Flat battery

[johnatg]

21 hours ago, Willex said:

The problem with my Lexus battery was sorted by a Lexus Centre yesterday and the problem was a dead cell. It was explained to me that the ‘starting’ battery has only that function, starting the engine,  - after that the computer and other batteries take over the management for running the car .... this includes charging the main ‘starting’ battery.

Lexus uk stated that they had posted information on their blog as to how to ensure that the problem of risking a flat battery could be eliminated by starting the car and letting it run for about an hour in a stationary position. Unfortunately, unless you went on to their blog then you would have been none the wiser, since no general information was received, - well not by me anyhow.

The Lexus centre have been magnificent, whilst Lexus Uk were not really helpful.

 

Just to be clear on the functions of the 12v Battery vs the hybrid Battery - as I understand it the 12v Battery is normally only used to switch on the computers - the actual starting power to turn the starter motor comes from the hybrid Battery unless the hybrid Battery is out of charge completely in which case the 12v Battery is used as the whole power source.

Some people have suggested on here that as the 12v Battery is not used for serious power when starting you can use some sort of small Battery - but that would not be wise in case it is needed for starter power.

As a general rule, It is never a good idea to run any car engine for a short time - if you start the car, drive it for a few miles at least unless you just need to shunt it out of the way on your drive or whatever. But try not to do that too often before going for a proper drive.

[Herbie]

I'm afraid I have to slightly disagree with John above.

The 12V Battery is used to power up the computers and pressurise the brakes, after which the traction Battery provides power for all the car's requirements, including charging the 12V Battery, just like an alternator would do in a conventional car.

If the traction Battery is flat, there is no way the 12V Battery can do anything about it. You have to call out the dealer or someone else who has the specialist knowledge and equipment to deal with it.

It's been quite a while since I looked so the following may be incorrect, but I seem to remember looking at the discharge curves for a pack of 8 AA batteries and if I remember correctly then I think they could supply 12V 28A for just under one minute. If my recollection is correct, then in theory at least, that would be enough to get the car into the READY state because my RX450h takes less than 20A to do that.

The 12V Battery can never be used for starting power as far as I know. This is because the hybrid system uses MG1 to spin the petrol engine up to 1000rpm before applying fuel and a spark to fire it.

[Steven Lockey]

@Herbie I think you are correct in normal operating conditions, but at least for certain cars, I think they can also start the engine off the normal Battery if it can't draw the power from the hybrid Battery. I noticed when my hybrid Battery was dodgy on my GS450h, it had issues starting when the 12V Battery was low which suggests it was using the 12V Battery sometimes. 

Also been told you can drive the GS450h with no hybrid Battery inside at all suggesting it must be able to start without it but I don't know how true that is 😉 

I think the lights, stereo and some other low power components also draw directly from the 12V Battery when the car is in standby mode as I accidentally left them running when I went for a walk in the forest, when I came back the 12V Battery was flat but the hybrid Battery had plenty of charge left. Just leaving the car turned off for 20 minutes seemed to charge the 12V from the hybrid Battery (volt-meter readings corrected) and the power level on the hybrid had decreased afterwards.

Everything worked fine since so that's the only thing that makes sense to me 😉  Seems a bit odd, but not the first odd thing I've found in Lexus world! Very nice cars but some very odd quirks!

[Herbie]

49 minutes ago, Steven Lockey said:

I think you are correct in normal operating conditions, but at least for certain cars, I think they can also start the engine off the normal battery if it can't draw the power from the hybrid battery. I noticed when my hybrid battery was dodgy on my GS450h, it had issues starting when the 12V battery was low which suggests it was using the 12V battery sometimes. 

I stand to be corrected if I'm wrong but.....

MG1 (Motor/Generator 1) is used to start the petrol engine and is a permanent magnet AC motor but as we know, the 12V Battery is DC so there has to be some conversion taking place and it's the hybrid system itself that does this.

If I remember correctly, a boost converter is used to provide 500V DC to the inverters for MG1 and that boost converter is fed by the traction Battery, so no traction Battery means no engine start.

49 minutes ago, Steven Lockey said:

I think the lights, stereo and some other low power components also draw directly from the 12V battery when the car is in standby mode as I accidentally left them running when I went for a walk in the forest, when I came back the 12V battery was flat but the hybrid battery had plenty of charge left.

That's correct and that's the reason why the car should be in READY mode rather than standby when you're sat in it listening to the radio or whatever.

If it's in standby then the computers will monitor the voltage and shut systems down to conserve energy and avoid that flat bettery. If the car is in READY mode, the petrol engine will fire up every time the voltage drops to a certain level so as to keep the Battery charged.

[NemesisUK]

This is for my RC300h but I would imagine it's the same for other Lexus hybrids.

 

HYBRID CONTROL SYSTEM CONTROL CONVERTER.pdf HYBRID CONTROL SYSTEM CONTROL INVERTER.pdf CHARGING SYSTEM SYSTEM DIAGRAM.pdf

[Thackeray]

Really useful documents from NemesisUK. Thanks for those.

7 hours ago, johnatg said:

the actual starting power to turn the starter motor comes from the hybrid battery unless the hybrid battery is out of charge completely in which case the 12v battery is used as the whole power source.

As usual, Herbie has come to the rescue with some clarification about how the electrical side of the car works. I have just a couple of other comments.

First of all, the traction Battery never runs down in normal everyday use. Its lower limit of charge (on my IS300h at least) is 40%. If the Battery declines to 40% while in Ready mode the engine starts. This usually shows as two bars on the dashboard energy meter. The engine runs until the Battery has 50% (three bars) and then it stops and the process repeats ad infinitum (or until the petrol runs out). So the car designers have built in this 40% reserve that will always be there if the car is not faulty. This means there will always be power for MG1 to start the engine, even if it declines a bit further to 37-38%. There's no need for a further backup from the 12v Battery.

When starting the car, MG1 uses a 650 volt supply. No doubt that might be possible from a 12 volt Battery but it sounds like a big demand on even a big 12 volt Battery.

A typical starter motor of 2 kw or so draws around 300 amps of power for a few seconds. But in the hybrid system the 12v circuit is limited to around 100 amps, which could give you a.maximum of 1.2 kw - unlikely to be enough to turn the engine slowly let alone at the higher speeds the hybrid system uses.

Conventional cars can get started with just a few slow turns of the engine and just a few initial sparks. By contrast the hybrid engine is set up not to spark until it's turning at around 1,000 rpm. This means you get a smooth unstressed start, unlike the typical jerky start in a conventional car. MG1 has around 40 or so kw of power and spins at around 4,000 rpm to turn the engine at 1,000 rpm - I might be wrong but it doesn't seem likely that 1.2kw of power from the 12v circuit would be enough to do that.

3 hours ago, Steven Lockey said:

Also been told you can drive the GS450h with no hybrid battery inside at all suggesting it must be able to start without it but I don't know how true that is

Not quite true, I think, as when the engine is not running you need the Battery either to start the car moving by supplying power to MG2 (which is linked directly to the road wheels) or to provide power to MG1 to start the engine when needed either before or after you start moving.

Nonetheless, it was interesting to see in the circuit diagram from NemesisUK that MG1 feeds MG2 directly. This is what I thought was the case (and mentioned in another recent thread). It means that the Battery is not necessary to keep the car going once it's moving. Purely hypothetically, (as I said in the other thread!) this means the car is entirely self-sufficient as regards electricity once it's moving and not dependent on the Battery except to get started. But in practical terms, you can't use the car unless it has the traction Battery as it can't get moving without it.

[Herbie]

I think you're right with your calculations William (@Thackeray) but it's a bit simpler than that.

There's no actual path for the 12V Battery to have any effect on MG1 and/or the petrol engine at all.

The voltage of the traction Battery varies by model and from the documents provided by Peter above, we know that the RC300h is nominally 230V, so we'll stick with that for the purposes of this discussion.

As you said above William, MG1 is supplied with 650V, which is derived by feeding the 230V from the traction Battery into the inverter. In other words, to be able to get 650V out, we have to squirt 230V in. If we could somehow supply the inverter with 12V (which I don't think can be done anyway) then the inverter would either not do anything at all, or it would squirt out a greatly reduced voltage, nowhere near the 650V needed to spin MG1 and the petrol engine.

It would be like trying to light up a 230V lamp with a 1.5V AA Battery - it just wouldn't happen.

[ColinBarber]

You cannot use the 12v Battery to start a Toyota hybrid vehicle if the traction Battery goes completely flat. This is why the AA/RAC will recover your vehicle to a Lexus dealer if you have a flat traction Battery as they don't have the ability to get it running. Each dealer has a THS charger, you can purchase one yourself but they are around £5k + the required lead for the vehicle in question.

The DC-DC converter that generates the vehicle's 12v system from the traction battery once in Ready state is made from solid state electronics designed to operate at specific voltages. It doesn't work in reverse, nor does it work at lower input voltage which then affects the output voltage. It cannot be thought of as a passive AC step down transformer which could be wired in reverse to step-up.

[Thackeray]

14 hours ago, Herbie said:

It would be like trying to light up a 230V lamp with a 1.5V AA battery - it just wouldn't happen

Nice analogy!

[royoftherovers]

2 hours ago, Thackeray said:

Nice analogy!

I`d be willing to bet that Dumbledore could light up a 230v lamp with a 1.5 AA Battery !