  I am one of them but by using smart charger once a week am able to soldier on for a time until I fit a replacement, shortly.  

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conclusion to this....   purchased new genuine battery  from Lexus parts direct ... about £200 with delivery.. good service as before.  Car now ok.

lessons learnt for me

- the car will start with about 9.5v on the Battery (in my case that was 2 days after re-charge.. (dead one for sure, needed replacement)
- only early sign of failing 12v Battery is the engine immediately firing up after car is started (ready) ... with new Battery it didn't fire until we put it into gear
- despite some good research by Barry, I couldn't find any alternative which gave me reassurance it wold fit and be safe ... so purchased genuine.

 

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