Stop start

[Mooly]

This is interesting (in general, not just this bit):

Toyota NR series engines.

Extract:

Quote

There are a few new features in starting system. Semi-automatic starting - it is enough to turn the key to START and release, after which the control system automatically keeps the starter on until the engine start. When the stop-start system is active, the control remembers cycle phase for each cylinder after engine stall, so at re-start the fuel and spark are supplied the cylinder which is able to join in the work immediately.

Charging system - with segment conductor alternators, 80-100A output, one-way clutch in pulley.
Continuously charging of battery is carried out during deceleration, but in steady-state mode cycles of charging and discharging of the battery are alternates for maximum efficiency. More complex control system required to use the battery temperature sensor and current sensor.

 

[tonybeau]

Well spotted, I wonder whether this applies to all in the range or just a selection.  It certainly looks complicated for us users but they must know what they are doing.

[Mooly]

I would think some variation of this is or has become standard...

Anyhow, as an electronics tech I got curious having read this and the comment by 'Honest John' (Daily Telegraph) and so I did a little test and observation today on what actually happens with my own car. I quickly made a test lead to allow connection of a DVM to the lighter socket... and as Mr Spock would say 'fascinating'.

Cold start having stood 24 hours.

1/ Key turned to 1st position (accessories) and a voltage of 12.40 was noted.

2/ Turning on ignition resulted in the voltage falling to 12.08. I was surprised that the voltage should dip so much with what is a relatively light loading but it was what it was. I operated the windows up and down a few times to significantly load the Battery but the voltage held at around 12.08.

3/ Starting resulted in a nominal 14.40 volts being generated. That is a fairly standard voltage and would allow the Battery to draw 'whatever it was capable of drawing' and would quickly bring the Battery to a high state of charge. The only limit to this current is the alternator capacity and the wiring resistance adding slight volt drop.

In other words a healthy battery could easily draw 20+ amps for a few moments and then see the current fall away.

A discharged but otherwise healthy battery could draw way over that value.

A deteriorated battery with high internal resistance would draw significantly less and consequently take longer to reach full charge.

4/ Now it gets interesting... after just a quarter mile or so the voltage fell back to around 12.5 volts. This shows the alternator is being actively controlled by the ECU and having its output voltage reduced. This lower voltage would not allow significant charging to occur.

5/ For the next 12 minutes or so of mixed driving the voltage was held at 12.5. Extra loading didn't alter things (such as heated window etc). 12.5 volts applied across the battery means that the alternator is supplying all the current demands needed, the battery essentially taking no current at this low voltage. The battery is never being discharged during this operation because maintaining the 12.5 volts prevents this.

It would appear this is done to aid economy and emissions. The less load on the alternator, the easier it is to turn mechanically and so reducing this loading to a bare minimum is beneficial on paper.

I noticed during this 'warm up' period that easing off the accelerator completely caused the voltage to rise again back to around 14.4 volts. So when decelerating the alternator can charge the battery and you get the bonus of slight 'engine braking' due to the alternator being harder to turn. Same at idle, 14.4 volts.

6/ After this 'warm up' period the alternator outputs approx. 14.4 volts continuously with no active control of the output. Again that makes sense. It would be assumed that the battery would now be of a high enough state of charge and so draws very little current. The alternator in this state is easy to turn and as before, supplies only the electrical demand required by the car itself.

There is on the battery negative terminal a little electronic device that actively monitors 24/7 current into and out of the battery, and this information can be used by the ECU to calculate the charge removed and the charge put back. So the ECU 'knows' when sufficient charge has been replaced in the battery during this 'warm up' phase and can then switch to the mode where the alternator output is varied.

Once past this phase, the battery is supplied with 14.4 volts constantly which would be a 'maintenance charge' to ensure it was fully charged.

(The above is all based on my interpretation of the observed behaviour)  

  

[tonybeau]

That certainly is fascinating and thanks for taking the trouble to do this.  This morning I tried to start the engine by briefly turning the key to start and releasing it straight away.  The technical bulletin referred to said that the system would keep turning the engine until it fired but that did not happen for me. 

[Mooly]

It was an interesting exercise for sure although I didn't try the starting procedure method.

This BMW document gives an idea as to what the Battery monitoring sensor can actually do:

Battery Monitoring

I'm still surprised my Battery dipped as low as it did before I actually started the car, and tbh if someone had just said their voltage was down to 12.08 volts with just the ignition on I would have been telling them their Battery was on the way out. Maybe its time to re-evaluate my thinking on all this. Stop/Start batteries are designed to be more 'cyclic' in their properties and to be able to withstand and be stored in a partial state of discharge and then to be able to be fully charged and then partial discharge and so on. 

  

[Mooly]

Ran the same test again today but this time gave the Battery a little workout for a couple of minutes by having the heated window on before starting. Voltage settled and held at 11.75 volts. Starting was instant, however this time there was no throttling of the alternator output, it remained on 14.4v for the duration, actually peaking at 14.55 on the motorway.

The stop/start failed to cut in at all today. Previous few days it has been pretty much every opportunity. Looks like the ECU 'knows' the Battery needs more recovery time and more charge input.

Charge acceptance of older batteries is an issue, they draw (the charge current taken) far less than a new Battery and also they tend to draw more (which seems conflicting)  'leakage' current. A new battery should see the current fall away very rapidly as it approached full charge.

This has got me wondering whether I should invest in a new battery at some point... at least before this one fails. 

 

[VSD]

Strange.....as I said, had the Battery tested at Toyota (1 day). 

Battery ok they said.

Since then.....stop start worked only once. Test effected over 1 month ago....

[tonybeau]

My Battery should be ok as well, the car is only 2 years old.

[Mooly]

It is a strange one for sure.

Today I just started and drove with no prior discharge apart from actually getting in and taking around 30 seconds to get settled. Alternator maintained 14.5 volts throughout, Stop/Start worked from the word go for the first couple of miles and then nothing during another 45 minutes driving.

[nielshm]

It's strange that you have so many difficulties with this system. Could it be, that pressing the clutch, put gearbox in neutral, release clutch, is a badly programmed routine, where many small things and parameters can go wrong, wich results in no function from start/stop?

The autobax works like a charm, enginetemp. higher than ~50°C, and the system kicks in, unless the Battery needs charging, or the A/C is trying to get inside tempratures right. 

[Mooly]

I really don't know the answer to why it behaves as it does although I wish I did. Badly programmed or just to cautious, I really don't know.

[Heidfirst]

I was reading the relevant page for Stop/Start (only fitted with the BMW diesels) in the Avensis manual the other night. 3-4 pages of conditions under which it may/may not work so it is complicated 😛.

[Mooly]

Complicated with a lot of things monitored I can understand, it is that it is not consistent that gets me. Today was another day of 60 minutes driving where it didn't cut in once.

[Heidfirst]

2 hours ago, Mooly said:

Complicated with a lot of things monitored I can understand, it is that it is not consistent that gets me.

It may not appear consistent to you but it will be acting within the rules of the algorithm that governs it so it probably is - just that you are not aware of what it is that has changed that resulted in it altering it's behaviour from previous.

[tonybeau]

If we were meant to know, the algorithm would have been printed on pages 2,3 and 4 of the user manual.  

[Mooly]

I guess so...

Had a further play today. I haven't a suitable modern Battery charger, only an old one from over 35 years ago but I do have current limited stabilised power supplies and so I set one of these to 14.4 volts and looked at what happened. I was surprised to see the Battery voltage rise very quickly to this value implying that it was essentially in a fair state of charge. The current fell to around 400 milliamps after only 5 minutes . I left this on for a couple of hours before driving off. Now 400ma over two hours is less than 1Ah put back and yet the starter was noticeably more lively PLUS the Stop/ Start worked every time today. I put it on a similar charge again for a couple of hours this afternoon.

I also noticed something else. Now when the Stop/Start restarts, the alternator output is held low (12.5v) for a few seconds. Can't say I had noticed that aspect of behaviour before but suspect it is to reduce loading on the engine following a restart and when you may want to set off quickly.

All very interesting though.