Thanks to Andy for the reassurance - I’m not worried so far, no problems. But we lack data on how extensive the problem is and what kind of use leads to battery failure.

Following a contribution above, apparently the only factor determining the level of charge is the time spent in ‘ready’ mode. The type of driving and environmental conditions are not relevant. That makes the calculations very easy. 

I have had the car 18 days and it has been driven just over 16 hours, with maybe another hour or so parked in ‘ready’ mode. In that time, the battery has gone from a charge of 12.8v to 12.2v. This means an average of about one hour per day in ‘ready’ mode is not enough to maintain the battery. It is, however, working normally at 12.2 although some have advised that is too low.

A trip of two hours plus twenty minutes in ‘ready’ two days ago did not shift the charge up from 12.2v. It was still reading 12.2 after the return trip which was broken into shorter segments. So four-five hours in ‘ready’ in a single day was not enough to increase the charge. I do wonder if the issue is about how effectively the battery gets charged rather than the battery itself.

In conclusion, I have not yet found what it takes in terms of normal use to increase the charge back to 12.8 or to maintain it at level. I will report here if I manage to work that out.

I hope this info helps someone, but if not please just tell me to shut up!

So I discovered something interesting today regarding charging the 12v battery via the hybrid battery. After my abortive attempt this morning to put the car into remote climatisation via the Lexus App in order to put some charge into the 12v battery (see my other post this am), I decided I’d have to go ‘old school 🤣’ and open up the car to do a manual recharge in ‘Ready’ mode. 

So at 10:00ish ‘Ready’ mode was initiated manually and as the battery was still charging @14.2v after 2hrs, I decided to continue to let it charge to see what happened. After 4.5hrs charging at 14.2v, the volts then dropped from 14.2v to 13.55v and continued at that level until I decided it had enough and I’d had enough!😂 So I believe the hybrid battery charging algorithm had now decided the battery was fully charged and activated a 13.55v ‘maintenance’ voltage, IMO a more sensible ‘maintenance’ voltage than the 12.8v I would get if the car was being driven. After closing the car up and waiting a couple of hours or so, I’m getting 12.88v settled voltage, which I would consider to be a fully charged state and pretty close to what I’d get after fully charging with the mains-powered Ctek smart charger. Comparing that to the 270 mile trip up to where I currently am situated, when the charging voltage dropped from 14.4v to 12.8v after 1.5hrs and never rose above that value (apart from a very short time following rest stops, see graph). Consequently, at the end of the journey, I ended up with battery volts around the 12.5v mark. So why can’t the car emulate the charging algorithm it uses in static ‘Ready’ mode to fully charge the 12v battery when the car is being driven? I’m pretty sure if I can’t get the battery fully charged after a  270 mile journey, no amount of driving is ever going to fully charge it! Incidentally, despite frequent visits to check on the car, I only saw the petrol engine kicking in once. I did have both the climatisation and the radio off though.

46 minutes ago, Emjay2 said:

Comparing that to the 270 mile trip up to where I currently am situated, when the charging voltage dropped from 14.4v to 12.8v after 1.5hrs and never rose above that value (apart from a very short time following rest stops, see graph). Consequently, at the end of the journey, I ended up with battery volts around the 12.5v mark. So why can’t the car emulate the charging algorithm it uses in static ‘Ready’ mode to fully charge the 12v battery when the car is being driven?

The actual voltage depends if the vehicle is performing basic battery charge control or constant voltage control and isn't just determined by the 12 v battery's SoC - the battery's temperate and the temperature of the hybrid inverter system will also be taken into account. It would have been interesting to know what voltage the system was at if you had left the vehicle for 30 mins to cool before driving again - it may well have continued to charge the battery. It will drop the voltage when it can to improve economy.

The vehicle will also perform a constant voltage charge every time the vehicle has passed a 20 hour cumulative driving time to remove effects of sulfation - all this makes it difficult to determine what is going on by just comparing a couple of reading.

The following is part of the system control description for the older RX series IV, but other hybrids work in a similar way.

 

15 hours ago, ColinBarber said:

It will drop the voltage when it can to improve economy.

Thanks for that Colin, it’s good to know there’s some logic behind the charging strategy, however, I for one, would gladly trade some loss in fuel economy for a better 12v SoC after a car trip. Unless he/she is going to be driving the car most days, I suspect Joe Public would as well.

Hit a new low:

Car still started though!

 

And today I went for the record:

Car started immediately.

It's good to know that the car starts with such a low voltage......however I would worry that the battery is going to be permanently damaged....

Anyone out there experienced this?

Six month old LBX.

4 days after a 20 mile daylight journey, wife went shopping.

Got in, closed the door, then Nothing.

Doors had locked, all windows stayed shut. A clear full-sunny day so the cabin started to heat up like a greenhouse. No phone to call for help.

Imagine the panic that set in.

20 minutes later I happened across her still on the drive in a panic.

Phoned the dealership and was talked through the emergency exit procedure to get her out into fresh air.

Flat battery ..but it let her in Then locked up.

Never has she had a car that would do this. And, understandably, she will never drive it again, knowing this can happen. 'Drive it more, buy a charger' the dealer says. Are they for real? Sell you a 40k plus car that needs coaxing like an old worn-out Fiesta?

At a great loss we are selling it and moving on.

 

You've had a sorry experience with the LBX, but another reminder to owner's to make themselves familiar with the handbook?

Perhaps you should have followed your wife's desired action back in October!

What was the resolution to the key failure?

 

On 12/22/2024 at 4:16 PM, Heuer said:

Hit a new low:

Car still started though!

 

I'm afraid this is not concusive, since you are showing the battery after charge period and after locking (typical pattern) for some time. Please show the voltage before unlocking/entering the car and with smart key off.

BTW: Even if a battery is replaced for a new one, it does not mean that it has full capability (due to storage time, assy time, etc.) I noticed that, but after connecting to a smart charger for 24h I noticed evident performance improvement.

On 3/5/2025 at 5:43 PM, 1957Phill said:

Anyone out there experienced this?

Six month old LBX.

4 days after a 20 mile daylight journey, wife went shopping.

Got in, closed the door, then Nothing.

Doors had locked, all windows stayed shut. A clear full-sunny day so the cabin started to heat up like a greenhouse. No phone to call for help.

Imagine the panic that set in.

20 minutes later I happened across her still on the drive in a panic.

Phoned the dealership and was talked through the emergency exit procedure to get her out into fresh air.

Flat battery ..but it let her in Then locked up.

Never has she had a car that would do this. And, understandably, she will never drive it again, knowing this can happen. 'Drive it more, buy a charger' the dealer says. Are they for real? Sell you a 40k plus car that needs coaxing like an old worn-out Fiesta?

At a great loss we are selling it and moving on.

 

This is partly down to the dealer not briefing you properly.   These electronic door handles are extremely safe and can easily be over ridden if the battery goes flat but only if you’ve been told what to do.  It’s a shame because all these troublesome cars with battery problems are fixed with a £75 battery and you can get on with enjoying a great little car.  

I too found that my brand new Takumi design went flat within 4 days if not driven.

For a new, expensive car this is hopeless. 'Just use it more, buy a charger' etc is an insult.

Why do we have to compensate for a weakness in this vehicle when so many other makes can be left for a couple of weeks, maybe at an airport, and still be started without a rescue call-out.

I sold it today for a £9,000 loss and am glad to see the back of it.

Next stop is to research a reliable car and forget this fiasco.

You could have fixed the problem for £75 (new Yuasa 45Ah battery) and still be £8,925 ahead. Round it down to £8,900 if you bought a trickle charger.

All hybrid's and EV's suffer this problem if not driven as they all have a small 12v battery for initiating the start sequence which is charged from the traction battery. Tesla's have a problem where even if you leave the car plugged in to a charger the 12v battery will go flat because once the traction battery is full charging for both is stopped. It takes about an hour of dismantling to get at a Tesla 12v battery to be able to recharge it once it goes flat. 

If you are a low mileage rural user go for an ICE powered car. The LBX was really meant to be an urban runabout and Lexus figured it would spend a lot of its time in stationary traffic at lights or in congestion which is when the 12v battery gets maximum charge.

Phill,

Great pity your Dealer failed you when he sold you a Hybrid------was it Derby ?

Tel

Battery care Leaflet lexus toyota.pdf

Not sure that this is in common circulation.  Even less clear to me is why it's not given at time of sale!  

Now to source a suitable solar panel for the type of battery!

I always thought once the system was in ready mode all the 12v systems were powered by the inverter, seems not.

2 hours ago, Beep said:

I always thought once the system was in ready mode all the 12v systems were powered by the inverter, seems not.

My understanding is that in Ready mode all the 12V systems are still powered from the 12V battery - the difference in Ready mode is that the DC-DC convertor now charges the 12V battery as needed from the hybrid battery, which will be at a rate greater than the 12V systems can run down the 12V battery. If the hybrid battery starts to get low the ICE then charges the hybrid battery, which us turn is providing power to the 12V battery. IMHO that leaflet does a poor job of explaining the interaction between the hybrid and 12V batteries.

39 minutes ago, wharfhouse said:

My understanding is that in Ready mode all the 12V systems are still powered from the 12V battery - the difference in Ready mode is that the DC-DC convertor now charges the 12V battery as needed from the hybrid battery, which will be at a rate greater than the 12V systems can run down the 12V battery. If the hybrid battery starts to get low the ICE then charges the hybrid battery, which us turn is providing power to the 12V battery. IMHO that leaflet does a poor job of explaining the interaction between the hybrid and 12V batteries.

I don’t think that’s the case it’s from the inverter, on you tube there are videos looking at the current flows around the system and the inverter amps will fluctuate due to lights being on etc, the battery plays little part.  Even if you look at the bm200 results it’s shows as the car is driven and the inverter is loaded, the battery charging voltage drops because the management system deprioritizes battery charging.  This leaflet is confusing.

12 minutes ago, Beep said:

I don’t think that’s the case it’s from the inverter, on you tube there are videos looking at the current flows around the system and the inverter amps will fluctuate due to lights being on etc, the battery plays little part.  Even if you look at the bm200 results it’s shows as the car is driven and the inverter is loaded, the battery charging voltage drops because the management system deprioritizes battery charging.  This leaflet is confusing.

You could be right - I thought I'd seen it described as I wrote somewhere but there is often a lot of misinformation flying around! We are agreed on the leaflet being confusing though 🤣

'12-volt battery' is mentioned 169 times in the 676 page manual.

Most of it refers to making sure accessories are not left on because of the risk of discharge and critical things suddenly not working.

8 hours ago, Glitch33 said:

'12-volt battery' is mentioned 169 times in the 676 page manual.

Most of it refers to making sure accessories are not left on because of the risk of discharge and critical things suddenly not working.

Ah many thanks. I was sat in the car with the manual in what turned out to be only a 30 min traffic jam where I just sat worrying about the battery and whether or not I would be ok to move off.  I turned all off stayed in ready mode but still watched as the battery level fell.  I days past I would have turned the engine off and just listened to the radio no worries. 

15 hours ago, Glitch33 said:

'12-volt battery' is mentioned 169 times in the 676 page manual.

Most of it refers to making sure accessories are not left on because of the risk of discharge and critical things suddenly not working.

I went looking and it seems there are 479 mentions but I have a life and from a quick look of some of those 676 pages, the att seems to be far more helpful - you will note its a Toyota not Lexus leaflet.  Lexus are seemingly in denial!  I would have thought that advice on how a hybrid works, limitations etc need to happen at the start of the process and not after a breakdown as was my experience.  I sat in a traffic jam wondering should I keep the car in ready mode and if so will the petrol last, is it now too late to turn the car off and then hopefully get it started again when the traffic eventually moves.  Not what I would have ever expected on a new £38k an 'Award' winning car as they publicity boasted. 

Battery care Leaflet lexus toyota.pdf

On 5/28/2025 at 7:36 AM, Stevelo said:

Ah many thanks. I was sat in the car with the manual in what turned out to be only a 30 min traffic jam where I just sat worrying about the battery and whether or not I would be ok to move off.  I turned all off stayed in ready mode but still watched as the battery level fell.  I days past I would have turned the engine off and just listened to the radio no worries. 

The Hybrid traction battery will charge the 12-volt battery when you're in Ready Mode so sitting in traffic with radio on shouldn't be a problem. 

The leaflet recommends sitting in the car for an hour each week in Ready Mode to keep the 12-volt battery topped up. That sounds ridiculous.

I will buy a solar panel trickle charger, plus a battery booster for emergencies. That cuts into the fuel cost savings. Still ridiculous to have to buy these but people say owning a hybrid is very different.

My car is supposed to have an upgraded battery so maybe I'll never have a problem. Only a couple more weeks to wait.

I wonder how low a 12v battery charge can go and the car still start without problems. I've seen my car start without any problems with the battery at 11.5v. So it was interesting to see the figures in the graphics below, with a car starting at 11.45v. This voltage only has to start the car's computer - I doubt it could start a conventional car with the need to turn its starter motor. 

But the state of charge figures don't make much sense. The first says 11.91% charge is 1% state of charge and the second says 11.45v is 0%. These percentages can't be accurate.

On 12/22/2024 at 3:16 PM, Heuer said:

Hit a new low:

Car still started though!

 

On 12/23/2024 at 4:31 PM, Heuer said:

And today I went for the record:

Car started immediately.

It's hard to find definitive information about AGM batteries as opinions seem to very a lot but the consistent two points I've come across are that you risk damaging the battery if you overcharge it; and secondly that AGM batteries tolerate deep discharge without damage. Maybe the car's charging algorithm aims to keep the charge below fully charged to extend the battery life. The hybrid battery is kept in a range of 40-80% and most often around 50-60%. Maybe the computer is doing something similar with the 12v battery in order to prolong its life.

I've seen charts of what percentage state of charge is represented by a given voltage on an AGM battery. (I think it was supplied by Herbie.) But I've now come across a different chart at this link. This page says that 11.9v would be 50%, which sounds perfectly adequate just to start the car's computer. Even the lower figure of 11.45v is put at around 25%, which the chart describes as "critical". But if it starts the car, the battery is immediately being recharged.

Has anyone managed to start the car at a lower voltage?

 

On 10/27/2024 at 5:01 PM, Herbie said:

Sorry Elaine, I've just re-read your post and realised that you have indeed already bought the battery monitor. Now that you know from the user manual just how little power it takes itself (1.0mA or to put it another way, 0.01 of an Amp) you can go back and have it fitted. Used in conjunction with the chart I hope you'll have much more confidence in the car.

Herb i use my DB2 to Check the State of my 12v Battery Daily and The Mean level is 12 ..In The three years i have had the Car I,ve Never had a Problem with the Battery..Certain Days it can Register 11.8...No problems Touch Wood..!!!!